Should I use precondition checks to check intermediate results?

Should I use precondition checks to check intermediate results? - java

Guava offers helper functions to check the preconditions but I could not find helper functions to check intermediate results.
private void foo(String param)
{
checkNotNull(param, "Required parameter is not set");
int x = get(param);
if (x == -1) {
throw new RuntimeException("This should never have happened and indicates a bug.");
}
}
Should I wrap the if (...) {....} part in my own helper?
Or should I use checkState from Guava?
Or should I view the failure of get() as a consequence of param and use checkArgument?
Should I use asserts in these cases?
Or am I missing something?

It's somewhere between a matter of preference and a matter of convention.
Generally, people will use asserts to indicate programming errors; that is, "if I did my job right, then a non-null param should never result in a -1 from get, regardless of user input or other outside forces." I treat them almost as comments that can optionally be verified at runtime.
On the other hand, if get might return -1 in some cases, but that input is invalid, then I would generally throw an IllegalArgumentException, and checkArgument is a perfectly reasonable way to do this. One drawback this has is that when you later catch that, it could have come from pretty much anywhere. Consider:
try {
baz();
bar();
foo(myInput);
} catch (IllegalArgumentException e) {
// Where did this come from!?
// It could have come from foo(myInput), or baz(), or bar(),
// or some method that any of them invoked, or really anywhere
// in that stack.
// It could be something totally unrelated to user input, just
// a bug somewhere in my code.
// Handle it somehow...
}
In cases where that matters -- for instance, you want to pop up a helpful note to the user that they're not allowed to enter -1 in their input form -- you may want to throw a custom exception so that you can more easily catch it later:
try {
baz();
bar();
foo(myInput);
} catch (BadUserInputException e) {
reportError("Bad input: " + e.getMessage());
log.info("recorded bad user input", e);
}
As for checkState, it doesn't really sound right to me. That exception usually implies that the problem was the state that this was in (or some other, more global state in the application). From the docs:
Signals that a method has been invoked at an illegal or inappropriate time.
In your case, a -1 is never appropriate, so checkState is misleading. Now, if it had been:
if (x == -1 && (!allowNegativeOne()) { ... }
...then that would be more appropriate, though it still has the drawback that IllegalArgumentException had above.
So, lastly, there's the question of whether you should just keep the if as it is, or use a helper method. That really comes down to taste, how complex the check is, and how often it's used (e.g. in other methods). If the check is as simple as x == -1 and that check isn't ever performed by other methods (so code reuse is not an issue), I would just keep the if.

If the get method is simply converting the string to an int, then it should do the validation there, preferably throwing an illegalArgumentException or some such RuntimeException. With the above you are also mixing levels of abstraction in your method. E.g. your checkNotNull abstracts away the checking of param for null, but the checking for param as an int is split across the get method and the foo method. Why not have one checkPreCondition type method? E.g.
private void paramShouldBeNonNullInt(String value) {
if (value == null) throw new IllegalArgumentException("value was null");
try {
Integer.parseInt(value)
} catch (NumberFormatException e) {
throw new IllegalArgumentException("value was not an integer");
}
}

First of all you need to make a distinction between contracts (e.g assertions/programming errors) and error handling (e.g. recoverable exceptions that could and should be caught and recovered from).
If you have the need to check an intermediate result, it seems like you don't trust the invoked service and you want to make sure your assumptions hold. Right? This should be expressed as an assertion, and Guava don't have very good support for that.
Have a look at valid4j. Found here https://github.com/helsing/valid4j and here http://www.valid4j.org.
I would then have expressed the code like this (using valid4j's support for hamcrest-matchers):
private int getFoo(String param) {
require(param, notNullValue()); // Violation means programming error at client
int x = get(param);
ensure(x, not(equalTo(-1)); // Violation means programming error at supplier
return x;
}

Some other excellent answers here.
From the Preconditions javadoc:
Precondition exceptions are used to signal that the calling method has made an error. (...) Postcondition or other invariant failures should not throw these types of exceptions.
So ...
Should I wrap the if (...) {....} part in my own helper?
No, existing facilities should be good enough.
Or should I use checkState from Guava?
Yes possibly: if parameters need to be loaded from a file before this method is called, then that would be part of the contract of how this class must be used.
Or should I view the failure of get() as a consequence of param and use checkArgument?
Yes possibly: e.g. if there was some formatting restriction on the syntax of parameters. (Although perhaps that would go inside get())
Should I use asserts in these cases?
Yes. If it's not a precondition check like the above, then normally I'd just use an assert here. Don't forget you can still add a message:
assert x != 1 : "Indicates a bug.";
I find this appropriate to document expectations and verify the internal / private implementation of a class or method.
If you want to make that a runtime check, you could do if (...) throw AssertionError but that's probably only necessary if you're working with dodgy code that you don't trust.

Related

Avoid duplicated code 'cause of null objects? [duplicate]

I use x != null to avoid NullPointerException. Is there an alternative?
if (x != null) {
// ...
}

This to me sounds like a reasonably common problem that junior to intermediate developers tend to face at some point: they either don't know or don't trust the contracts they are participating in and defensively overcheck for nulls. Additionally, when writing their own code, they tend to rely on returning nulls to indicate something thus requiring the caller to check for nulls.
To put this another way, there are two instances where null checking comes up:
Where null is a valid response in terms of the contract; and
Where it isn't a valid response.
(2) is easy. As of Java 1.7 you can use Objects.requireNonNull(foo). (If you are stuck with a previous version then assertions may be a good alternative.)
"Proper" usage of this method would be like below. The method returns the object passed into it and throws a NullPointerException if the object is null. This means that the returned value is always non-null. The method is primarily intended for validating parameters.
public Foo(Bar bar) {
this.bar = Objects.requireNonNull(bar);
}
It can also be used like an assertion though since it throws an exception if the object is null. In both uses, a message can be added which will be shown in the exception. Below is using it like an assertion and providing a message.
Objects.requireNonNull(someobject, "if someobject is null then something is wrong");
someobject.doCalc();
Generally throwing a specific exception like NullPointerException when a value is null but shouldn't be is favorable to throwing a more general exception like AssertionError. This is the approach the Java library takes; favoring NullPointerException over IllegalArgumentException when an argument is not allowed to be null.
(1) is a little harder. If you have no control over the code you're calling then you're stuck. If null is a valid response, you have to check for it.
If it's code that you do control, however (and this is often the case), then it's a different story. Avoid using nulls as a response. With methods that return collections, it's easy: return empty collections (or arrays) instead of nulls pretty much all the time.
With non-collections it might be harder. Consider this as an example: if you have these interfaces:
public interface Action {
void doSomething();
}
public interface Parser {
Action findAction(String userInput);
}
where Parser takes raw user input and finds something to do, perhaps if you're implementing a command line interface for something. Now you might make the contract that it returns null if there's no appropriate action. That leads the null checking you're talking about.
An alternative solution is to never return null and instead use the Null Object pattern:
public class MyParser implements Parser {
private static Action DO_NOTHING = new Action() {
public void doSomething() { /* do nothing */ }
};
public Action findAction(String userInput) {
// ...
if ( /* we can't find any actions */ ) {
return DO_NOTHING;
}
}
}
Compare:
Parser parser = ParserFactory.getParser();
if (parser == null) {
// now what?
// this would be an example of where null isn't (or shouldn't be) a valid response
}
Action action = parser.findAction(someInput);
if (action == null) {
// do nothing
} else {
action.doSomething();
}
to
ParserFactory.getParser().findAction(someInput).doSomething();
which is a much better design because it leads to more concise code.
That said, perhaps it is entirely appropriate for the findAction() method to throw an Exception with a meaningful error message -- especially in this case where you are relying on user input. It would be much better for the findAction method to throw an Exception than for the calling method to blow up with a simple NullPointerException with no explanation.
try {
ParserFactory.getParser().findAction(someInput).doSomething();
} catch(ActionNotFoundException anfe) {
userConsole.err(anfe.getMessage());
}
Or if you think the try/catch mechanism is too ugly, rather than Do Nothing your default action should provide feedback to the user.
public Action findAction(final String userInput) {
/* Code to return requested Action if found */
return new Action() {
public void doSomething() {
userConsole.err("Action not found: " + userInput);
}
}
}

If you use (or planning to use) a Java IDE like JetBrains IntelliJ IDEA, Eclipse or Netbeans or a tool like findbugs then you can use annotations to solve this problem.
Basically, you've got #Nullable and #NotNull.
You can use in method and parameters, like this:
#NotNull public static String helloWorld() {
return "Hello World";
}
or
#Nullable public static String helloWorld() {
return "Hello World";
}
The second example won't compile (in IntelliJ IDEA).
When you use the first helloWorld() function in another piece of code:
public static void main(String[] args)
{
String result = helloWorld();
if(result != null) {
System.out.println(result);
}
}
Now the IntelliJ IDEA compiler will tell you that the check is useless, since the helloWorld() function won't return null, ever.
Using parameter
void someMethod(#NotNull someParameter) { }
if you write something like:
someMethod(null);
This won't compile.
Last example using #Nullable
#Nullable iWantToDestroyEverything() { return null; }
Doing this
iWantToDestroyEverything().something();
And you can be sure that this won't happen. :)
It's a nice way to let the compiler check something more than it usually does and to enforce your contracts to be stronger. Unfortunately, it's not supported by all the compilers.
In IntelliJ IDEA 10.5 and on, they added support for any other #Nullable #NotNull implementations.
See blog post More flexible and configurable #Nullable/#NotNull annotations.

If null-values are not allowed
If your method is called externally, start with something like this:
public void method(Object object) {
if (object == null) {
throw new IllegalArgumentException("...");
}
Then, in the rest of that method, you'll know that object is not null.
If it is an internal method (not part of an API), just document that it cannot be null, and that's it.
Example:
public String getFirst3Chars(String text) {
return text.subString(0, 3);
}
However, if your method just passes the value on, and the next method passes it on etc. it could get problematic. In that case you may want to check the argument as above.
If null is allowed
This really depends. If find that I often do something like this:
if (object == null) {
// something
} else {
// something else
}
So I branch, and do two completely different things. There is no ugly code snippet, because I really need to do two different things depending on the data. For example, should I work on the input, or should I calculate a good default value?
It's actually rare for me to use the idiom "if (object != null && ...".
It may be easier to give you examples, if you show examples of where you typically use the idiom.

