Question

Possible Duplicate:
Most efficient way to cast List<SubClass> to List<BaseClass>
why polymorphism doesn't treat generic collections and plain arrays the same way?

If I have an abstract base class BaseClass and I write a function expecting Collection<BaseClass> as its argument, calls to that function with a Collection<SubClass> of a class SubClass extends BaseClass fail to compile.

In the base class:

public void addLots(Collection<BaseClass> collection) {
    for(BaseClass yourbase : collection) {
        us.add(yourbase) //what you say!!
    }
}

And in the subclass:

public void addMoreLots(Collection<SubClass> collection) {
    addLots(collection); //FAILS TO COMPILE
}

Now I think I can see why this is: Collection<SubClass> is NOT a subclass of Collection<BaseClass>. What is the correct method of making this call?

Answer 1

Collections are checked only during the compilation time, not during the run time, so this is done in order to protect a collection taking in the wrong type of object in.

Try it this way…

public <T extends BaseClass> void addLots(Collection<T> collection){ }

OR

public void addLots(Collection<? extends BaseClass> collection) {}