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Which is better way of having a null check?

I came across these two ways of having a null check for a string object.
Given a string object String str = "example";
If(str.someMethod() != null ) or
If (null != str.someMethod())
Why do we prefer the 2nd one ?
What is the exact reason behind this, is it related to performance ?

In your example, it makes absolutely no difference which you do (other than style), because the reason for Yoda checks is to avoid accidentally doing an assignment (but keep reading for why this doesn't matter in Java), and you can't assign to the result of calling a method.
One of the nice things about Java is that even if you were testing str, e.g.:
if (str == null)
vs.
if (null == str)
there would still be no difference, whereas in some of the languages with syntax derived from B (such as C, C++, D, JavaScript, etc.), people do the second (a "Yoda test") to minimize the odds of this bug:
if (str = null) // Not an issue in Java
In C or JavaScript, for instance, that would assign null to str, then evaluate the result, coerce it to boolean, and not branch. But in Java, that's a syntax error the compiler tells you about.
Java doesn't do that kind of boolean conversion, so the only reason for using Yoda checks in Java is if you're testing booleans, e.c.
boolean flag;
// ...
if (flag == false)
There, you might conceivably do this by accident:
if (flag = false)
But since using == and != with booleans is completely unnecessary (you'd just use if (flag) or if (!flag)), in the real world you don't need Yoda checks with Java at all.
That doesn't mean people don't still use them, as a matter of their own personal style. There's just no objective reason to, in Java.

It makes no difference performance-wise, however the Yoda programming pattern have some advantages when it comes to the world of programming skills.
In your example it would not matter as both cases would throw a NullPointerException (since you're invoking someMethod` of a null instance reference).
However, say that you wanted to check if str is null. In the first case, you'd write if (str == null) and in the second if (null == str). Both are the same. Now say that you have accidently used = instead of ==. In Java, it would not matter as the compiler wouldn't let you as the expression doesn't evalute to a boolean value. But other languages let you do that, more specifically languages that are compiler-free and only use an interperter. In that case, if you write if (str = null) you'll be assigning null to string and overriding its' current value, which would result in buggy behavior and you chasing after your tail for quite some time. However, if you'd write if (null = str) you'll get an error saying you cannot assign a value to null and thus save yourself a lot of time and effort. Again, this is not relevant to JAVA.
An example which might be relevant for Java, is the use of method invocation on constant values. For example, if (str.equals("constantString"). If str is null you'll get a NullPointerException. However, if you use a Yoad pattern and write if ("constantString".equals(str)) you'll get false as ConstantString does not equal null. This of course is only relevant for comparison, and not say contains etc.

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

null == foo versus foo == null [duplicate]

This question already has answers here:
Which is more effective: if (null == variable) or if (variable == null)? [duplicate]
(9 answers)
Closed 9 years ago.
This may just be a style question, but I'm reading a Java coding book ('Programming Android') and the writer all declares null first before a variable method, a practice I am not familiar with. For example:
if (null == foo) {
//code here
}
or
if (null != foo) {
//code here
}
instead of
if (foo == null) {
//code here
}
I can't see how the order would make a difference semantically/syntactically, or am I wrong here? Really just curious.

It's probably a habit left over from C/C++. In C, you would put constants on the left, because if you mistyped = instead of == there would be an error because you can't assign something to a constant. In Java, this is unnecessary because if (foo = null) also gives an error, which says that an object reference isn't a boolean.

This is a holdover from C/C++. It was advantages to put the value on the left of the == operator in case you accidently used the assignment = operator. The C compiler will catch the 14 = var as an error, but var = 14 will compile, when you meant to type var == 14. There is not much reason to do this in Java, but some still do it.

Sometimes order saves you from null pointer exception e.g. if a String variable is coming from somewhere and you compare it like this:
if(foo.equals("foo")){
}
then you might get Null pointer exception. On the other hand if you do it like this:
if("foo".equals(foo)){
}
then you not only achieve your purpose but you also avoid a null pointer exception in case String foo was null.

No difference.
Second one is merely because C/C++ where programmers always did assignment instead of comparing.
E.g.
// no compiler complaint at all for C/C++
// while in Java, this is illegal.
if(a = 2) {
}
// this is illegal in C/C++
// and thus become best practice, from C/C++ which is not applicable to Java at all.
if(2 = a) {
}
While java compiler will generate compilation error..
There is no really different between two form. There is no performance issue but there are following notes:
First form is readable for code reader, because people usually read
codes Left-To-Right.
Second form is better for code writer, because in java = operator is
for assignment and == operator is for test equivalent, but people
usually using in if statement = instead of ==, by second approch
developer getting Compile-Time-Error because null can't use in
Left-Side of a assignment statement.
ADDED
if (object = null) {
The convention of putting the constant on the left side of == isn't
really useful in Java since Java requires that the expression in an if
evaluate to a boolean value, so unless the constant is a boolean,
you'd get a compilation error either way you put the arguments. (and
if it is a boolean, you shouldn't be using == anyway...)

