How to call a generic method with two bounds (for a single argument) if one bound needs a cast

[Christof Wiesmeier]

I want to specify two types for a method's formal parameter. That's still simple: Make it a generic method with a type parameter like <T extends MyClass & MyInterface>.

Calling that method having an expression of type MySubClass (which extends MyClass and implements MyInterface) works fine.

But how can i call the method if i have an expression of (compile-time) type MyClass which i know is also of type MyInterface? If i cast to MyInterface, the compiler does not treat it as type "MyClass & MyInterface" (like it should?), but only MyInterface.

Here's the broken code:

    
public class Generics_2Bounds {
	static interface MyInterface {
		void i();
	}

	static class MyClass {
		void c() { System.out.println("c1"); }
	}

	static class MySubClass extends MyClass implements MyInterface {
		public void i() { System.out.println("i"); }

		void c() { System.out.println("c2"); }
	}

	static <T extends MyClass & MyInterface> void f(T o) {
		o.c();
		o.i();
	}

	static void callF0(MySubClass o) {
		f(o);
	}

	static void callF1(MyClass o) {
		// ... do something for all kinds of MyClass's

		if (o instanceof MyInterface)
			f((MyInterface) o); // compiler error: <T>f(T) in Generics_2Bounds 
			  // cannot be applied to (Generics_2Bounds.MyInterface)
	}

	static void callF2(MyInterface o) {
		// ... do something for all kinds of MyInterfaces

		if (o instanceof MyClass)
			f((MyClass) o); // compiler error: <T>f(T) in Generics_2Bounds
			  // cannot be applied to (Generics_2Bounds.MyClass)
	}
}

I want method f to work for all MyClass subclasses which implement MyInterface, so i don't want to simply declare: void f(MySubClass o).

The only solution i can think of is using an extra adapter class like this:

    
	static class Adapter extends MyClass implements MyInterface {
		final private MyClass _c;
		final private MyInterface _i;
		
		public Adapter(MyClass c, MyInterface i) { this._c = c; this._i = i; }
		
		public void i() { _i.i(); }
		public void f() { _c.c(); }
	}

and change callF1 and callF2 to

	static void callF1(MyClass o) {
		// ... do something for all kinds of MyClass's
		
		if (o instanceof MyInterface)
			f(new Adapter(o, (MyInterface) o));
	}

	static void callF2(MyInterface o) {
		// ... do something for all kinds of MyInterfaces

		if (o instanceof MyClass)
			f(new Adapter((MyClass) o, o));
	}

Is there a better way to fix methods callF1 and callF2?
Thanks!