Wow, I almost hate to add another answer when we have 57 different ways to recommend the NullObject pattern, but I think that some people interested in this question may like to know that there is a proposal on the table for Java 7 to add "null-safe handling"—a streamlined syntax for if-not-equal-null logic.
The example given by Alex Miller looks like this:
public String getPostcode(Person person) {
return person?.getAddress()?.getPostcode();
}
The ?. means only de-reference the left identifier if it is not null, otherwise evaluate the remainder of the expression as null. Some people, like Java Posse member Dick Wall and the voters at Devoxx really love this proposal, but there is opposition too, on the grounds that it will actually encourage more use of null as a sentinel value.
Update: An official proposal for a null-safe operator in Java 7 has been submitted under Project Coin. The syntax is a little different than the example above, but it's the same notion.
Update: The null-safe operator proposal didn't make it into Project Coin. So, you won't be seeing this syntax in Java 7.

If undefined values are not permitted:
You might configure your IDE to warn you about potential null dereferencing. E.g. in Eclipse, see Preferences > Java > Compiler > Errors/Warnings/Null analysis.
If undefined values are permitted:
If you want to define a new API where undefined values make sense, use the Option Pattern (may be familiar from functional languages). It has the following advantages:
It is stated explicitly in the API whether an input or output exists or not.
The compiler forces you to handle the "undefined" case.
Option is a monad, so there is no need for verbose null checking, just use map/foreach/getOrElse or a similar combinator to safely use the value (example).
Java 8 has a built-in Optional class (recommended); for earlier versions, there are library alternatives, for example Guava's Optional or FunctionalJava's Option. But like many functional-style patterns, using Option in Java (even 8) results in quite some boilerplate, which you can reduce using a less verbose JVM language, e.g. Scala or Xtend.
If you have to deal with an API which might return nulls, you can't do much in Java. Xtend and Groovy have the Elvis operator ?: and the null-safe dereference operator ?., but note that this returns null in case of a null reference, so it just "defers" the proper handling of null.

Only for this situation -
Not checking if a variable is null before invoking an equals method (a string compare example below):
if ( foo.equals("bar") ) {
// ...
}
will result in a NullPointerException if foo doesn't exist.
You can avoid that if you compare your Strings like this:
if ( "bar".equals(foo) ) {
// ...
}

With Java 8 comes the new java.util.Optional class that arguably solves some of the problem. One can at least say that it improves the readability of the code, and in the case of public APIs make the API's contract clearer to the client developer.
They work like that:
An optional object for a given type (Fruit) is created as the return type of a method. It can be empty or contain a Fruit object:
public static Optional<Fruit> find(String name, List<Fruit> fruits) {
for (Fruit fruit : fruits) {
if (fruit.getName().equals(name)) {
return Optional.of(fruit);
}
}
return Optional.empty();
}
Now look at this code where we search a list of Fruit (fruits) for a given Fruit instance:
Optional<Fruit> found = find("lemon", fruits);
if (found.isPresent()) {
Fruit fruit = found.get();
String name = fruit.getName();
}
You can use the map() operator to perform a computation on--or extract a value from--an optional object. orElse() lets you provide a fallback for missing values.
String nameOrNull = find("lemon", fruits)
.map(f -> f.getName())
.orElse("empty-name");
Of course, the check for null/empty value is still necessary, but at least the developer is conscious that the value might be empty and the risk of forgetting to check is limited.
In an API built from scratch using Optional whenever a return value might be empty, and returning a plain object only when it cannot be null (convention), the client code might abandon null checks on simple object return values...
Of course Optional could also be used as a method argument, perhaps a better way to indicate optional arguments than 5 or 10 overloading methods in some cases.
Optional offers other convenient methods, such as orElse that allow the use of a default value, and ifPresent that works with lambda expressions.
I invite you to read this article (my main source for writing this answer) in which the NullPointerException (and in general null pointer) problematic as well as the (partial) solution brought by Optional are well explained: Java Optional Objects.

Depending on what kind of objects you are checking you may be able to use some of the classes in the apache commons such as: apache commons lang and apache commons collections
Example:
String foo;
...
if( StringUtils.isBlank( foo ) ) {
///do something
}
or (depending on what you need to check):
String foo;
...
if( StringUtils.isEmpty( foo ) ) {
///do something
}
The StringUtils class is only one of many; there are quite a few good classes in the commons that do null safe manipulation.
Here follows an example of how you can use null vallidation in JAVA when you include apache library(commons-lang-2.4.jar)
public DOCUMENT read(String xml, ValidationEventHandler validationEventHandler) {
Validate.notNull(validationEventHandler,"ValidationHandler not Injected");
return read(new StringReader(xml), true, validationEventHandler);
}
And if you are using Spring, Spring also has the same functionality in its package, see library(spring-2.4.6.jar)
Example on how to use this static classf from spring(org.springframework.util.Assert)
Assert.notNull(validationEventHandler,"ValidationHandler not Injected");

If you consider an object should not be null (or it is a bug) use an assert.
If your method doesn't accept null params say it in the javadoc and use an assert.
You have to check for object != null only if you want to handle the case where the object may be null...
There is a proposal to add new annotations in Java7 to help with null / notnull params:
http://tech.puredanger.com/java7/#jsr308

I'm a fan of "fail fast" code. Ask yourself - are you doing something useful in the case where the parameter is null? If you don't have a clear answer for what your code should do in that case... i.e. - it should never be null in the first place, then ignore it and allow a NullPointerException to be thrown. The calling code will make just as much sense of an NPE as it would an IllegalArgumentException, but it'll be easier for the developer to debug and understand what went wrong if an NPE is thrown rather than your code attempting to execute some other unexpected contingency logic - which ultimately results in the application failing anyway.

Sometimes, you have methods that operate on its parameters that define a symmetric operation:
a.f(b); <-> b.f(a);
If you know b can never be null, you can just swap it. It is most useful for equals:
Instead of foo.equals("bar"); better do "bar".equals(foo);.

Rather than Null Object Pattern -- which has its uses -- you might consider situations where the null object is a bug.
When the exception is thrown, examine the stack trace and work through the bug.

The Google collections framework offers a good and elegant way to achieve the null check.
There is a method in a library class like this:
static <T> T checkNotNull(T e) {
if (e == null) {
throw new NullPointerException();
}
return e;
}
And the usage is (with import static):
...
void foo(int a, Person p) {
if (checkNotNull(p).getAge() > a) {
...
}
else {
...
}
}
...
Or in your example:
checkNotNull(someobject).doCalc();

Null is not a 'problem'. It is an integral part of a complete modeling tool set. Software aims to model the complexity of the world and null bears its burden. Null indicates 'No data' or 'Unknown' in Java and the like. So it is appropriate to use nulls for these purposes. I don't prefer the 'Null object' pattern; I think it rise the 'who will guard
the guardians' problem.
If you ask me what is the name of my girlfriend I'll tell you that I have no girlfriend. In the Java language I'll return null.
An alternative would be to throw meaningful exception to indicate some problem that can't be (or don't want to be) solved right there and delegate it somewhere higher in the stack to retry or report data access error to the user.
For an 'unknown question' give 'unknown answer'. (Be null-safe where this is correct from business point of view) Checking arguments for null once inside a method before usage relieves multiple callers from checking them before a call.
public Photo getPhotoOfThePerson(Person person) {
if (person == null)
return null;
// Grabbing some resources or intensive calculation
// using person object anyhow.
}
Previous leads to normal logic flow to get no photo of a non-existent girlfriend from my photo library.
getPhotoOfThePerson(me.getGirlfriend())
And it fits with new coming Java API (looking forward)
getPhotoByName(me.getGirlfriend()?.getName())
While it is rather 'normal business flow' not to find photo stored into the DB for some person, I used to use pairs like below for some other cases
public static MyEnum parseMyEnum(String value); // throws IllegalArgumentException
public static MyEnum parseMyEnumOrNull(String value);
And don't loathe to type <alt> + <shift> + <j> (generate javadoc in Eclipse) and write three additional words for you public API. This will be more than enough for all but those who don't read documentation.
/**
* #return photo or null
*/
or
/**
* #return photo, never null
*/
This is rather theoretical case and in most cases you should prefer java null safe API (in case it will be released in another 10 years), but NullPointerException is subclass of an Exception. Thus it is a form of Throwable that indicates conditions that a reasonable application might want to catch (javadoc)! To use the first most advantage of exceptions and separate error-handling code from 'regular' code (according to creators of Java) it is appropriate, as for me, to catch NullPointerException.
public Photo getGirlfriendPhoto() {
try {
return appContext.getPhotoDataSource().getPhotoByName(me.getGirlfriend().getName());
} catch (NullPointerException e) {
return null;
}
}
Questions could arise:
Q. What if getPhotoDataSource() returns null?
A. It is up to business logic. If I fail to find a photo album I'll show you no photos. What if appContext is not initialized? This method's business logic puts up with this. If the same logic should be more strict then throwing an exception it is part of the business logic and explicit check for null should be used (case 3). The new Java Null-safe API fits better here to specify selectively what implies and what does not imply to be initialized to be fail-fast in case of programmer errors.
Q. Redundant code could be executed and unnecessary resources could be grabbed.
A. It could take place if getPhotoByName() would try to open a database connection, create PreparedStatement and use the person name as an SQL parameter at last. The approach for an unknown question gives an unknown answer (case 1) works here. Before grabbing resources the method should check parameters and return 'unknown' result if needed.
Q. This approach has a performance penalty due to the try closure opening.
A. Software should be easy to understand and modify firstly. Only after this, one could think about performance, and only if needed! and where needed! (source), and many others).
PS. This approach will be as reasonable to use as the separate error-handling code from "regular" code principle is reasonable to use in some place. Consider the next example:
public SomeValue calculateSomeValueUsingSophisticatedLogic(Predicate predicate) {
try {
Result1 result1 = performSomeCalculation(predicate);
Result2 result2 = performSomeOtherCalculation(result1.getSomeProperty());
Result3 result3 = performThirdCalculation(result2.getSomeProperty());
Result4 result4 = performLastCalculation(result3.getSomeProperty());
return result4.getSomeProperty();
} catch (NullPointerException e) {
return null;
}
}
public SomeValue calculateSomeValueUsingSophisticatedLogic(Predicate predicate) {
SomeValue result = null;
if (predicate != null) {
Result1 result1 = performSomeCalculation(predicate);
if (result1 != null && result1.getSomeProperty() != null) {
Result2 result2 = performSomeOtherCalculation(result1.getSomeProperty());
if (result2 != null && result2.getSomeProperty() != null) {
Result3 result3 = performThirdCalculation(result2.getSomeProperty());
if (result3 != null && result3.getSomeProperty() != null) {
Result4 result4 = performLastCalculation(result3.getSomeProperty());
if (result4 != null) {
result = result4.getSomeProperty();
}
}
}
}
}
return result;
}
PPS. For those fast to downvote (and not so fast to read documentation) I would like to say that I've never caught a null-pointer exception (NPE) in my life. But this possibility was intentionally designed by the Java creators because NPE is a subclass of Exception. We have a precedent in Java history when ThreadDeath is an Error not because it is actually an application error, but solely because it was not intended to be caught! How much NPE fits to be an Error than ThreadDeath! But it is not.
Check for 'No data' only if business logic implies it.
public void updatePersonPhoneNumber(Long personId, String phoneNumber) {
if (personId == null)
return;
DataSource dataSource = appContext.getStuffDataSource();
Person person = dataSource.getPersonById(personId);
if (person != null) {
person.setPhoneNumber(phoneNumber);
dataSource.updatePerson(person);
} else {
Person = new Person(personId);
person.setPhoneNumber(phoneNumber);
dataSource.insertPerson(person);
}
}
and
public void updatePersonPhoneNumber(Long personId, String phoneNumber) {
if (personId == null)
return;
DataSource dataSource = appContext.getStuffDataSource();
Person person = dataSource.getPersonById(personId);
if (person == null)
throw new SomeReasonableUserException("What are you thinking about ???");
person.setPhoneNumber(phoneNumber);
dataSource.updatePerson(person);
}
If appContext or dataSource is not initialized unhandled runtime NullPointerException will kill current thread and will be processed by Thread.defaultUncaughtExceptionHandler (for you to define and use your favorite logger or other notification mechanizm). If not set, ThreadGroup#uncaughtException will print stacktrace to system err. One should monitor application error log and open Jira issue for each unhandled exception which in fact is application error. Programmer should fix bug somewhere in initialization stuff.

