Java Generics: Type cast issue (ClassCastException)

Question

I'm having trouble figuring out how to properly cast a generic object in java to a type that extends the generic object.

For example, say I some setup like the following:

public class Parameters extends SomeCustomMap<String, String>
{
   ...
}

public class SomeCustomMap<K, V> implements Map<K, V>
{
    public SomeCustomMap<K, V> getSubSet(...)
    {
        SomeCustomMap<K, V> subset;

        ...

        return subset;
    }
}

class ExampleApp
{
    private void someMethod()
    {
        Parameters params;
        Parameters paramsSubSet;

        try
        {
            ...

            paramsSubSet = (Parameters) params.getSubSet(...);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }
}

Running code similar to the above consistently throws a ClassCastException, the likes of which I do not fully understand. Any assitence for how to correctly set up a scenario similar to the above would be appreciated! Namely, how might I properly cast the the SomeCustomMap object that is returned from the params.getSubSet(...) method back to a Parameters object?

Thanks in advance!

Answer 1

You can try something like this:

public <T extends SomeCustomMap<K, V>> T getSubSet(...){
    T subset = (T)this.clone();
    subset.clear();

    return subset;
}

creation looks a little funny - feel free to change it to whatever you want :)

As a bonus you will not need to cast :)

paramsSubSet = params.getSubSet(...)

Answer 2

Your Problem is that the Subset returned by getSubSet is a of instance SomeCustomMap and not of Parameters.

This problem does not deal with generics. You will get the same problem if you did not use generics.

I don't know how you create an instance of subset but maybe you could use the template desing pattern and some generics to fix your problem.

Answer 3

Though I've commented asking for more information, based on what you've posted so far, I think getSubSet is constructing a SomeCustomMap to return (with new SomeCustomMap) somewhere. If you don't override getSubSet in Parameters, then Parameters.getSubset will return a SomeCustomMap (the base class), not a Parameters, so your typecast to Parameters fails.

(Hot tip, if you override getSubSet in the Parameters class, you can change the return type to Parameters and avoid the typecast.)

Answer 4

Generics don't inherently have anything to do with casting (save that due to the nature of erasure, generic parameters cannot be checked during a cast).

If you're getting a ClassCastException in this case, it means that the object returned really is not an instance of Parameters. Just before you cast, try calling

System.out.println(params.getSubSet(...).getClass());

and see what the actual run-time class of the subset is. Chances are the problem lies elsewhere, as your expectation that the subset is a Parameters object is almost certainly not correct at runtime - it's a SomeCustomMap or some other subclass thereof.

Answer 5

As others have explained, the issue is that the actual object you are constructing in getSubSet() is not an instance of Parameters.

Here's one possible workaround. I don't love it, but it is a way to declare the method in SomeCustomMap but have its return value be typed correctly for any subclass.

public static <T extends SomeCustomMap<K, V>> getSubSet(T fullSet)
{
    T subset;

    ... (use fullSet instead of this)

    return subset;
}