Question

I’m ran into an interesting issue today. Consider the following code

public static class Parent {}
public static class Child extends Parent {}

Set<Child> childs = new HashSet();
Set<Parent> parents = (Set<Parent>)childs; //Error: inconvertible types

Parent parent = (Parent)new Child(); //works?!

Why wouldn’t a cast like that work? I would expect that an implicit cast wouldn’t work due to the various rules of generics, but why can’t an explicit cast work?

Answer 1

The cast doesn’t work because Java generics are not covariant.

If the compiler allowed this:

List<Child> children = new ArrayList();
List<Parent> parents = (List<Parent>)children;

then what would happen in this case?

parents.add(new Parent());
Child c = children.get(0);

The last line would attempt to assign Parent to a Child — but a Parent is not a Child!

All Child are Parent (since Child extends Parent) but all Parent are not Child.