Java 7 has a new java.util.Objects utility class on which there is a requireNonNull() method. All this does is throw a NullPointerException if its argument is null, but it cleans up the code a bit. Example:
Objects.requireNonNull(someObject);
someObject.doCalc();
The method is most useful for checking just before an assignment in a constructor, where each use of it can save three lines of code:
Parent(Child child) {
if (child == null) {
throw new NullPointerException("child");
}
this.child = child;
}
becomes
Parent(Child child) {
this.child = Objects.requireNonNull(child, "child");
}

Ultimately, the only way to completely solve this problem is by using a different programming language:
In Objective-C, you can do the equivalent of invoking a method on nil, and absolutely nothing will happen. This makes most null checks unnecessary, but it can make errors much harder to diagnose.
In Nice, a Java-derived language, there are two versions of all types: a potentially-null version and a not-null version. You can only invoke methods on not-null types. Potentially-null types can be converted to not-null types through explicit checking for null. This makes it much easier to know where null checks are necessary and where they aren't.

Common "problem" in Java indeed.
First, my thoughts on this:
I consider that it is bad to "eat" something when NULL was passed where NULL isn't a valid value. If you're not exiting the method with some sort of error then it means nothing went wrong in your method which is not true. Then you probably return null in this case, and in the receiving method you again check for null, and it never ends, and you end up with "if != null", etc..
So, IMHO, null must be a critical error which prevents further execution (that is, where null is not a valid value).
The way I solve this problem is this:
First, I follow this convention:
All public methods / API always check its arguments for null
All private methods do not check for null since they are controlled methods (just let die with nullpointer exception in case it wasn't handled above)
The only other methods which do not check for null are utility methods. They are public, but if you call them for some reason, you know what parameters you pass. This is like trying to boil water in the kettle without providing water...
And finally, in the code, the first line of the public method goes like this:
ValidationUtils.getNullValidator().addParam(plans, "plans").addParam(persons, "persons").validate();
Note that addParam() returns self, so that you can add more parameters to check.
Method validate() will throw checked ValidationException if any of the parameters is null (checked or unchecked is more a design/taste issue, but my ValidationException is checked).
void validate() throws ValidationException;
The message will contain the following text if, for example, "plans" is null:
"Illegal argument value null is encountered for parameter [plans]"
As you can see, the second value in the addParam() method (string) is needed for the user message, because you cannot easily detect passed-in variable name, even with reflection (not subject of this post anyway...).
And yes, we know that beyond this line we will no longer encounter a null value so we just safely invoke methods on those objects.
This way, the code is clean, easy maintainable and readable.

Asking that question points out that you may be interested in error handling strategies. How and where to handle errors is a pervasive architectural question. There are several ways to do this.
My favorite: allow the Exceptions to ripple through - catch them at the 'main loop' or in some other function with the appropriate responsibilities. Checking for error conditions and handling them appropriately can be seen as a specialized responsibility.
Sure do have a look at Aspect Oriented Programming, too - they have neat ways to insert if( o == null ) handleNull() into your bytecode.

In addition to using assert you can use the following:
if (someobject == null) {
// Handle null here then move on.
}
This is slightly better than:
if (someobject != null) {
.....
.....
.....
}

Just don't ever use null. Don't allow it.
In my classes, most fields and local variables have non-null default values, and I add contract statements (always-on asserts) everywhere in the code to make sure this is being enforced (since it's more succinct, and more expressive than letting it come up as an NPE and then having to resolve the line number, etc.).
Once I adopted this practice, I noticed that the problems seemed to fix themselves. You'd catch things much earlier in the development process just by accident and realize you had a weak spot.. and more importantly.. it helps encapsulate different modules' concerns, different modules can 'trust' each other, and no more littering the code with if = null else constructs!
This is defensive programming and results in much cleaner code in the long run. Always sanitize the data, e.g. here by enforcing rigid standards, and the problems go away.
class C {
private final MyType mustBeSet;
public C(MyType mything) {
mustBeSet=Contract.notNull(mything);
}
private String name = "<unknown>";
public void setName(String s) {
name = Contract.notNull(s);
}
}
class Contract {
public static <T> T notNull(T t) { if (t == null) { throw new ContractException("argument must be non-null"); return t; }
}
The contracts are like mini-unit tests which are always running, even in production, and when things fail, you know why, rather than a random NPE you have to somehow figure out.

Guava, a very useful core library by Google, has a nice and useful API to avoid nulls. I find UsingAndAvoidingNullExplained very helpful.
As explained in the wiki:
Optional<T> is a way of replacing a nullable T reference with a
non-null value. An Optional may either contain a non-null T reference
(in which case we say the reference is "present"), or it may contain
nothing (in which case we say the reference is "absent"). It is never
said to "contain null."
Usage:
Optional<Integer> possible = Optional.of(5);
possible.isPresent(); // returns true
possible.get(); // returns 5

This is a very common problem for every Java developer. So there is official support in Java 8 to address these issues without cluttered code.
Java 8 has introduced java.util.Optional<T>. It is a container that may or may not hold a non-null value. Java 8 has given a safer way to handle an object whose value may be null in some of the cases. It is inspired from the ideas of Haskell and Scala.
In a nutshell, the Optional class includes methods to explicitly deal with the cases where a value is present or absent. However, the advantage compared to null references is that the Optional<T> class forces you to think about the case when the value is not present. As a consequence, you can prevent unintended null pointer exceptions.
In above example we have a home service factory that returns a handle to multiple appliances available in the home. But these services may or may not be available/functional; it means it may result in a NullPointerException. Instead of adding a null if condition before using any service, let's wrap it in to Optional<Service>.
WRAPPING TO OPTION<T>
Let's consider a method to get a reference of a service from a factory. Instead of returning the service reference, wrap it with Optional. It lets the API user know that the returned service may or may not available/functional, use defensively
public Optional<Service> getRefrigertorControl() {
Service s = new RefrigeratorService();
//...
return Optional.ofNullable(s);
}
As you see Optional.ofNullable() provides an easy way to get the reference wrapped. There are another ways to get the reference of Optional, either Optional.empty() & Optional.of(). One for returning an empty object instead of retuning null and the other to wrap a non-nullable object, respectively.
SO HOW EXACTLY IT HELPS TO AVOID A NULL CHECK?
Once you have wrapped a reference object, Optional provides many useful methods to invoke methods on a wrapped reference without NPE.
Optional ref = homeServices.getRefrigertorControl();
ref.ifPresent(HomeServices::switchItOn);
Optional.ifPresent invokes the given Consumer with a reference if it is a non-null value. Otherwise, it does nothing.
#FunctionalInterface
public interface Consumer<T>
Represents an operation that accepts a single input argument and returns no result. Unlike most other functional interfaces, Consumer is expected to operate via side-effects.
It is so clean and easy to understand. In the above code example, HomeService.switchOn(Service) gets invoked if the Optional holding reference is non-null.
We use the ternary operator very often for checking null condition and return an alternative value or default value. Optional provides another way to handle the same condition without checking null. Optional.orElse(defaultObj) returns defaultObj if the Optional has a null value. Let's use this in our sample code:
public static Optional<HomeServices> get() {
service = Optional.of(service.orElse(new HomeServices()));
return service;
}
Now HomeServices.get() does same thing, but in a better way. It checks whether the service is already initialized of not. If it is then return the same or create a new New service. Optional<T>.orElse(T) helps to return a default value.
Finally, here is our NPE as well as null check-free code:
import java.util.Optional;
public class HomeServices {
private static final int NOW = 0;
private static Optional<HomeServices> service;
public static Optional<HomeServices> get() {
service = Optional.of(service.orElse(new HomeServices()));
return service;
}
public Optional<Service> getRefrigertorControl() {
Service s = new RefrigeratorService();
//...
return Optional.ofNullable(s);
}
public static void main(String[] args) {
/* Get Home Services handle */
Optional<HomeServices> homeServices = HomeServices.get();
if(homeServices != null) {
Optional<Service> refrigertorControl = homeServices.get().getRefrigertorControl();
refrigertorControl.ifPresent(HomeServices::switchItOn);
}
}
public static void switchItOn(Service s){
//...
}
}
The complete post is NPE as well as Null check-free code … Really?.

I like articles from Nat Pryce. Here are the links:
Avoiding Nulls with Polymorphic Dispatch
Avoiding Nulls with "Tell, Don't Ask" Style
In the articles there is also a link to a Git repository for a Java Maybe Type which I find interesting, but I don't think it alone could decrease the
checking code bloat. After doing some research on the Internet, I think != null code bloat could be decreased mainly by careful design.

I've tried the NullObjectPattern but for me is not always the best way to go. There are sometimes when a "no action" is not appropiate.
NullPointerException is a Runtime exception that means it's developers fault and with enough experience it tells you exactly where is the error.
Now to the answer:
Try to make all your attributes and its accessors as private as possible or avoid to expose them to the clients at all. You can have the argument values in the constructor of course, but by reducing the scope you don't let the client class pass an invalid value. If you need to modify the values, you can always create a new object. You check the values in the constructor only once and in the rest of the methods you can be almost sure that the values are not null.
Of course, experience is the better way to understand and apply this suggestion.
Byte!

