Java, anonymous inner class definition

Question

I've seen a couple of examples similar to this in Java, and am hoping someone can explain what is happening. It seems like a new class can be defined inline, which seems really weird to me.

The first printout line is expected, since it is simply the toString. However the 2nd seems like the function can be overriden inline.

Is there a technical term for this?
Or any documentation which goes into more depth?

If I have the following code:

public class Apple {
    public String toString() {
        return "original apple";
    }
}

public class Driver {
    public static void main(String[] args) {
        System.out.println("first: " + new Apple());
        System.out.println("second: " + 
            new Apple() {
                public String toString() {
                    return "modified apple";
                }
            }
        );
    }
}

The code outputs:

first: original apple
second: modified apple

Solution 1

It is an anonymous inner class. You can find some more information about it at the Java documentation link for Inner Classes. EDIT I am adding a better link describing anonymous inner classes, as the Java documentation leaves something to be desired. /EDIT

Most people will use Anonymous inner classes to define listeners on the fly. Consider this scenario:

I have a Button, and when I click it I want it to display something to the console. But I do not want to have to create a new class in a different file, and I don't want to have to define an inner class later in this file, I want the logic to be immediately available right here.

class Example {
    Button button = new SomeButton();

    public void example() {
        button.setOnClickListener(new OnClickListener() {
            public void onClick(SomeClickEvent clickEvent) {
                System.out.println("A click happened at " + clickEvent.getClickTime());
            }
        });
    }

    interface OnClickListener {
        void onClick(SomeClickEvent clickEvent);
    }

    interface Button {
        void setOnClickListener(OnClickListener ocl);
    }
}

The example is somewhat contrived and obviously not complete, but I think it gets the idea across.

Solution 2

What is happening in your code is that you are implicitly subclassing Apple with an anonymous inner class and overriding its toString() method.

Comments

[ARF]

I just want to add that the 'this' value inside the onClick method refers to the Button instance, if you want to refer to the Example instance inside the onClick method you can use 'Example.this'. Static inner classes do not have this reference though.