Anonymous Inner Class Handlers

An anonymous inner class is an inner class without a name. It combines defining an inner class and creating an instance of the class into one step. Inner-class handlers can be shortened using anonymous inner classes. The inner class in ControlCircle.java here can be replaced by an anonymous inner class as shown below.

The syntax for an anonymous inner class is shown below

new SuperClassName/InterfaceName() {
// Implement or override methods in superclass or interface
// Other methods if necessary
}

Since an anonymous inner class is a special kind of inner class, it is treated like an inner class with the following features:

• An anonymous inner class must always extend a superclass or implement an interface, but it cannot have an explicit extends or implements clause.
• An anonymous inner class must implement all the abstract methods in the superclass or in the interface.
• An anonymous inner class always uses the no-arg constructor from its superclass to create an instance. If an anonymous inner class implements an interface, the constructor is Object().
• An anonymous inner class is compiled into a class named OuterClassName$n.class. For example, if the outer class Test has two anonymous inner classes, they are compiled into Test$1.class and Test$2.class.

The program below gives an example that handles the events from four buttons, as shown in Figure below.

package application;
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class AnonymousHandlerDemo extends Application {
    @Override // Override the start method in the Application class
    public void start(Stage primaryStage) {
        // Hold four buttons in an HBox 
        HBox hBox = new HBox();
        hBox.setSpacing(10);
        hBox.setAlignment(Pos.CENTER);
        Button btNew = new Button("New");
        Button btOpen = new Button("Open");
        Button btSave = new Button("Save");
        Button btPrint = new Button("Print");
        hBox.getChildren().addAll(btNew, btOpen, btSave, btPrint);

        // Create and register the handler
        btNew.setOnAction(new EventHandler<ActionEvent>() {
            @Override // Override the handle method
            public void handle(ActionEvent e) {
                System.out.println("Process New");
            }
        });

        btOpen.setOnAction(new EventHandler<ActionEvent>() {
            @Override // Override the handle method
            public void handle(ActionEvent e) {
                System.out.println("Process Open");
            }
        });

        btSave.setOnAction(new EventHandler<ActionEvent>() {
            @Override // Override the handle method
            public void handle(ActionEvent e) {
                System.out.println("Process Save");
            }
        });

        btPrint.setOnAction(new EventHandler<ActionEvent>() {
            @Override // Override the handle method
            public void handle(ActionEvent e) {
                System.out.println("Process Print");
            }
        });

        // Create a scene and place it in the stage
        Scene scene = new Scene(hBox, 300, 50);
        primaryStage.setTitle("AnonymousHandlerDemo"); // Set title
        primaryStage.setScene(scene); // Place the scene in the stage
        primaryStage.show(); // Display the stage
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}

The program creates four handlers using anonymous inner classes (lines 25–51). Without using anonymous inner classes, you would have to create four separate classes. An anonymous handler works the same way as that of an inner class handler. The program is condensed using an anonymous inner class.

The anonymous inner classes in this example are compiled into
AnonymousHandlerDemo$1.class, AnonymousHandlerDemo$2.class,
AnonymousHandlerDemo$3.class, and AnonymousHandlerDemo$4.class.