Probably the best alternative for Java 8 or newer is to use the Optional class.
Optional stringToUse = Optional.of("optional is there");
stringToUse.ifPresent(System.out::println);
This is especially handy for long chains of possible null values. Example:
Optional<Integer> i = Optional.ofNullable(wsObject.getFoo())
.map(f -> f.getBar())
.map(b -> b.getBaz())
.map(b -> b.getInt());
Example on how to throw exception on null:
Optional optionalCarNull = Optional.ofNullable(someNull);
optionalCarNull.orElseThrow(IllegalStateException::new);
Java 7 introduced the Objects.requireNonNull method which can be handy when something should be checked for non-nullness. Example:
String lowerVal = Objects.requireNonNull(someVar, "input cannot be null or empty").toLowerCase();

May I answer it more generally!
We usually face this issue when the methods get the parameters in the way we not expected (bad method call is programmer's fault). For example: you expect to get an object, instead you get a null. You expect to get an String with at least one character, instead you get an empty String ...
So there is no difference between:
if(object == null){
//you called my method badly!
}
or
if(str.length() == 0){
//you called my method badly again!
}
They both want to make sure that we received valid parameters, before we do any other functions.
As mentioned in some other answers, to avoid above problems you can follow the Design by contract pattern. Please see http://en.wikipedia.org/wiki/Design_by_contract.
To implement this pattern in java, you can use core java annotations like javax.annotation.NotNull or use more sophisticated libraries like Hibernate Validator.
Just a sample:
getCustomerAccounts(#NotEmpty String customerId,#Size(min = 1) String accountType)
Now you can safely develop the core function of your method without needing to check input parameters, they guard your methods from unexpected parameters.
You can go a step further and make sure that only valid pojos could be created in your application. (sample from hibernate validator site)
public class Car {
#NotNull
private String manufacturer;
#NotNull
#Size(min = 2, max = 14)
private String licensePlate;
#Min(2)
private int seatCount;
// ...
}

I highly disregard answers that suggest using the null objects in every situation. This pattern may break the contract and bury problems deeper and deeper instead of solving them, not mentioning that used inappropriately will create another pile of boilerplate code that will require future maintenance.
In reality if something returned from a method can be null and the calling code has to make decision upon that, there should an earlier call that ensures the state.
Also keep in mind, that null object pattern will be memory hungry if used without care. For this - the instance of a NullObject should be shared between owners, and not be an unigue instance for each of these.
Also I would not recommend using this pattern where the type is meant to be a primitive type representation - like mathematical entities, that are not scalars: vectors, matrices, complex numbers and POD(Plain Old Data) objects, which are meant to hold state in form of Java built-in types. In the latter case you would end up calling getter methods with arbitrary results. For example what should a NullPerson.getName() method return?
It's worth considering such cases in order to avoid absurd results.

Never initialise variables to null.
If (1) is not possible, initialise all collections and arrays to empty collections/arrays.
Doing this in your own code and you can avoid != null checks.
Most of the time null checks seem to guard loops over collections or arrays, so just initialise them empty, you won't need any null checks.
// Bad
ArrayList<String> lemmings;
String[] names;
void checkLemmings() {
if (lemmings != null) for(lemming: lemmings) {
// do something
}
}
// Good
ArrayList<String> lemmings = new ArrayList<String>();
String[] names = {};
void checkLemmings() {
for(lemming: lemmings) {
// do something
}
}
There is a tiny overhead in this, but it's worth it for cleaner code and less NullPointerExceptions.

This is the most common error occurred for most of the developers.
We have number of ways to handle this.
Approach 1:
org.apache.commons.lang.Validate //using apache framework
notNull(Object object, String message)
Approach 2:
if(someObject!=null){ // simply checking against null
}
Approach 3:
#isNull #Nullable // using annotation based validation
Approach 4:
// by writing static method and calling it across whereever we needed to check the validation
static <T> T isNull(someObject e){
if(e == null){
throw new NullPointerException();
}
return e;
}

Java 8 has introduced a new class Optional in java.util package.
Advantages of Java 8 Optional:
1.) Null checks are not required.
2.) No more NullPointerException at run-time.
3.) We can develop clean and neat APIs.
Optional - A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.
For more details find here oracle docs :-
https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html

When should an IllegalArgumentException be thrown?

I'm worried that this is a runtime exception so it should probably be used sparingly.
Standard use case:
void setPercentage(int pct) {
if( pct < 0 || pct > 100) {
throw new IllegalArgumentException("bad percent");
}
}
But that seems like it would force the following design:
public void computeScore() throws MyPackageException {
try {
setPercentage(userInputPercent);
}
catch(IllegalArgumentException exc){
throw new MyPackageException(exc);
}
}
To get it back to being a checked exception.
Okay, but let's go with that. If you give bad input, you get a runtime error. So firstly that's actually a fairly difficult policy to implement uniformly, because you could have to do the very opposite conversion:
public void scanEmail(String emailStr, InputStream mime) {
try {
EmailAddress parsedAddress = EmailUtil.parse(emailStr);
}
catch(ParseException exc){
throw new IllegalArgumentException("bad email", exc);
}
}
And worse - while checking 0 <= pct && pct <= 100 the client code could be expected to do statically, this is not so for more advanced data such as an email address, or worse, something that has to be checked against a database, therefore in general client code cannot pre-validate.
So basically what I'm saying is I don't see a meaningful consistent policy for the use of IllegalArgumentException. It seems it should not be used and we should stick to our own checked exceptions. What is a good use case to throw this?

The API doc for IllegalArgumentException:
Thrown to indicate that a method has been passed an illegal or inappropriate argument.
From looking at how it is used in the JDK libraries, I would say:
It seems like a defensive measure to complain about obviously bad input before the input can get into the works and cause something to fail halfway through with a nonsensical error message.
It's used for cases where it would be too annoying to throw a checked exception (although it makes an appearance in the java.lang.reflect code, where concern about ridiculous levels of checked-exception-throwing is not otherwise apparent).
I would use IllegalArgumentException to do last ditch defensive argument checking for common utilities (trying to stay consistent with the JDK usage). Or where the expectation is that a bad argument is a programmer error, similar to an NullPointerException. I wouldn't use it to implement validation in business code. I certainly wouldn't use it for the email example.

When talking about "bad input", you should consider where the input is coming from.
Is the input entered by a user or another external system you don't control, you should expect the input to be invalid, and always validate it. It's perfectly ok to throw a checked exception in this case. Your application should 'recover' from this exception by providing an error message to the user.
If the input originates from your own system, e.g. your database, or some other parts of your application, you should be able to rely on it to be valid (it should have been validated before it got there). In this case it's perfectly ok to throw an unchecked exception like an IllegalArgumentException, which should not be caught (in general you should never catch unchecked exceptions). It is a programmer's error that the invalid value got there in the first place ;) You need to fix it.

Throwing runtime exceptions "sparingly" isn't really a good policy -- Effective Java recommends that you use checked exceptions when the caller can reasonably be expected to recover. (Programmer error is a specific example: if a particular case indicates programmer error, then you should throw an unchecked exception; you want the programmer to have a stack trace of where the logic problem occurred, not to try to handle it yourself.)
If there's no hope of recovery, then feel free to use unchecked exceptions; there's no point in catching them, so that's perfectly fine.
It's not 100% clear from your example which case this example is in your code, though.

Treat IllegalArgumentException as a preconditions check, and consider the design principle: A public method should both know and publicly document its own preconditions.
I would agree this example is correct:
void setPercentage(int pct) {
if( pct < 0 || pct > 100) {
throw new IllegalArgumentException("bad percent");
}
}
If EmailUtil is opaque, meaning there's some reason the preconditions cannot be described to the end-user, then a checked exception is correct. The second version, corrected for this design:
import com.someoneelse.EmailUtil;
public void scanEmail(String emailStr, InputStream mime) throws ParseException {
EmailAddress parsedAddress = EmailUtil.parseAddress(emailStr);
}
If EmailUtil is transparent, for instance maybe it's a private method owned by the class under question, IllegalArgumentException is correct if and only if its preconditions can be described in the function documentation. This is a correct version as well:
/** #param String email An email with an address in the form abc#xyz.com
* with no nested comments, periods or other nonsense.
*/
public String scanEmail(String email)
if (!addressIsProperlyFormatted(email)) {
throw new IllegalArgumentException("invalid address");
}
return parseEmail(emailAddr);
}
private String parseEmail(String emailS) {
// Assumes email is valid
boolean parsesJustFine = true;
// Parse logic
if (!parsesJustFine) {
// As a private method it is an internal error if address is improperly
// formatted. This is an internal error to the class implementation.
throw new AssertError("Internal error");
}
}
This design could go either way.
If preconditions are expensive to describe, or if the class is intended to be used by clients who don't know whether their emails are valid, then use ParseException. The top level method here is named scanEmail which hints the end user intends to send unstudied email through so this is likely correct.
If preconditions can be described in function documentation, and the class does not intent for invalid input and therefore programmer error is indicated, use IllegalArgumentException. Although not "checked" the "check" moves to the Javadoc documenting the function, which the client is expected to adhere to. IllegalArgumentException where the client can't tell their argument is illegal beforehand is wrong.
A note on IllegalStateException: This means "this object's internal state (private instance variables) is not able to perform this action." The end user cannot see private state so loosely speaking it takes precedence over IllegalArgumentException in the case where the client call has no way to know the object's state is inconsistent. I don't have a good explanation when it's preferred over checked exceptions, although things like initializing twice, or losing a database connection that isn't recovered, are examples.

As specified in oracle official tutorial , it states that:
If a client can reasonably be expected to recover from an exception,
make it a checked exception. If a client cannot do anything to recover
from the exception, make it an unchecked exception.
If I have an Application interacting with database using JDBC , And I have a method that takes the argument as the int item and double price. The price for corresponding item is read from database table. I simply multiply the total number of item purchased with the price value and return the result. Although I am always sure at my end(Application end) that price field value in the table could never be negative .But what if the price value comes out negative? It shows that there is a serious issue with the database side. Perhaps wrong price entry by the operator. This is the kind of issue that the other part of application calling that method can't anticipate and can't recover from it. It is a BUG in your database. So , and IllegalArguementException() should be thrown in this case which would state that the price can't be negative.
I hope that I have expressed my point clearly..

Any API should check the validity of the every parameter of any public method before executing it:
void setPercentage(int pct, AnObject object) {
if( pct < 0 || pct > 100) {
throw new IllegalArgumentException("pct has an invalid value");
}
if (object == null) {
throw new IllegalArgumentException("object is null");
}
}
They represent 99.9% of the times errors in the application because it is asking for impossible operations so in the end they are bugs that should crash the application (so it is a non recoverable error).
In this case and following the approach of fail fast you should let the application finish to avoid corrupting the application state.

