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(...)