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Exception to throw when instance variables are null

I'm working on making a codebase complaint with a list of PMD rules. These include that you're not allowed to throw or catch NullPointerExceptions. I found this method which requires 2 instance variables to be non-null in order to run. It throws a NullPointerException otherwise:
public String build() {
if (apiKey == null || url == null) {
throw new NullPointerException(
"Cannot build: API key and URL must be set");
}
What should it be throwing instead? it seems to me that NullPointerException would be the bast call. The only other thing that comes to mind is IllegalArgumentException, but these aren't really arguments, so I don't think it really fits.

IllegalStateException seems entirely appropriate here:
Signals that a method has been invoked at an illegal or inappropriate time.
That describes the situation reasonably clearly, doesn't it?

IllegalStateException springs to mind.
One thing to consider here is whether this should be a checked or an unchecked exception. You could also consider making your own explicit Exception for this case if it can usefully be handled differently from the generic IllegalStateException.

Should exceptions be checked at lower level if higher level takes care of them

This is a quote from Effective java.
"Where possible, the best way to deal with exceptions from lower layers is to avoid them, by ensuring that lower-level methods succeed. Sometimes you can do this by checking the validity of the higher-level method’s parameters before passing them on to lower layers."
Consider an object called "AccessControlContext actx" which is propogated from handler to lower levels. We can do a higher level check that "actx != null" but does it need to done in lower level again ?
Eg in psuedocode:
class RestServlet {
void verify Servlet(AccessControlContext actx) {
// check made at higher level according to effective java
if (actx == null) { throw exception; }
// do something
Verify.checkAccessControl(actx); // calling lower leve
// do something
}
}
class Verify {
static checkAccessControl(actx) {
// DO I NEED NULL POINTER CHECK HERE AGAIN ?
}
}
Although question is nested inside the comment, reiterating with elaboration.
A redundant check at lower level ensures defensive coding but - it is redundant.
It may be well spelled out in the javadocs that it does not accept null, but that does not solve the purpose of having a bug free code. Should we dedupe exception checks ?
NOTE: This was a random example. My question is not specific to this example. Its a broader question which seeks to understand how much to duplicate exceptions.

When it comes to validating parameters, in my opinion you can't do it too often. The way I'm used to working, parameters are validated in all layers of the code. If a method can't handle a null value (it would lead to a nullpointer for instance), it should be up to the method itself to make sure that the parameters passed aren't null. The same goes for methods where a bad parameter value would cause and exception further down in the layers.
The way I see it, if a bad parameter would cause an exception, it doesn't matter whether or not it is an exception thrown from a parameter validation at a high level or a parameter validation at low level, it is anyways better than the exception it would cause if there were no validation at all (or even worse, no exception and an erroneous execution.)
By letting each method be responsible for validating its input, even if it is just a passthrough-method, one can make sure that any new method calling an existing method will be prevented from passing along a bad parameter, since it would lead to a validation exception, no matter if the method being called is a high level or low level method.
So, in your example I would validate the input in both methods, with accompanying tests verifying that the validation behaves as it should. Input validation could also be as easy as a one-liner, check out the Validate class of apache commons lang: http://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/Validate.html
For instance:
import org.apache.commons.lang.Validate;
import org.apache.commons.lang.StringUtils;
public class Something {
public void doSomething(final Integer someInt, final String someString) {
Validate.notNull(someInt, "Can't proceed when someInt is null!");
Validate.isTrue(!StringUtils.isEmpty(someString, "Can't proceed when someString is null or empty!");
//do stuff
}
}
If the validation fails, an IllegalArgumentException with the message defined as the second parameter will be thrown. Tests could then look like this:
public class SomethingTest {
private Something something;
#Rule
public ExpectedException expectedException = ExpectedException.none();
#Before
public void setup() {
something = new Something();
}
#Test
public void doSomethingShouldFailIfSomeIntIsNull() {
expectedException.expect(IllegalArgumentException.class);
expectedException.expectMessage("someInt is null");
something.doSomething(null, "hello");
}
#Test
public void doSomethingShouldFailIfSomeStringIsEmpty() {
expectedException.expect(IllegalArgumentException.class);
expectedException.expectMessage("someString is null or empty");
something.doSomething(123, "");
}
//more tests..
}

I'm not sure that I'd be happy with this approach, what if checkAccessControl is called from many places? You have to rely on each caller doing the null check - it's not a safe way to proceed. I'd be putting the null check in to the checkAccessControl method, probably throwing IllegalArgumentException if a null was detected.
So, my broad approach would be, if your method relies on specific values being presented to it ( eg a parameter can't be null ), I would make it the responsibility of the method to validate it.
You can of course still ask the caller to validate the argument for you ( via javadocs for example ), but you can't enforce it.