On Asserts and Exceptions; Java

In this specific scenarios, are asserts more appropriate then exceptions?
It is my understanding that assert should be used when program is FUBAR to a degree where it can not recover and will exit.
I also was told to always throw exceptions for clarity and error message handling.
Is there a fine line between when to use each? Is there an example where assert must be used in place of exception unconditionally?
public void subscribe(DataConsumer c) throws IllegalArgumentException {
if (c == null) {
// Almost certainly FUBAR
throw new IllegalArgumentException("Can't subscribe null as a DataConsumer. Object not initialized");
}
if (dataConsumerList == null) {
// Definetely FUBAR
throw new IllegalArgumentException("Nothing to subscribe to. DataConsumerList is null");
}
dataConsumerList.add(c);
}

Personally I'm not keen on using assertions for this sort of thing, simply because they can be turned off. Some places use assertions when running tests, but then disable them for production for the sake of speed. To me this is like taking your driving test as normal, but then removing your seatbelt when you get on the motorway.
An assertion is only going to throw an exception anyway, of course. If you absolutely want to take the JVM down right now, you'd need to use something like Runtime.halt.
So I'm a fan of exceptions here, although I'd typically use a NullPointerException when given a null argument, and if dataConsumerList is part of your state then I would personally use IllegalStateException to differentiate that situation. (It's a shame that Java doesn't have the same ArgmentNullException that .NET has, given what a common check it is.)
Guava has the useful Preconditions class which lets you write this more concisely:
public void subscribe(DataConsumer c) throws IllegalArgumentException {
Preconditions.checkNotNull(c,
"Can't subscribe null as a DataConsumer. Object not initialized");
Preconditions.checkState(dataConsumerList != null,
"Nothing to subscribe to. DataConsumerList is null");
dataConsumerList.add(c);
}

General rule (copied from here)
assertions should protect from (not always obvious) mistakes of the
developer, e.g. using a pointer despite its being NULL.
exceptions are a way to handle errors that may legitimately occur at
runtime, e.g. the failure of trying to connect to some server (which may
not respond for various reasons).
And there is a better way of writing the above code using google-guava Preconditions.checkNotNull() class.
public void subscribe(DataConsumer c) throws IllegalArgumentException
{
checkNotNull(c, "Can't subscribe null as a DataConsumer. Object not initialized");
checkNotNull(dataConsumerList , "Nothing to subscribe to. DataConsumerList is null");
dataConsumerList.add(c);
}

If you could put this in English terms, use assert for "gotta" (Got to, Must) and exceptions for "otta" (Ought to, should).
Use the assert for show-stopping, critical conditions that must be true for the execution to continue. Examples might be that a division happens correctly (think of the Intel chip floating point bug) or that your database connection is not null after you have correctly opened it. If these have occurred, then program execution should not continue.
Use the throw for foreseeable errors that your method may handle. The throw is a part of a contract that declares to you and other programmers that certain types of errors may be encountered (and that it's not your responsibility).
In your example, my guess is that a null consumer or an empty list should never happen under normal circumstances. If my guess is correct, then you would want to use an assert here, declaring that subscribe() will handle it.
If my guess is wrong and a null consumer happens, say 1 out of 50 times, then the throw would be better and you would be declaring that subscribe() forms a contract with a calling method, whereby the calling method handles the error.

The Java technote Programming With Assertions contain this explicit line in with regards to usage:
Do not use assertions for argument checking in public methods.
That should be a pretty definitive answer to your question.

What is a good "Error Checking" Pattern (Java)?

I'll explain what I mean by input error checking.
Say you have a function doSomething(x).
If the function completes successfully doSomething does something and returns nothing. However, if there are errors I'd like to be notified. That is what I mean by error checking.
I'm looking for, in general, the best way to check for errors. I've thought of the following solutions, each with a potential problem.
Flag error checking. If doSomething(x) completes successfully return null. Otherwise, it returns a boolean or an error string. Problem: Side effects.
Throwing an exception. Throw an exception if doSomething(x) encounters an error. Problem: If you are performing error checking for parameters only, throwing an IllegalArgumentExceptionseems inappropriate.
Validating input prior to function call. If the error checking is only meant for the parameters of the function, then you can call a validator function before calling the doSomething(x) function. Problem: What if a client of the class forgets to call the validator function before calling doSomething(x)?
I often encounter this problem and any help or a point in the right direction would be much appreciated.

Throw an exception is the best way.
If you are performing error checking for parameters only, throwing an
IllegalArgumentException seems inappropriate.
Why? That's the purpose of this Exception.

Flag error checking
This is appropriate in some cases, depending on what you mean by an "error".
An example from the API: If you try to add an object to a Set, which already contains another object which equals the new object, the add method sort of "fails" and indicates this by returning false. (Note that we're on a level at which it's technically not even an "error"!)
2.Throwing an exception
This is the default option.
Question is now, should you go for a checked exception (which you need a throws declaration or try/catch clauses) or an unchecked exception (an exception that extends RuntimeException). There are a few rules of thumbs here.
From Java Practices -> Checked versus unchecked exceptions:
Unchecked exceptions: Represent defects in the program (bugs) - often invalid arguments passed to a non-private method.
Checked exceptions: Represent invalid conditions in areas outside the immediate control of the program (invalid user input, database problems, network outages, absent files)
Note that IllegalArgumentException is an unchecked exception, perfectly suitable for throwing when arguments are not as they should be.
If you want to throw a checked exception, you could A) roll your own by extending Exception, B) use some existing checked exception or C) "chain" a runtime exception in, for instance, an IOException: throw new IOException(new IllegalArgumentException("reason goes here..."));
3.Validating input prior to function call
Relying on the fact that the client should have sanitized / checked his arguments before the call seems like a bad idea to me.

Your second suggestion ("Throwing an exception") is the best choice. The other two options rely on the invoker either doing something before ("Validating input prior to function call") or after ("Flag error checking") the call to the method. Either way, the extra task is not mandated by the compiler so someone invoking the function isn't forced to call the "extra thing" so problems are not spotted till run-time.
As for "Throwing an Exception" and your suggested 'problem', well the answer is throw appropriate exception types for the code. If the input parameters are invalid, then throw an InvalidArgumentException (since that's the appropriate error). If the exception is for functionality (e.g. cannot open network connection), use another exception type or create your own.

I agree with throwing exceptions. I want to add another option that combines #2 and #3 - the proxy pattern. That way your code stays fairly cohesive - validation in one place and business logic in another. This makes sense if you have a large set of calls that need to be validated.
Create a proxy to handle validation. Have it delegate all calls to the actual implementation of your business logic interface after it validates, otherwise it can throw exceptions if something does not validate.

I decide which method to use usually on the type of interface.
User interface (GUI): I validate before calling business methods, because the user wants to know what was wrong.
On technical interfaces between components or systems, the interface should have been tested and work properly in this case I throw exceptions.

Exceptions is the way to go. Your stated problem with exceptions can be mitigated by the proper implementation of exception throwing / handling. Use exceptions to your advantage by validating parameters at the lowest level that you need them and throwing an exception if the validation fails. This allows you to avoid redundantly checking for validity at multiple levels in the call stack. Throw the exception at the bottom and let the stack unroll to the appropriate place for handling the error.

The method you choose depends on the situation, and they are not mutually exclusive so you can mix them all in the same solution (although whether that's a good idea really depends on your situation).
Choose this method if you want a very simple method for handling errors. This method might be OK for situations where the calling function can accept any value the called function returns. There might be situations where business logic dictates this as an OK choice, such as returning a specific message string when a resource cannot be properly located, or a server does not respond. Generally, I don't use this or see this technique in Java very much, as exceptions are a better mechanism for error handling.
Throw an exception when your function runs into un defined behaviour. If you have a math function that can only operate on positive integers and someone passes -1, you should thrown an InvalidArguementException. If your function is given the ID of a product in a database, but the product cannot be found by a query, you could throw a custom ProductNotFound exception.
Validating input is a good idea, I would say it should be done by the called function, rather than the caller - unless the caller can avoid an exception from the callee by validating the input before passing it. If you work in a language that supports Design By Contract, validating input would be done as the function's precondition.
I usually use #2 and #3. I haven't written code with error flags for a while. The exception to that might be a function that returned an enum, where one possible value indicated an error code. That was driven more by a business rule than anything else.
And generally, try to keep it simple.

Throw a custom checked exception.
doSomething(WithX x ) throws BusinessRuleViolatedException

Input validation is surprisingly complicated and all three of the suggested approaches in the original post are needed and sometimes more. Exceptions are appropriate when input is outside the bounds of business logic, if it is corrupt or cannot be read for example.
Flag checking quickly becomes an anti-pattern if you have more than one or two flags to check, and can be replaced with a slightly specialized version of the visitor pattern. I do not know the exact name of this specific pattern, but I'll informally call it the "validator list pattern" and will describe it in more detail below.
Checking input early and failing fast is usually good, but not always possible. Often there is a lot of input validation, all input received from outside of your control should be treated as hostile and requires validation. Good program design and architecture will help make it clear when exactly this needs to happen.
'The Validator List Pattern'
As an example, let's first describe in code the "Validation Flag" anti-pattern, and then we'll transform it to the "validation list" pattern.
public Optional<String> checkForErrorsUsingFlags(
ObjectToCheck objToCheck ) {
// the small series of checks and if statements represent the
// anti-pattern. Hard to test and many other problems crop up.
String errMsg = checkForError1( objToCheck );
if(errMsg != null ) {
return Optional.of(errMsg);
}
errMsg = checkForError2( objToCheck );
if(errMsg != null ) {
return Optional.of(errMsg);
}
return Optional.empty();
}
/**** client usage ****/
ObjectToCheck obj = doSomethingToReadInput(obj);
Optional<String> error = checkForErrors( obj);
if (error.isPresent()) {
// invalid input, throw object away and request input again
} else {
// do stuff, we have a valid input
}
To fix, start by creating a common interface that will represent a single validator. Then each check is converted to a validator instance. Finally we create a list of validators and pass it to the validator code.
/** The common validator interface each validator will use */
private interface MyValidator {
public boolean isValid(ObjectToCheck obj);
public String getErrorMessage(ObjectToCheck obj);
}
// this method should look familiar to the above, now we
// have a list of validators as an additional parameter
public Optional<String> checkForErrors( ObjectToCheck objToCheck,
List<MyValidator> validators ) {
for(MyValidator validator : validators ) {
if (!validator.isValid(objToCheck)) {
String errMsg = validator.getErrorMessage(objToCheck);
return Optional.of(errMsg);
}
}
return Optional.empty();
}
/****** client usage *****/
// now in this pattern, the client controls when the validators
// are created, and which ones are used.
MyValidator validator1 = new MyValidator() {
#Override
public boolean isValid(ObjectToCheck obj) {
return checkForError1( objToCheck ) != null;
}
#Override
public boolean getErrorMessage(ObjectToCheck obj) {
return checkForError1( objToCheck );
}
}
// note: above we call checkForError1 twice, not optimal.
// typical in real examples this can be avoided,
// and the error message generation logic split from the detection
// logic often simplifies things.
MyValidator validator2 = new MyValidator() { ... }
List<MyValidator> validators =
ImmutableList.of( validator1, validator2);
Optional<String> error = checkForErrors(objToCheck, validators);
if (error.isPresent()) {
// invalid input, throw object away and request input again
} else {
// do stuff, we have a valid input
}
Now to test, create a series of mock validators and check that each one has their validate called. You can stub validator results and ensure the correct behavior is taken. Then you also have access to each validator individually so you can test them one by one on their own.
Cheers - hope that helps, happy coding.

