This appears to create an object from an interface; how does it work?

Question

interface Int {
    public void show();
}

public class Test {     
    public static void main(String[] args) {
        Int t1 = new Int() {
            public void show() {
                System.out.println("message");
            }
        };

        t1.show();
    }
}

Answer 1

You're defining an anonymous class that implements the interface Int, and immediately creating an object of type thatAnonymousClassYouJustMade.

Answer 2

This notation is shorthand for

Int t1 = new MyIntClass();

// Plus this class declaration added to class Test
private static class MyIntClass implements Int
    public void show() {
        System.out.println("message");
    }
}

So in the end you're creating an instance of a concrete class, whose behavior you defined inline.

You can do this with abstract classes too, by providing implementations for all the abstract methods inline.