Java RuntimeException

I am having trouble understanding how a RuntimeException works. I have searched through Google.
The application I want to use one in is a cash register program that just simulates two cash registers working. I will use two classes: one with the input and output and the other with all my methods and such.
In my assignment description, it said to make sure that your methods catch any violated preconditions ny throwing a RuntimeException with an informative error.
I am not sure how to implement this - I am not looking for syntax here but just how to set something like this up, seeing as I don't have a full understanding of how the RuntimeException works or what preconditions for a method even are. How could I implement one?

A precondition is something that your method documents must be true before the method will execute properly. For example, you could have a precondition that an argument must be positive. (Another example is that the object must be in a certain state.)
RuntimeException is a subclass of Exception which is special in that exceptions of type RuntimeException (and its subclasses) are unchecked - you don't need to declare that you throw them as part of the method signature. One example of a RuntimeException is IllegalArgumentException.
So if we have a method which will only work if it's called with a positive number, you might have:
public void foo(int x) {
if (x <= 0) {
throw new IllegalArgumentException("x must be positive");
}
// Rest of the code here
}
This protects the rest of the method from trying to operate in a situation it was never intended for.

I think you need to increase your know-how level with reading some articles before the implementation which is more further step. Firstly you can research about Throwable class which is the superclass of all errors and exceptions in Java. You can check this question also for it.
Then Exceptions need to be visited. Oracle tutorials tells more about run-time handling. Or you can check this article also which illustrates the title into a real-life example.
Also about Error and Exception hierarchy this graphic makes it easier to understand:

Choosing between NullPointerException and IllegalArgumentException for signalling error at object construction

I have a question about best practices in Java. I'm writing a class, which relies on an object at construction pretty heavily. In fact, if the object passed in at construction is null, most of the functionality wouldn't make sense. I thought it best to throw an exception in the case where null is passed as an argument at object construction, but I don't know what the most appropriate exception would be in this case. Should I throw a NullPointer, IllegalArgument, Instatiation, or even an Initialization exception.
I've seen these all used throughout the Java source codebase, and I don't seem to be able to distinguish why a NullPointer was used in some cases and IllegalArgument used in others. I personally would have thought that if a bad argument is passed in at construction, some initialization error should occur.

Firstly, you should absolutely throw an exception. It's simply the Right Thing To Do.
As for whether you should throw IllegalArgumentException or NullPointerException - Josh Bloch talks about this in Effective Java, and generally concludes that NullPointerException is a reasonable choice. I'm not entirely sure I agree (I would probably have picked IllegalArgumentException), but ultimately it's unlikely to matter: you shouldn't be catching either of these exceptions directly, and the stack trace will show you where there's a problem anyway.
(It's a shame Java doesn't have the equivalent to .NET's ArgumentNullException, which means exactly what it sounds like.)
Personally I'm a big fan of Guava's Preconditions class:
public class Foo {
private final Bar bar;
public Foo(Bar bar) {
this.bar = Preconditions.checkNotNull(bar);
}
}
(Sometimes used with a static import of checkNotNull.)

IllegalArgument makes it clear, to the developer who's used the code, what the problem is and how to fix it I would say. It also makes it clear that the method CANNOT work without that object.
NullPointer means they have to think a little bit about what the problem is (not much) and may mean the code is cleaner, but like I said above. It's not immediately clear, from a documentation point-of-view whether your code can work with a null value or not.

If you think the platform exceptions do not give the client the right amount of information to understand and fix the issue, you can create your own exception that's more appropriate in that regard, say like .NET's ArgumentNullException; which will tell the client both what the problem is and how to fix it.

NullPointerException is used when you are expecting a valid object but get null instead. This is especially pertinent when trying to call methods on a null object. This is the most common use for it. However, your case may also warrant it.
IllegalArgumentException is used when you received an argument into a method that is not what you expect or of the wrong type/style/etc. This, in my view, would be more appropriate in your case. You expect a valid object, but you received null as an argument. This argument value is invalid - so I would throw IllegalArgumentException and in the message of the exception specify what's wrong, e.g.:
public MyClass(InputObject obj) {
if(obj == null) {
throw new IllegalArgumentException("null passed to MyClass constructor");
}
...
}

Is it good practice to make the constructor throw an exception? [duplicate]

This question already has answers here:
When to throw an exception?
(33 answers)
Closed 4 years ago.
Is it a good practice to make the constructor throw an exception?
For example I have a class Person and I have age as its only attribute. Now
I provide the class as
class Person{
int age;
Person(int age) throws Exception{
if (age<0)
throw new Exception("invalid age");
this.age = age;
}
public void setAge(int age) throws Exception{
if (age<0)
throw new Exception("invalid age");
this.age = age;
}
}

