Question

It’s been a few years since I’ve been heavily into Java. Coming back to it I’m seeing this pattern all over the place:

ratingBar.setOnRatingBarChangeListener(new OnRatingBarChangeListener() {
  public void onRatingChanged(RatingBar ratingBar, float rating, boolean fromUser) {
    // do work
  }
});

This looks more like Functional programming to me. It’s a nice pattern but how is it possible to pass a method like this? In the old days a class was a class, and once compiled there was little you could do to it.

My questions are:

1. Can anyone let me know what this pattern is called?
2. How can I write a class that can be instantiated in this way.
3. Are there any other useful examples of functional patterns that have made their way into Java?
4. What do I need to Google to read more about this?

Thanks.

Answer 1

This passes an anonymous class, not a method.

This is a typical pattern, not just in Swing programming, but anywhere you need (hopefully) short, “throw-away” implementations of an interface or class that doesn’t need to be re-used, instead of creating a full-blown implementation.

Any class/interface can be instantiated like this, there’s nothing special about it:

public interface Foo {
    String foo();
}

…

public class Main {
    public static void main(String[] args) {
        System.out.println(new Foo() {
            public String foo() {
                return "plugh";
            }
        });
    }
}

Anonymous inner classes get their own class files, too, even though their source is embedded.

In this example, a Main$1.class file will be generated for the anonymous inner class, in addition to the expected Main.class file.