Question

Why doesn’t this work?

public class FooImpl implements Foo { /* ... */ }

public class Main {
    public static <T> Collection<T> getList(Class<? extends T> itemClass) { /* ... */ }

    public static void main(String[] args) {
        Collection<Foo> foos = getList(FooImpl.class);
    }
}

On the line where foos is declared, I’m getting “Incompatible types. Required: Collection<Foo>, found: Collection<FooImpl>” error. Any idea why?

Answer 1

Try this :

Collection<Foo> foos = Main.<Foo>getList(FooImpl.class);

When you create your getList() method, it says that it will be Typed with T. And it also says that it will need a parameter of a subtype of T (a class of subtype of T to be precise).

As you never specify what will T be, getList suppose that it will be FooImpl so getList() returns a Collection of FooImpl.

With the solution I gave you, you specify that T is Foo, so the parameter will need to be a subtype of Foo. FooImpl for example.

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Resources :

• JLS – Generic methods