Anonymous and Interfaces

Using anonymous classes with interfaces is different than a class, because the anonymous class is implementing the interface.

• The variable type will be the interface.
• Use () after the interface name, like calling a constructor (even though there is no such thing as an interface constructor).
• Implement all abstract methods in the anonymous class's body.

Implementing the Interface

Consider this interface, from java.lang.

public interface Runnable {
  public abstract void run();
}

To implement this interface, classes must implement a run() method.

// Top-level class
public class RunnableClass implements Runnable {

  public void run() {
    System.out.println("Running...");
    // ...
  }

}

An anonymous Runnable implementation looks like:

Runnable r = new Runnable() {
  public void run() {
    System.out.println("Running...");
  }
};

Passing an Anonymous Instance in a Method Call

The method below takes a Runnable method parameter.

public void runIt(Runnable r) {
  r.run();
}

We can pass an anonymous instance when we call the method.

runIt(new Runnable() {

  public void run() {
    System.out.println("Running...");
  }

});

We simply passed the class definition and instantiation to the method.

If we have a method that takes a generic type, like java.util.Comparator<T>, we add the type before ().

This method takes a List<Planet> and Comparator<Planet>.

public void compareIt(List<Planet> planets, Comparator<Planet> p) {
  // ...
}

We pass the List and the anonymous Comparator.

compareIt(planets, new Comparator<Planet>() {

  @Override
  public int compare(Planet o1, Planet o2) {
    int result = 0;
    // ...
    return result;
  }
});

Drill

InnerClasses/com.example.innerclasses.drills.PrintSortedPlanetsAnonymous

• Change this class to pass an anonymous Comparator<Planet> instead of a member class instance.

(Solution: PrintSortedPlanetsAnonymous.java)

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