Throwing exceptions in a constructor is not bad practice. In fact, it is the only reasonable way for a constructor to indicate that there is a problem; e.g. that the parameters are invalid.
I also think that throwing checked exceptions can be OK1, assuming that the checked exception is 1) declared, 2) specific to the problem you are reporting, and 3) it is reasonable to expect the caller to deal with a checked exception for this2.
However explicitly declaring or throwing java.lang.Exception is almost always bad practice.
You should pick an exception class that matches the exceptional condition that has occurred. If you throw Exception it is difficult for the caller to separate this exception from any number of other possible declared and undeclared exceptions. This makes error recovery difficult, and if the caller chooses to propagate the Exception, the problem just spreads.
1 - Some people may disagree, but IMO there is no substantive difference between this case and the case of throwing exceptions in methods. The standard checked vs unchecked advice applies equally to both cases.
2 - For example, the existing FileInputStream constructors will throw FileNotFoundException if you try to open a file that does not exist. Assuming that it is reasonable for FileNotFoundException to be a checked exception3, then the constructor is the most appropriate place for that exception to be thrown. If we threw the FileNotFoundException the first time that (say) a read or write call was made, that is liable to make application logic more complicated.
3 - Given that this is one of the motivating examples for checked exceptions, if you don't accept this you are basically saying that all exceptions should be unchecked. That is not practical ... if you are going to use Java.
Someone suggested using assert for checking arguments. The problem with this is that checking of assert assertions can be turned on and off via a JVM command-line setting. Using assertions to check internal invariants is OK, but using them to implement argument checking that is specified in your javadoc is not a good idea ... because it means your method will only strictly implement the specification when assertion checking is enabled.
The second problem with assert is that if an assertion fails, then AssertionError will be thrown, and received wisdom is that it is a bad idea to attempt to catch Error and any of its subtypes.

I've always considered throwing checked exceptions in the constructor to be bad practice, or at least something that should be avoided.
The reason for this is that you cannot do this :
private SomeObject foo = new SomeObject();
Instead you must do this :
private SomeObject foo;
public MyObject() {
try {
foo = new SomeObject()
} Catch(PointlessCheckedException e) {
throw new RuntimeException("ahhg",e);
}
}
At the point when I'm constructing SomeObject I know what it's parameters are
so why should I be expected to wrap it in a try catch?
Ahh you say but if I'm constructing an object from dynamic parameters I don't know if they're valid or not.
Well, you could... validate the parameters before passing them to the constructor. That would be good practice.
And if all you're concerned about is whether the parameters are valid then you can use IllegalArgumentException.
So instead of throwing checked exceptions just do
public SomeObject(final String param) {
if (param==null) throw new NullPointerException("please stop");
if (param.length()==0) throw new IllegalArgumentException("no really, please stop");
}
Of course there are cases where it might just be reasonable to throw a checked exception
public SomeObject() {
if (todayIsWednesday) throw new YouKnowYouCannotDoThisOnAWednesday();
}
But how often is that likely?

As mentioned in another answer here, in Guideline 7-3 of the Java Secure Coding Guidelines, throwing an exception in the constructor of a non-final class opens a potential attack vector:
Guideline 7-3 / OBJECT-3: Defend against partially initialized
instances of non-final classes When a constructor in a non-final class
throws an exception, attackers can attempt to gain access to partially
initialized instances of that class. Ensure that a non-final class
remains totally unusable until its constructor completes successfully.
From JDK 6 on, construction of a subclassable class can be prevented
by throwing an exception before the Object constructor completes. To
do this, perform the checks in an expression that is evaluated in a
call to this() or super().
// non-final java.lang.ClassLoader
public abstract class ClassLoader {
protected ClassLoader() {
this(securityManagerCheck());
}
private ClassLoader(Void ignored) {
// ... continue initialization ...
}
private static Void securityManagerCheck() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkCreateClassLoader();
}
return null;
}
}
For compatibility with older releases, a potential solution involves
the use of an initialized flag. Set the flag as the last operation in
a constructor before returning successfully. All methods providing a
gateway to sensitive operations must first consult the flag before
proceeding:
public abstract class ClassLoader {
private volatile boolean initialized;
protected ClassLoader() {
// permission needed to create ClassLoader
securityManagerCheck();
init();
// Last action of constructor.
this.initialized = true;
}
protected final Class defineClass(...) {
checkInitialized();
// regular logic follows
...
}
private void checkInitialized() {
if (!initialized) {
throw new SecurityException(
"NonFinal not initialized"
);
}
}
}
Furthermore, any security-sensitive uses of such classes should check
the state of the initialization flag. In the case of ClassLoader
construction, it should check that its parent class loader is
initialized.
Partially initialized instances of a non-final class can be accessed
via a finalizer attack. The attacker overrides the protected finalize
method in a subclass and attempts to create a new instance of that
subclass. This attempt fails (in the above example, the
SecurityManager check in ClassLoader's constructor throws a security
exception), but the attacker simply ignores any exception and waits
for the virtual machine to perform finalization on the partially
initialized object. When that occurs the malicious finalize method
implementation is invoked, giving the attacker access to this, a
reference to the object being finalized. Although the object is only
partially initialized, the attacker can still invoke methods on it,
thereby circumventing the SecurityManager check. While the initialized
flag does not prevent access to the partially initialized object, it
does prevent methods on that object from doing anything useful for the
attacker.
Use of an initialized flag, while secure, can be cumbersome. Simply
ensuring that all fields in a public non-final class contain a safe
value (such as null) until object initialization completes
successfully can represent a reasonable alternative in classes that
are not security-sensitive.
A more robust, but also more verbose, approach is to use a "pointer to
implementation" (or "pimpl"). The core of the class is moved into a
non-public class with the interface class forwarding method calls. Any
attempts to use the class before it is fully initialized will result
in a NullPointerException. This approach is also good for dealing with
clone and deserialization attacks.
public abstract class ClassLoader {
private final ClassLoaderImpl impl;
protected ClassLoader() {
this.impl = new ClassLoaderImpl();
}
protected final Class defineClass(...) {
return impl.defineClass(...);
}
}
/* pp */ class ClassLoaderImpl {
/* pp */ ClassLoaderImpl() {
// permission needed to create ClassLoader
securityManagerCheck();
init();
}
/* pp */ Class defineClass(...) {
// regular logic follows
...
}
}

