how to convert generics in java?

Question

What is the correct way to convert generics in java?

interface A {
}

class B implements A {

}

class C {
  public Set<B> returnSomeB(){
    //some logic
  }
}

C c = new C();
Set<A> = c.returnSomeB();

Set<A> = c.returnSomeB(); this line would give me a compile time error, what's the most proper way to seamlessly convert this since class B is a concrete class of A interface?

Answer 1

A variable of type Set<A> can only hold a Set<A> object, not a Set<B> , even though B is a subtype of A .

The reason is this: What if you stored a Set<B> object in a Set<A> variable, then added an object of type A (but not B ) to it? It would fit all the right argument types, but the end result would be a violation of Java's type safety.

To get around this, you can use wildcards. Instead of declaring a variable of type Set<A> , declare one of type Set<? extends A> Set<? extends A> .

Answer 2

Try bounding your type parameter to the widest possible scope of acceptable values:

interface A {
}

interface B extends A {
}

class C {
    public static void main(String[] args) {
        final Set<A> foo = C.returnSomeB();
        final Set<B> bar = C.returnSomeB(); 
    }

    public static <T extends A> Set<T> returnSomeB() {
        return null;
    }
}