How to get rid of this generics warning?

Question

I am trying to mock a generic interface, and whenever I mock it, I gets this warning:

The expression of type GenericInterface needs unchecked conversion to conform to GenericInterface<String>

My interface is

interface GenericInterface<T>{
    public T get();
}

and my test is

@Test
public void testGenericMethod(){
    GenericInterface<String> mockedInterface = EasyMock.createMock(GenericInterface.class);
}

I get warning at the first line in test case.

How do I remove this generic warning?

Answer 1

The correct steps to get rid of the warning is:

• First and foremost, prove that the unchecked cast is safe, and document why
• Only then perform the unchecked cast, and annotate @SuppressWarnings("unchecked") on the variable declaration (not on the whole method)

So something like this:

// this cast is correct because...
@SuppressWarnings("unchecked")
GenericInterface<String> mockedInterface =
    (GenericInterface<String>) EasyMock.createMock(GenericInterface.class);

---

Guidelines

The following is excerpt from Effective Java 2nd Edition: Item 24: Eliminate unchecked warnings:

• Eliminate every unchecked warning that you can.
• If you can't eliminate a warning, and you can prove that the code that provoked the warning is typesafe, then (and only then) suppress the warning with @SuppressWarning("unchecked") annotation.
• Always use the SuppressWarning annotation on the smallest scope possible.
• Every time you use an @SuppressWarning("unchecked") annotation, add a comment saying why it's safe to do so.

Related questions

• What is SuppressWarnings (“unchecked”) in Java?
• How do I address unchecked cast warnings?
• Type safety: Unchecked cast

---

Refactoring the cast

It is also possible in most cases to perform the unchecked cast inside a generified createMock. It looks something like this:

static <E> Set<E> newSet(Class<? extends Set> klazz) {
    try {
        // cast is safe because newly instantiated set is empty
        @SuppressWarnings("unchecked")
        Set<E> set = (Set<E>) klazz.newInstance();
        return set;
    } catch (InstantiationException e) {
        throw new IllegalArgumentException(e);
    } catch (IllegalAccessException e) {
        throw new IllegalArgumentException(e);          
    }
}

Then elsewhere you can simply do:

// compiles fine with no unchecked cast warnings!
Set<String> names = newSet(HashSet.class);
Set<Integer> nums = newSet(TreeSet.class);

See also

• Java Tutorials/Generics
• Angelika Langer's Java Generics FAQ

Answer 2

The problem here is that EasyMock.createMock() is going to return an object of type GenericInterface and not GenericInterface<String>. You could use the @SupressWarnings annotation to ignore the warning, or you could try and explicit cast to GenericInterface<String> (I think this just gives a different warning though.)

Answer 3

• http://www.velocityreviews.com/forums/t147848-generics-in-1-5-nonsense-warnings.html
• http://forums.sun.com/thread.jspa?threadID=635608

seem to discuss the same warning...

i guess @SuppressWarnings might be the key to happiness in that case

Answer 4

If you are really stuck up on avoiding the compiler warning, you can declare an interface in your test just for the purpose of avoiding it.

 interface MockGenericInterface extends GenericInterface<String> {}

Then you can do:

 GenericInterface<String> mockedInterface = EasyMock.createMock(MockGenericInterface.class);