Java cast cast exception with SortedSet

Question

I'm trying to understand why this code won't compile.
I've got a class implementing an interface. The last method won't compile for some reason.

It will not simply allow me to cast the set as a set, but does allow it to return the single object fine.

Can someone please explain to me why this is? thanks.

public class Testing2 {

    public SortedSet<ITesting> iTests = new TreeSet<ITesting>();
    public SortedSet<Testing> tests = new TreeSet<Testing>();

    public ITesting iTest = null;
    public ITesting test = new Testing();

    // Returns the implementing class as expected
    public ITesting getITesting(){
        return this.test;
    }

    // This method will not compile
    // Type mismatch: cannot convert from SortedSet<Testing> to SortedSet<ITesting>
    public SortedSet<ITesting> getITests(){
        return this.tests;
    }

}

Would you edit your question to include the exact compiler message? edit: Also, it looks like Testing implements ITesting? — Aaron Kurtzhals, Feb 28 '13 at 21:44
Looks like a duplicate of http://stackoverflow.com/questions/897935/when-do-java-generics-require-extends-t-instead-of-t-and-is-there-any-down — Aaron Kurtzhals, Feb 28 '13 at 21:49

score 6 · Accepted Answer · answered Feb 28 '13 at 21:43

Simply, a SortedSet<Testing> is not a SortedSet<ITesting>. For example:

SortedSet<Testing> testing = new TreeMap<Testing>();
// Imagine if this compiled...
SortedSet<ITesting> broken = testing;
broken.add(new SomeOtherImplementationOfITesting());

Now your SortedSet<Testing> would contain an element which isn't a Testing. That would be bad.

What you can do is this:

SortedSet<? extends ITesting> working = testing;

... because then you can only get values out of the set.

So this should work:

public SortedSet<? extends ITesting> getITests(){
    return this.tests;
}

score 1 · Answer 2 · answered Feb 28 '13 at 21:43

Assuming ITesting is a super type of Testing. Generic types are not polymorphic.Thus SortedSet<ITesting> is not a super type of SortedSet<Testing>.Polymorphism simply doesn't apply for generic types. you probably need to use wildcards with lowerbounds ? extends ITesting as your return type.

public SortedSet<? extends ITesting> getITests(){
    return this.tests;
}

score 0 · Answer 3 · answered Feb 28 '13 at 21:43

0

You have a typo in your declartion:

public SortedSet<Testing> tests = new TreeSet<Testing>();

Should be ITesting there if you want the method to return an ITesting, or you need the method to return:

SortedSet<Testing>

answered Feb 28 '13 at 21:43

Michael

6,141
2
20
21

score 0 · Answer 4 · answered Feb 28 '13 at 21:43

0

I think you want this instead:

public SortedSet<Testing> getTests(){
    return this.tests;
}

Right now you're trying to return tests, which is declared as a SortedSet<Testing> and not SortedSet<ITesting>.

answered Feb 28 '13 at 21:43

thegrinner

11,546
5
41
64

Java cast cast exception with SortedSet

4 Answers4