You do not need to throw a checked exception. This is a bug within the control of the program, so you want to throw an unchecked exception. Use one of the unchecked exceptions already provided by the Java language, such as IllegalArgumentException, IllegalStateException or NullPointerException.
You may also want to get rid of the setter. You've already provided a way to initiate age through the constructor. Does it need to be updated once instantiated? If not, skip the setter. A good rule, do not make things more public than necessary. Start with private or default, and secure your data with final. Now everyone knows that Person has been constructed properly, and is immutable. It can be used with confidence.
Most likely this is what you really need:
class Person {
private final int age;
Person(int age) {
if (age < 0)
throw new IllegalArgumentException("age less than zero: " + age);
this.age = age;
}
// setter removed

This is totally valid, I do it all the time. I usually use IllegalArguemntException if it is a result of parameter checking.
In this case I wouldn't suggest asserts because they are turned off in a deployment build and you always want to stop this from happening, but they are valid if your group does ALL it's testing with asserts turned on and you think the chance of missing a parameter problem at runtime is more acceptable than throwing an exception that is maybe more likely to cause a runtime crash.
Also, an assert would be more difficult for the caller to trap, this is easy.
You probably want to list it as a "throws" in your method's javadocs along with the reason so that callers aren't surprised.

I have never considered it to be a bad practice to throw an exception in the constructor. When the class is designed, you have a certain idea in mind of what the structure for that class should be. If someone else has a different idea and tries to execute that idea, then you should error accordingly, giving the user feedback on what the error is. In your case, you might consider something like
if (age < 0) throw new NegativeAgeException("The person you attempted " +
"to construct must be given a positive age.");
where NegativeAgeException is an exception class that you constructed yourself, possibly extending another exception like IndexOutOfBoundsException or something similar.
Assertions don't exactly seem to be the way to go, either, since you're not trying to discover bugs in your code. I would say terminating with an exception is absolutely the right thing to do here.

I am not for throwing Exceptions in the constructor since I am considering this as non-clean. There are several reasons for my opinion.
As Richard mentioned you cannot initialize an instance in an easy manner. Especially in tests it is really annoying to build a test-wide object only by surrounding it in a try-catch during initialization.
Constructors should be logic-free. There is no reason at all to encapsulate logic in a constructor, since you are always aiming for the Separation of Concerns and Single Responsibility Principle. Since the concern of the constructor is to "construct an object" it should not encapsulate any exception handling if following this approach.
It smells like bad design. Imho if I am forced to do exception handling in the constructor I am at first asking myself if I have any design frauds in my class. It is necessary sometimes, but then I outsource this to a builder or factory to keep the constructor as simple as possible.
So if it is necessary to do some exception handling in the constructor, why would you not outsource this logic to a Builder of Factory? It might be a few more lines of code but gives you the freedom to implement a far more robust and well suited exception handling since you can outsource the logic for the exception handling even more and are not sticked to the constructor, which will encapsulate too much logic. And the client does not need to know anything about your constructing logic if you delegate the exception handling properly.

It is bad practice to throw Exception, as that requires anyone who calls your constructor to catch Exception which is a bad practice.
It is a good idea to have a constructor (or any method) throw an exception, generally speaking IllegalArgumentException, which is unchecked, and thus the compiler doesn't force you to catch it.
You should throw checked exceptions (things that extend from Exception, but not RuntimeException) if you want the caller to catch it.