There is no difference, and
if (foo == null)
enter code here
is the prefered way; however in C, you would put constants to the left since there would be an error if you used = instead of ==

What is the value of the test !"".equals(someString)

In a project I've been trying to familiarise myself with, I ran across a method that looks like this:
public boolean testString(String string){
return string != null && !"".equals(string);
}
What is the value of testing the string for emptiness this way instead of with the variable first? I understand why we see constant-first (Yoda syntax) in C, but is there any reason to do so with method calls in Java?
note: I do understand about NullPointerException, which is not possible in this instance. I'm looking for a value to doing it this way in this case particularly.

In this context it makes little difference, as it already tested for null. Usually you do it this way to make sure you don't call a member on a null-reference (resulting in a NullPointerException), i.e.
"test".equals(myString)
will never throw a null pointer exception whereas
myString.equals("test")
will if myString is null. So basically, the first test makes sure it's a string (not null) AND it's equal to "test".

For two strings it doesn't matter much, but when there is a non-final type involved it can be a micro-optimization.
If the left hand side is a non-overridden concrete type, then the dispatch becomes static.
Consider what the JIT has to do for
Object o;
String s;
o.equals(s)
vs
s.equals(o)
In the first, the JIT has to find the actual equals method used, whereas in the second, it knows that it can only by String.equals.
I adopted the habit of doing
"constant value" == variableName
in other languages, since it means that the code will fail to parse if I mis-type = instead of ==.
And when I learned Java, I kept that order preference.

The usual reason for using "constant string".equals(variable) is that this works properly even if variable is null (unlike variable.equals("constant string")). In your case, however, since you are testing that string != null in a short-circuit boolean test, it's entirely a matter of style (or habit).

If they just did this:
!"".equals(string);
then they're avoiding the possibility of a NullPointerException, which is pretty smart. However, they're checking for null right before this condition, which is technically not necessary.

Is it running any tools like checkstyle? if it is, putting the variable first will result in checkstyle failing. Another reason is that if you put the empty string first it will take away the possibility of getting a null exception if the variable is null because the expression will always evaluate to false. If you had the variable first and the variable was null it will throw an exception.

It is more than a coder preference. If the purpose of the method was only to check that string is not an empty String (without caring whether its a null) then it makes sense to have the constant first to avoid a NullPointerException.
e.g. This method will return the boolean outcome. false in case string is null.
public boolean testString(String string){
return !"".equals(string);
}
while this one may throw a runtime exception if string is null
public boolean testString(String string){
return !string.equals("");
}

No, it is unnatural, and harder to read. It triggers a pause for most readers, and may wastefully consume lots of resources on stackoverlow.com.
(Better use string.isEmtpy() anyway)
There are no fixed rules tho, sometime this is easier to read
if( null != foobar(blahblahblah, blahblahblah, blahblahblah) )
than
if( foobar(blahblahblah, blahblahblah, blahblahblah) != null )

This question can be answered on a number of levels:
What does the example mean?
As other answers have explained, !"".equals(str) tests if str is an non-empty string. In general, the <stringLiteral>.equals(str) idiom is a neat way of testing a string that deals with the null case without an explicit test. (If str is null then the expression evaluates to false.
Is this particular example best practice?
In general no. The !"".equals(str) part deals with the case where str is null, so the preceding null test is redundant.
However, if str was null in the vast majority of cases, this usage would possibly be faster.
What is a better way to do this from a code-style perspective?
return "".equals(str);
or
return str != null && !str.isEmpty();
However, the second approach doesn't work with Java versions prior to 1.6 ... because isEmpty() is a recent API extension.
What is the optimal way to do this?
My gut feeling is that return str != null && !str.isEmpty(); will be fastest. The String.isEmpty() method is implemented as a one-line test, and is small enough that the JIT compiler will inline it. The String.equals(Object) method is a lot more complicated, and too big to be inlined.

Miško Hevery (see his videos on youtube) calls this type of overkill "paranoid programming" :-)
Probably in this video:
http://www.youtube.com/watch?v=wEhu57pih5w
See also here: http://misko.hevery.com/2009/02/09/to-assert-or-not-to-assert/

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