Trail: Learning the Java Language
Lesson: Classes and Objects
Section: Nested Classes
Anonymous Classes
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Anonymous Classes

Anonymous classes enable you to make your code more concise. They enable you to declare and instantiate a class at the same time. They are like local classes except that they do not have a name. Use them if you need to use a local class only once.

This section covers the following topics:

Declaring Anonymous Classes

While local classes are class declarations, anonymous classes are expressions, which means that you define the class in another expression. The following example, HelloWorldAnonymousClasses, uses anonymous classes in the initialization statements of the local variables frenchGreeting and spanishGreeting, but uses a local class for the initialization of the variable englishGreeting:


public class HelloWorldAnonymousClasses {
  
    interface HelloWorld {
        public void greet();
        public void greetSomeone(String someone);
    }
  
    public void sayHello() {
        
        class EnglishGreeting implements HelloWorld {
            String name = "world";
            public void greet() {
                greetSomeone("world");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Hello " + name);
            }
        }
      
        HelloWorld englishGreeting = new EnglishGreeting();
        
        HelloWorld frenchGreeting = new HelloWorld() {
            String name = "tout le monde";
            public void greet() {
                greetSomeone("tout le monde");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Salut " + name);
            }
        };
        
        HelloWorld spanishGreeting = new HelloWorld() {
            String name = "mundo";
            public void greet() {
                greetSomeone("mundo");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Hola, " + name);
            }
        };
        englishGreeting.greet();
        frenchGreeting.greetSomeone("Fred");
        spanishGreeting.greet();
    }

    public static void main(String... args) {
        HelloWorldAnonymousClasses myApp =
            new HelloWorldAnonymousClasses();
        myApp.sayHello();
    }            
}

Syntax of Anonymous Classes

As mentioned previously, an anonymous class is an expression. The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code.

Consider the instantiation of the frenchGreeting object:

        HelloWorld frenchGreeting = new HelloWorld() {
            String name = "tout le monde";
            public void greet() {
                greetSomeone("tout le monde");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Salut " + name);
            }
        };

The anonymous class expression consists of the following:

Because an anonymous class definition is an expression, it must be part of a statement. In this example, the anonymous class expression is part of the statement that instantiates the frenchGreeting object. (This explains why there is a semicolon after the closing brace.)

Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class

Like local classes, anonymous classes can capture variables; they have the same access to local variables of the enclosing scope:

Anonymous classes also have the same restrictions as local classes with respect to their members:

Note that you can declare the following in anonymous classes:

However, you cannot declare constructors in an anonymous class.

Examples of Anonymous Classes

Anonymous classes are often used in graphical user interface (GUI) applications.

Consider the JavaFX example HelloWorld.java (from the section Hello World, JavaFX Style from Getting Started with JavaFX). This sample creates a frame that contains a Say 'Hello World' button. The anonymous class expression is highlighted:

import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
 
public class HelloWorld extends Application {
    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Hello World!");
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnAction(new EventHandler<ActionEvent>() {
 
            @Override
            public void handle(ActionEvent event) {
                System.out.println("Hello World!");
            }
        });
        
        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }
}

In this example, the method invocation btn.setOnAction specifies what happens when you select the Say 'Hello World' button. This method requires an object of type EventHandler<ActionEvent>. The EventHandler<ActionEvent> interface contains only one method, handle. Instead of implementing this method with a new class, the example uses an anonymous class expression. Notice that this expression is the argument passed to the btn.setOnAction method.

Because the EventHandler<ActionEvent> interface contains only one method, you can use a lambda expression instead of an anonymous class expression. See the section Lambda Expressions for more information.

Anonymous classes are ideal for implementing an interface that contains two or more methods. The following JavaFX example is from the section Customization of UI Controls. The highlighted code creates a text field that only accepts numeric values. It redefines the default implementation of the TextField class with an anonymous class by overriding the replaceText and replaceSelection methods inherited from the TextInputControl class.

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Insets;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.GridPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class CustomTextFieldSample extends Application {
    
    final static Label label = new Label();
 
    @Override
    public void start(Stage stage) {
        Group root = new Group();
        Scene scene = new Scene(root, 300, 150);
        stage.setScene(scene);
        stage.setTitle("Text Field Sample");
 
        GridPane grid = new GridPane();
        grid.setPadding(new Insets(10, 10, 10, 10));
        grid.setVgap(5);
        grid.setHgap(5);
 
        scene.setRoot(grid);
        final Label dollar = new Label("$");
        GridPane.setConstraints(dollar, 0, 0);
        grid.getChildren().add(dollar);
        
        final TextField sum = new TextField() {
            @Override
            public void replaceText(int start, int end, String text) {
                if (!text.matches("[a-z, A-Z]")) {
                    super.replaceText(start, end, text);                     
                }
                label.setText("Enter a numeric value");
            }
 
            @Override
            public void replaceSelection(String text) {
                if (!text.matches("[a-z, A-Z]")) {
                    super.replaceSelection(text);
                }
            }
        };
 
        sum.setPromptText("Enter the total");
        sum.setPrefColumnCount(10);
        GridPane.setConstraints(sum, 1, 0);
        grid.getChildren().add(sum);
        
        Button submit = new Button("Submit");
        GridPane.setConstraints(submit, 2, 0);
        grid.getChildren().add(submit);
        
        submit.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent e) {
                label.setText(null);
            }
        });
        
        GridPane.setConstraints(label, 0, 1);
        GridPane.setColumnSpan(label, 3);
        grid.getChildren().add(label);
        
        scene.setRoot(grid);
        stage.show();
    }
 
    public static void main(String[] args) {
        launch(args);
    }
}

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