How can I call the most specific method using generics?

Question

Having the following example:

public class Test {

    public static class A {}

    public static void main(String[] args) {
        A a = new A();
        m1(a);
    }

    public static <T> void m1(T t) {
        // t.getClass().getSimpleName() is A
        // t instanceof A is true
        m2(t);
    }

    /* Not called */
    public static void m2(A a) {
        System.out.println("A");
    }

    public static void m2(Object o) {
        // o.getClass().getSimpleName() is A
        // o instanceof A is true
        System.out.println("O");
    }

}

I don't understand why m2(Object o) is chosen instead of m2(A a). As you can see, when m2(t) is called, t "is an A".

Output:

• actual

  O
  

• expected

  A
  

How can I use generics for the situation above so that m2(A a) is chosen?

---

Edit:

I'd like to have a general solution that will work even if I add a type B (similar to A).

...
public static void main(String[] args) {
    A a = new A();
    m1(a);
    B b = new B();
    m1(b);
}
...
public static void m2(B b) {
    System.out.println("B");
}
...

Output:

• actual

  O
  O
  

• expected

  A
  B
  

Answer 1

You have to do:

public static <T extends A> void m1(T t) {
    m2(t);
}

Otherwise the compiler cannot infer that the passed parameter is compliant with m2(A a) and with pick m2(Object o) instead.

Answer 2

You are looking for double dispatch which Java does not support. I do not think that generics can help here, but there's the visitor design pattern with which you can emulate it:

public class Test {
    public static interface Visitable {
        void accept(Visitor visitor);
    }

    public static class A implements Visitable {
        @Override
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

    public static class B implements Visitable {
        @Override
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

    public static interface Visitor {
        void visit(A a);

        void visit(B b);
    }

    public static class PrintingVisitor implements Visitor {
        @Override
        public void visit(A a) {
            System.out.println("A");
        }

        @Override
        public void visit(B b) {
            System.out.println("B");
        }
    }

    public static void main(String[] args) {
        Visitable visitable = new A();
        m(visitable);
        visitable = new B();
        m(visitable);
    }

    public static void m(Visitable visitable) {
        visitable.accept(new PrintingVisitor());
    }
}