Avoiding NullPointerException in Java

I use x != null to avoid NullPointerException. Is there an alternative?
if (x != null) {
// ...
}

This to me sounds like a reasonably common problem that junior to intermediate developers tend to face at some point: they either don't know or don't trust the contracts they are participating in and defensively overcheck for nulls. Additionally, when writing their own code, they tend to rely on returning nulls to indicate something thus requiring the caller to check for nulls.
To put this another way, there are two instances where null checking comes up:
Where null is a valid response in terms of the contract; and
Where it isn't a valid response.
(2) is easy. As of Java 1.7 you can use Objects.requireNonNull(foo). (If you are stuck with a previous version then assertions may be a good alternative.)
"Proper" usage of this method would be like below. The method returns the object passed into it and throws a NullPointerException if the object is null. This means that the returned value is always non-null. The method is primarily intended for validating parameters.
public Foo(Bar bar) {
this.bar = Objects.requireNonNull(bar);
}
It can also be used like an assertion though since it throws an exception if the object is null. In both uses, a message can be added which will be shown in the exception. Below is using it like an assertion and providing a message.
Objects.requireNonNull(someobject, "if someobject is null then something is wrong");
someobject.doCalc();
Generally throwing a specific exception like NullPointerException when a value is null but shouldn't be is favorable to throwing a more general exception like AssertionError. This is the approach the Java library takes; favoring NullPointerException over IllegalArgumentException when an argument is not allowed to be null.
(1) is a little harder. If you have no control over the code you're calling then you're stuck. If null is a valid response, you have to check for it.
If it's code that you do control, however (and this is often the case), then it's a different story. Avoid using nulls as a response. With methods that return collections, it's easy: return empty collections (or arrays) instead of nulls pretty much all the time.
With non-collections it might be harder. Consider this as an example: if you have these interfaces:
public interface Action {
void doSomething();
}
public interface Parser {
Action findAction(String userInput);
}
where Parser takes raw user input and finds something to do, perhaps if you're implementing a command line interface for something. Now you might make the contract that it returns null if there's no appropriate action. That leads the null checking you're talking about.
An alternative solution is to never return null and instead use the Null Object pattern:
public class MyParser implements Parser {
private static Action DO_NOTHING = new Action() {
public void doSomething() { /* do nothing */ }
};
public Action findAction(String userInput) {
// ...
if ( /* we can't find any actions */ ) {
return DO_NOTHING;
}
}
}
Compare:
Parser parser = ParserFactory.getParser();
if (parser == null) {
// now what?
// this would be an example of where null isn't (or shouldn't be) a valid response
}
Action action = parser.findAction(someInput);
if (action == null) {
// do nothing
} else {
action.doSomething();
}
to
ParserFactory.getParser().findAction(someInput).doSomething();
which is a much better design because it leads to more concise code.
That said, perhaps it is entirely appropriate for the findAction() method to throw an Exception with a meaningful error message -- especially in this case where you are relying on user input. It would be much better for the findAction method to throw an Exception than for the calling method to blow up with a simple NullPointerException with no explanation.
try {
ParserFactory.getParser().findAction(someInput).doSomething();
} catch(ActionNotFoundException anfe) {
userConsole.err(anfe.getMessage());
}
Or if you think the try/catch mechanism is too ugly, rather than Do Nothing your default action should provide feedback to the user.
public Action findAction(final String userInput) {
/* Code to return requested Action if found */
return new Action() {
public void doSomething() {
userConsole.err("Action not found: " + userInput);
}
}
}

If you use (or planning to use) a Java IDE like JetBrains IntelliJ IDEA, Eclipse or Netbeans or a tool like findbugs then you can use annotations to solve this problem.
Basically, you've got #Nullable and #NotNull.
You can use in method and parameters, like this:
#NotNull public static String helloWorld() {
return "Hello World";
}
or
#Nullable public static String helloWorld() {
return "Hello World";
}
The second example won't compile (in IntelliJ IDEA).
When you use the first helloWorld() function in another piece of code:
public static void main(String[] args)
{
String result = helloWorld();
if(result != null) {
System.out.println(result);
}
}
Now the IntelliJ IDEA compiler will tell you that the check is useless, since the helloWorld() function won't return null, ever.
Using parameter
void someMethod(#NotNull someParameter) { }
if you write something like:
someMethod(null);
This won't compile.
Last example using #Nullable
#Nullable iWantToDestroyEverything() { return null; }
Doing this
iWantToDestroyEverything().something();
And you can be sure that this won't happen. :)
It's a nice way to let the compiler check something more than it usually does and to enforce your contracts to be stronger. Unfortunately, it's not supported by all the compilers.
In IntelliJ IDEA 10.5 and on, they added support for any other #Nullable #NotNull implementations.
See blog post More flexible and configurable #Nullable/#NotNull annotations.

If null-values are not allowed
If your method is called externally, start with something like this:
public void method(Object object) {
if (object == null) {
throw new IllegalArgumentException("...");
}
Then, in the rest of that method, you'll know that object is not null.
If it is an internal method (not part of an API), just document that it cannot be null, and that's it.
Example:
public String getFirst3Chars(String text) {
return text.subString(0, 3);
}
However, if your method just passes the value on, and the next method passes it on etc. it could get problematic. In that case you may want to check the argument as above.
If null is allowed
This really depends. If find that I often do something like this:
if (object == null) {
// something
} else {
// something else
}
So I branch, and do two completely different things. There is no ugly code snippet, because I really need to do two different things depending on the data. For example, should I work on the input, or should I calculate a good default value?
It's actually rare for me to use the idiom "if (object != null && ...".
It may be easier to give you examples, if you show examples of where you typically use the idiom.

Wow, I almost hate to add another answer when we have 57 different ways to recommend the NullObject pattern, but I think that some people interested in this question may like to know that there is a proposal on the table for Java 7 to add "null-safe handling"—a streamlined syntax for if-not-equal-null logic.
The example given by Alex Miller looks like this:
public String getPostcode(Person person) {
return person?.getAddress()?.getPostcode();
}
The ?. means only de-reference the left identifier if it is not null, otherwise evaluate the remainder of the expression as null. Some people, like Java Posse member Dick Wall and the voters at Devoxx really love this proposal, but there is opposition too, on the grounds that it will actually encourage more use of null as a sentinel value.
Update: An official proposal for a null-safe operator in Java 7 has been submitted under Project Coin. The syntax is a little different than the example above, but it's the same notion.
Update: The null-safe operator proposal didn't make it into Project Coin. So, you won't be seeing this syntax in Java 7.

If undefined values are not permitted:
You might configure your IDE to warn you about potential null dereferencing. E.g. in Eclipse, see Preferences > Java > Compiler > Errors/Warnings/Null analysis.
If undefined values are permitted:
If you want to define a new API where undefined values make sense, use the Option Pattern (may be familiar from functional languages). It has the following advantages:
It is stated explicitly in the API whether an input or output exists or not.
The compiler forces you to handle the "undefined" case.
Option is a monad, so there is no need for verbose null checking, just use map/foreach/getOrElse or a similar combinator to safely use the value (example).
Java 8 has a built-in Optional class (recommended); for earlier versions, there are library alternatives, for example Guava's Optional or FunctionalJava's Option. But like many functional-style patterns, using Option in Java (even 8) results in quite some boilerplate, which you can reduce using a less verbose JVM language, e.g. Scala or Xtend.
If you have to deal with an API which might return nulls, you can't do much in Java. Xtend and Groovy have the Elvis operator ?: and the null-safe dereference operator ?., but note that this returns null in case of a null reference, so it just "defers" the proper handling of null.

Only for this situation -
Not checking if a variable is null before invoking an equals method (a string compare example below):
if ( foo.equals("bar") ) {
// ...
}
will result in a NullPointerException if foo doesn't exist.
You can avoid that if you compare your Strings like this:
if ( "bar".equals(foo) ) {
// ...
}

With Java 8 comes the new java.util.Optional class that arguably solves some of the problem. One can at least say that it improves the readability of the code, and in the case of public APIs make the API's contract clearer to the client developer.
They work like that:
An optional object for a given type (Fruit) is created as the return type of a method. It can be empty or contain a Fruit object:
public static Optional<Fruit> find(String name, List<Fruit> fruits) {
for (Fruit fruit : fruits) {
if (fruit.getName().equals(name)) {
return Optional.of(fruit);
}
}
return Optional.empty();
}
Now look at this code where we search a list of Fruit (fruits) for a given Fruit instance:
Optional<Fruit> found = find("lemon", fruits);
if (found.isPresent()) {
Fruit fruit = found.get();
String name = fruit.getName();
}
You can use the map() operator to perform a computation on--or extract a value from--an optional object. orElse() lets you provide a fallback for missing values.
String nameOrNull = find("lemon", fruits)
.map(f -> f.getName())
.orElse("empty-name");
Of course, the check for null/empty value is still necessary, but at least the developer is conscious that the value might be empty and the risk of forgetting to check is limited.
In an API built from scratch using Optional whenever a return value might be empty, and returning a plain object only when it cannot be null (convention), the client code might abandon null checks on simple object return values...
Of course Optional could also be used as a method argument, perhaps a better way to indicate optional arguments than 5 or 10 overloading methods in some cases.
Optional offers other convenient methods, such as orElse that allow the use of a default value, and ifPresent that works with lambda expressions.
I invite you to read this article (my main source for writing this answer) in which the NullPointerException (and in general null pointer) problematic as well as the (partial) solution brought by Optional are well explained: Java Optional Objects.

