Why does Java let you cast to a collection?

istovatis :

I have a simple foo class, and I am able to cast to a collection interface (either Map or List) without any compiler error. Note that Foo class does not implement any interface or extends any other class.

public class Foo {

    public List<String> getCollectionCast() {
        return (List<String>) this;    // No compiler error
    }

    public Map<String, String> getCollection2Cast() {
        return (Map<String, String>) this;    // No compiler error
    }

    public Other getCast() {
        return (Other)this;     // Incompatible types. Cannot cast Foo to Other
    }

    public  static class Other {
        // Just for casting demo
    }

}

Why does the Java compiler not return incompatible types error when I try to cast the Foo class to a collection?

Foo does not implement Collection. I would expect an incompatible types error, because given the current Foo class signature, this cannot be a Collection.

T.J. Crowder :

It's not because they're collection classes, it's because they're interfaces. Foo doesn't implement them, but subclasses of it could. So it's not a compile-time error, since those methods may be valid for subclasses. At runtime, if this isn't of a class that implements those interfaces, naturally it's a runtime error.

If you change List<String> to ArrayList<String>, you'll get a compiler-time error for that, too, since a Foo subclass could implement List, but can't extend ArrayList (since Foo doesn't). Similarly, if you make Foo final, the compiler will give you an error for your interface casts because it knows they can never be true (since Foo can't have subclasses, and doesn't implement those interfaces).