Depending on what kind of objects you are checking you may be able to use some of the classes in the apache commons such as: apache commons lang and apache commons collections
Example:
String foo;
...
if( StringUtils.isBlank( foo ) ) {
///do something
}
or (depending on what you need to check):
String foo;
...
if( StringUtils.isEmpty( foo ) ) {
///do something
}
The StringUtils class is only one of many; there are quite a few good classes in the commons that do null safe manipulation.
Here follows an example of how you can use null vallidation in JAVA when you include apache library(commons-lang-2.4.jar)
public DOCUMENT read(String xml, ValidationEventHandler validationEventHandler) {
Validate.notNull(validationEventHandler,"ValidationHandler not Injected");
return read(new StringReader(xml), true, validationEventHandler);
}
And if you are using Spring, Spring also has the same functionality in its package, see library(spring-2.4.6.jar)
Example on how to use this static classf from spring(org.springframework.util.Assert)
Assert.notNull(validationEventHandler,"ValidationHandler not Injected");

If you consider an object should not be null (or it is a bug) use an assert.
If your method doesn't accept null params say it in the javadoc and use an assert.
You have to check for object != null only if you want to handle the case where the object may be null...
There is a proposal to add new annotations in Java7 to help with null / notnull params:
http://tech.puredanger.com/java7/#jsr308

I'm a fan of "fail fast" code. Ask yourself - are you doing something useful in the case where the parameter is null? If you don't have a clear answer for what your code should do in that case... i.e. - it should never be null in the first place, then ignore it and allow a NullPointerException to be thrown. The calling code will make just as much sense of an NPE as it would an IllegalArgumentException, but it'll be easier for the developer to debug and understand what went wrong if an NPE is thrown rather than your code attempting to execute some other unexpected contingency logic - which ultimately results in the application failing anyway.

Sometimes, you have methods that operate on its parameters that define a symmetric operation:
a.f(b); <-> b.f(a);
If you know b can never be null, you can just swap it. It is most useful for equals:
Instead of foo.equals("bar"); better do "bar".equals(foo);.

Rather than Null Object Pattern -- which has its uses -- you might consider situations where the null object is a bug.
When the exception is thrown, examine the stack trace and work through the bug.

The Google collections framework offers a good and elegant way to achieve the null check.
There is a method in a library class like this:
static <T> T checkNotNull(T e) {
if (e == null) {
throw new NullPointerException();
}
return e;
}
And the usage is (with import static):
...
void foo(int a, Person p) {
if (checkNotNull(p).getAge() > a) {
...
}
else {
...
}
}
...
Or in your example:
checkNotNull(someobject).doCalc();

Null is not a 'problem'. It is an integral part of a complete modeling tool set. Software aims to model the complexity of the world and null bears its burden. Null indicates 'No data' or 'Unknown' in Java and the like. So it is appropriate to use nulls for these purposes. I don't prefer the 'Null object' pattern; I think it rise the 'who will guard
the guardians' problem.
If you ask me what is the name of my girlfriend I'll tell you that I have no girlfriend. In the Java language I'll return null.
An alternative would be to throw meaningful exception to indicate some problem that can't be (or don't want to be) solved right there and delegate it somewhere higher in the stack to retry or report data access error to the user.
For an 'unknown question' give 'unknown answer'. (Be null-safe where this is correct from business point of view) Checking arguments for null once inside a method before usage relieves multiple callers from checking them before a call.
public Photo getPhotoOfThePerson(Person person) {
if (person == null)
return null;
// Grabbing some resources or intensive calculation
// using person object anyhow.
}
Previous leads to normal logic flow to get no photo of a non-existent girlfriend from my photo library.
getPhotoOfThePerson(me.getGirlfriend())
And it fits with new coming Java API (looking forward)
getPhotoByName(me.getGirlfriend()?.getName())
While it is rather 'normal business flow' not to find photo stored into the DB for some person, I used to use pairs like below for some other cases
public static MyEnum parseMyEnum(String value); // throws IllegalArgumentException
public static MyEnum parseMyEnumOrNull(String value);
And don't loathe to type <alt> + <shift> + <j> (generate javadoc in Eclipse) and write three additional words for you public API. This will be more than enough for all but those who don't read documentation.
/**
* #return photo or null
*/
or
/**
* #return photo, never null
*/
This is rather theoretical case and in most cases you should prefer java null safe API (in case it will be released in another 10 years), but NullPointerException is subclass of an Exception. Thus it is a form of Throwable that indicates conditions that a reasonable application might want to catch (javadoc)! To use the first most advantage of exceptions and separate error-handling code from 'regular' code (according to creators of Java) it is appropriate, as for me, to catch NullPointerException.
public Photo getGirlfriendPhoto() {
try {
return appContext.getPhotoDataSource().getPhotoByName(me.getGirlfriend().getName());
} catch (NullPointerException e) {
return null;
}
}
Questions could arise:
Q. What if getPhotoDataSource() returns null?
A. It is up to business logic. If I fail to find a photo album I'll show you no photos. What if appContext is not initialized? This method's business logic puts up with this. If the same logic should be more strict then throwing an exception it is part of the business logic and explicit check for null should be used (case 3). The new Java Null-safe API fits better here to specify selectively what implies and what does not imply to be initialized to be fail-fast in case of programmer errors.
Q. Redundant code could be executed and unnecessary resources could be grabbed.
A. It could take place if getPhotoByName() would try to open a database connection, create PreparedStatement and use the person name as an SQL parameter at last. The approach for an unknown question gives an unknown answer (case 1) works here. Before grabbing resources the method should check parameters and return 'unknown' result if needed.
Q. This approach has a performance penalty due to the try closure opening.
A. Software should be easy to understand and modify firstly. Only after this, one could think about performance, and only if needed! and where needed! (source), and many others).
PS. This approach will be as reasonable to use as the separate error-handling code from "regular" code principle is reasonable to use in some place. Consider the next example:
public SomeValue calculateSomeValueUsingSophisticatedLogic(Predicate predicate) {
try {
Result1 result1 = performSomeCalculation(predicate);
Result2 result2 = performSomeOtherCalculation(result1.getSomeProperty());
Result3 result3 = performThirdCalculation(result2.getSomeProperty());
Result4 result4 = performLastCalculation(result3.getSomeProperty());
return result4.getSomeProperty();
} catch (NullPointerException e) {
return null;
}
}
public SomeValue calculateSomeValueUsingSophisticatedLogic(Predicate predicate) {
SomeValue result = null;
if (predicate != null) {
Result1 result1 = performSomeCalculation(predicate);
if (result1 != null && result1.getSomeProperty() != null) {
Result2 result2 = performSomeOtherCalculation(result1.getSomeProperty());
if (result2 != null && result2.getSomeProperty() != null) {
Result3 result3 = performThirdCalculation(result2.getSomeProperty());
if (result3 != null && result3.getSomeProperty() != null) {
Result4 result4 = performLastCalculation(result3.getSomeProperty());
if (result4 != null) {
result = result4.getSomeProperty();
}
}
}
}
}
return result;
}
PPS. For those fast to downvote (and not so fast to read documentation) I would like to say that I've never caught a null-pointer exception (NPE) in my life. But this possibility was intentionally designed by the Java creators because NPE is a subclass of Exception. We have a precedent in Java history when ThreadDeath is an Error not because it is actually an application error, but solely because it was not intended to be caught! How much NPE fits to be an Error than ThreadDeath! But it is not.
Check for 'No data' only if business logic implies it.
public void updatePersonPhoneNumber(Long personId, String phoneNumber) {
if (personId == null)
return;
DataSource dataSource = appContext.getStuffDataSource();
Person person = dataSource.getPersonById(personId);
if (person != null) {
person.setPhoneNumber(phoneNumber);
dataSource.updatePerson(person);
} else {
Person = new Person(personId);
person.setPhoneNumber(phoneNumber);
dataSource.insertPerson(person);
}
}
and
public void updatePersonPhoneNumber(Long personId, String phoneNumber) {
if (personId == null)
return;
DataSource dataSource = appContext.getStuffDataSource();
Person person = dataSource.getPersonById(personId);
if (person == null)
throw new SomeReasonableUserException("What are you thinking about ???");
person.setPhoneNumber(phoneNumber);
dataSource.updatePerson(person);
}
If appContext or dataSource is not initialized unhandled runtime NullPointerException will kill current thread and will be processed by Thread.defaultUncaughtExceptionHandler (for you to define and use your favorite logger or other notification mechanizm). If not set, ThreadGroup#uncaughtException will print stacktrace to system err. One should monitor application error log and open Jira issue for each unhandled exception which in fact is application error. Programmer should fix bug somewhere in initialization stuff.

Java 7 has a new java.util.Objects utility class on which there is a requireNonNull() method. All this does is throw a NullPointerException if its argument is null, but it cleans up the code a bit. Example:
Objects.requireNonNull(someObject);
someObject.doCalc();
The method is most useful for checking just before an assignment in a constructor, where each use of it can save three lines of code:
Parent(Child child) {
if (child == null) {
throw new NullPointerException("child");
}
this.child = child;
}
becomes
Parent(Child child) {
this.child = Objects.requireNonNull(child, "child");
}

Ultimately, the only way to completely solve this problem is by using a different programming language:
In Objective-C, you can do the equivalent of invoking a method on nil, and absolutely nothing will happen. This makes most null checks unnecessary, but it can make errors much harder to diagnose.
In Nice, a Java-derived language, there are two versions of all types: a potentially-null version and a not-null version. You can only invoke methods on not-null types. Potentially-null types can be converted to not-null types through explicit checking for null. This makes it much easier to know where null checks are necessary and where they aren't.

Common "problem" in Java indeed.
First, my thoughts on this:
I consider that it is bad to "eat" something when NULL was passed where NULL isn't a valid value. If you're not exiting the method with some sort of error then it means nothing went wrong in your method which is not true. Then you probably return null in this case, and in the receiving method you again check for null, and it never ends, and you end up with "if != null", etc..
So, IMHO, null must be a critical error which prevents further execution (that is, where null is not a valid value).
The way I solve this problem is this:
First, I follow this convention:
All public methods / API always check its arguments for null
All private methods do not check for null since they are controlled methods (just let die with nullpointer exception in case it wasn't handled above)
The only other methods which do not check for null are utility methods. They are public, but if you call them for some reason, you know what parameters you pass. This is like trying to boil water in the kettle without providing water...
And finally, in the code, the first line of the public method goes like this:
ValidationUtils.getNullValidator().addParam(plans, "plans").addParam(persons, "persons").validate();
Note that addParam() returns self, so that you can add more parameters to check.
Method validate() will throw checked ValidationException if any of the parameters is null (checked or unchecked is more a design/taste issue, but my ValidationException is checked).
void validate() throws ValidationException;
The message will contain the following text if, for example, "plans" is null:
"Illegal argument value null is encountered for parameter [plans]"
As you can see, the second value in the addParam() method (string) is needed for the user message, because you cannot easily detect passed-in variable name, even with reflection (not subject of this post anyway...).
And yes, we know that beyond this line we will no longer encounter a null value so we just safely invoke methods on those objects.
This way, the code is clean, easy maintainable and readable.

Asking that question points out that you may be interested in error handling strategies. How and where to handle errors is a pervasive architectural question. There are several ways to do this.
My favorite: allow the Exceptions to ripple through - catch them at the 'main loop' or in some other function with the appropriate responsibilities. Checking for error conditions and handling them appropriately can be seen as a specialized responsibility.
Sure do have a look at Aspect Oriented Programming, too - they have neat ways to insert if( o == null ) handleNull() into your bytecode.

In addition to using assert you can use the following:
if (someobject == null) {
// Handle null here then move on.
}
This is slightly better than:
if (someobject != null) {
.....
.....
.....
}

Just don't ever use null. Don't allow it.
In my classes, most fields and local variables have non-null default values, and I add contract statements (always-on asserts) everywhere in the code to make sure this is being enforced (since it's more succinct, and more expressive than letting it come up as an NPE and then having to resolve the line number, etc.).
Once I adopted this practice, I noticed that the problems seemed to fix themselves. You'd catch things much earlier in the development process just by accident and realize you had a weak spot.. and more importantly.. it helps encapsulate different modules' concerns, different modules can 'trust' each other, and no more littering the code with if = null else constructs!
This is defensive programming and results in much cleaner code in the long run. Always sanitize the data, e.g. here by enforcing rigid standards, and the problems go away.
class C {
private final MyType mustBeSet;
public C(MyType mything) {
mustBeSet=Contract.notNull(mything);
}
private String name = "<unknown>";
public void setName(String s) {
name = Contract.notNull(s);
}
}
class Contract {
public static <T> T notNull(T t) { if (t == null) { throw new ContractException("argument must be non-null"); return t; }
}
The contracts are like mini-unit tests which are always running, even in production, and when things fail, you know why, rather than a random NPE you have to somehow figure out.

Guava, a very useful core library by Google, has a nice and useful API to avoid nulls. I find UsingAndAvoidingNullExplained very helpful.
As explained in the wiki:
Optional<T> is a way of replacing a nullable T reference with a
non-null value. An Optional may either contain a non-null T reference
(in which case we say the reference is "present"), or it may contain
nothing (in which case we say the reference is "absent"). It is never
said to "contain null."
Usage:
Optional<Integer> possible = Optional.of(5);
possible.isPresent(); // returns true
possible.get(); // returns 5

This is a very common problem for every Java developer. So there is official support in Java 8 to address these issues without cluttered code.
Java 8 has introduced java.util.Optional<T>. It is a container that may or may not hold a non-null value. Java 8 has given a safer way to handle an object whose value may be null in some of the cases. It is inspired from the ideas of Haskell and Scala.
In a nutshell, the Optional class includes methods to explicitly deal with the cases where a value is present or absent. However, the advantage compared to null references is that the Optional<T> class forces you to think about the case when the value is not present. As a consequence, you can prevent unintended null pointer exceptions.
In above example we have a home service factory that returns a handle to multiple appliances available in the home. But these services may or may not be available/functional; it means it may result in a NullPointerException. Instead of adding a null if condition before using any service, let's wrap it in to Optional<Service>.
WRAPPING TO OPTION<T>
Let's consider a method to get a reference of a service from a factory. Instead of returning the service reference, wrap it with Optional. It lets the API user know that the returned service may or may not available/functional, use defensively
public Optional<Service> getRefrigertorControl() {
Service s = new RefrigeratorService();
//...
return Optional.ofNullable(s);
}
As you see Optional.ofNullable() provides an easy way to get the reference wrapped. There are another ways to get the reference of Optional, either Optional.empty() & Optional.of(). One for returning an empty object instead of retuning null and the other to wrap a non-nullable object, respectively.
SO HOW EXACTLY IT HELPS TO AVOID A NULL CHECK?
Once you have wrapped a reference object, Optional provides many useful methods to invoke methods on a wrapped reference without NPE.
Optional ref = homeServices.getRefrigertorControl();
ref.ifPresent(HomeServices::switchItOn);
Optional.ifPresent invokes the given Consumer with a reference if it is a non-null value. Otherwise, it does nothing.
#FunctionalInterface
public interface Consumer<T>
Represents an operation that accepts a single input argument and returns no result. Unlike most other functional interfaces, Consumer is expected to operate via side-effects.
It is so clean and easy to understand. In the above code example, HomeService.switchOn(Service) gets invoked if the Optional holding reference is non-null.
We use the ternary operator very often for checking null condition and return an alternative value or default value. Optional provides another way to handle the same condition without checking null. Optional.orElse(defaultObj) returns defaultObj if the Optional has a null value. Let's use this in our sample code:
public static Optional<HomeServices> get() {
service = Optional.of(service.orElse(new HomeServices()));
return service;
}
Now HomeServices.get() does same thing, but in a better way. It checks whether the service is already initialized of not. If it is then return the same or create a new New service. Optional<T>.orElse(T) helps to return a default value.
Finally, here is our NPE as well as null check-free code:
import java.util.Optional;
public class HomeServices {
private static final int NOW = 0;
private static Optional<HomeServices> service;
public static Optional<HomeServices> get() {
service = Optional.of(service.orElse(new HomeServices()));
return service;
}
public Optional<Service> getRefrigertorControl() {
Service s = new RefrigeratorService();
//...
return Optional.ofNullable(s);
}
public static void main(String[] args) {
/* Get Home Services handle */
Optional<HomeServices> homeServices = HomeServices.get();
if(homeServices != null) {
Optional<Service> refrigertorControl = homeServices.get().getRefrigertorControl();
refrigertorControl.ifPresent(HomeServices::switchItOn);
}
}
public static void switchItOn(Service s){
//...
}
}
The complete post is NPE as well as Null check-free code … Really?.

I like articles from Nat Pryce. Here are the links:
Avoiding Nulls with Polymorphic Dispatch
Avoiding Nulls with "Tell, Don't Ask" Style
In the articles there is also a link to a Git repository for a Java Maybe Type which I find interesting, but I don't think it alone could decrease the
checking code bloat. After doing some research on the Internet, I think != null code bloat could be decreased mainly by careful design.

I've tried the NullObjectPattern but for me is not always the best way to go. There are sometimes when a "no action" is not appropiate.
NullPointerException is a Runtime exception that means it's developers fault and with enough experience it tells you exactly where is the error.
Now to the answer:
Try to make all your attributes and its accessors as private as possible or avoid to expose them to the clients at all. You can have the argument values in the constructor of course, but by reducing the scope you don't let the client class pass an invalid value. If you need to modify the values, you can always create a new object. You check the values in the constructor only once and in the rest of the methods you can be almost sure that the values are not null.
Of course, experience is the better way to understand and apply this suggestion.
Byte!

Probably the best alternative for Java 8 or newer is to use the Optional class.
Optional stringToUse = Optional.of("optional is there");
stringToUse.ifPresent(System.out::println);
This is especially handy for long chains of possible null values. Example:
Optional<Integer> i = Optional.ofNullable(wsObject.getFoo())
.map(f -> f.getBar())
.map(b -> b.getBaz())
.map(b -> b.getInt());
Example on how to throw exception on null:
Optional optionalCarNull = Optional.ofNullable(someNull);
optionalCarNull.orElseThrow(IllegalStateException::new);
Java 7 introduced the Objects.requireNonNull method which can be handy when something should be checked for non-nullness. Example:
String lowerVal = Objects.requireNonNull(someVar, "input cannot be null or empty").toLowerCase();

May I answer it more generally!
We usually face this issue when the methods get the parameters in the way we not expected (bad method call is programmer's fault). For example: you expect to get an object, instead you get a null. You expect to get an String with at least one character, instead you get an empty String ...
So there is no difference between:
if(object == null){
//you called my method badly!
}
or
if(str.length() == 0){
//you called my method badly again!
}
They both want to make sure that we received valid parameters, before we do any other functions.
As mentioned in some other answers, to avoid above problems you can follow the Design by contract pattern. Please see http://en.wikipedia.org/wiki/Design_by_contract.
To implement this pattern in java, you can use core java annotations like javax.annotation.NotNull or use more sophisticated libraries like Hibernate Validator.
Just a sample:
getCustomerAccounts(#NotEmpty String customerId,#Size(min = 1) String accountType)
Now you can safely develop the core function of your method without needing to check input parameters, they guard your methods from unexpected parameters.
You can go a step further and make sure that only valid pojos could be created in your application. (sample from hibernate validator site)
public class Car {
#NotNull
private String manufacturer;
#NotNull
#Size(min = 2, max = 14)
private String licensePlate;
#Min(2)
private int seatCount;
// ...
}

I highly disregard answers that suggest using the null objects in every situation. This pattern may break the contract and bury problems deeper and deeper instead of solving them, not mentioning that used inappropriately will create another pile of boilerplate code that will require future maintenance.
In reality if something returned from a method can be null and the calling code has to make decision upon that, there should an earlier call that ensures the state.
Also keep in mind, that null object pattern will be memory hungry if used without care. For this - the instance of a NullObject should be shared between owners, and not be an unigue instance for each of these.
Also I would not recommend using this pattern where the type is meant to be a primitive type representation - like mathematical entities, that are not scalars: vectors, matrices, complex numbers and POD(Plain Old Data) objects, which are meant to hold state in form of Java built-in types. In the latter case you would end up calling getter methods with arbitrary results. For example what should a NullPerson.getName() method return?
It's worth considering such cases in order to avoid absurd results.

Never initialise variables to null.
If (1) is not possible, initialise all collections and arrays to empty collections/arrays.
Doing this in your own code and you can avoid != null checks.
Most of the time null checks seem to guard loops over collections or arrays, so just initialise them empty, you won't need any null checks.
// Bad
ArrayList<String> lemmings;
String[] names;
void checkLemmings() {
if (lemmings != null) for(lemming: lemmings) {
// do something
}
}
// Good
ArrayList<String> lemmings = new ArrayList<String>();
String[] names = {};
void checkLemmings() {
for(lemming: lemmings) {
// do something
}
}
There is a tiny overhead in this, but it's worth it for cleaner code and less NullPointerExceptions.

This is the most common error occurred for most of the developers.
We have number of ways to handle this.
Approach 1:
org.apache.commons.lang.Validate //using apache framework
notNull(Object object, String message)
Approach 2:
if(someObject!=null){ // simply checking against null
}
Approach 3:
#isNull #Nullable // using annotation based validation
Approach 4:
// by writing static method and calling it across whereever we needed to check the validation
static <T> T isNull(someObject e){
if(e == null){
throw new NullPointerException();
}
return e;
}

Java 8 has introduced a new class Optional in java.util package.
Advantages of Java 8 Optional:
1.) Null checks are not required.
2.) No more NullPointerException at run-time.
3.) We can develop clean and neat APIs.
Optional - A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.
For more details find here oracle docs :-
https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html

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