Trail: Learning the Java Language

Lesson: Classes and Objects

Section: Nested Classes

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don’t take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases. See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

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:

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();
    }            
}

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:

• The new operator

• The name of an interface to
  implement or a class to extend. In this example, the anonymous
  class is implementing the interface HelloWorld.

• Parentheses that contain the
  arguments to a constructor, just like a normal class instance
  creation expression. Note: When you implement an
  interface, there is no constructor, so you use an empty pair of
  parentheses, as in this example.

• A body, which is a class
  declaration body. More specifically, in the body, method
  declarations are allowed but statements are not.

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.)

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

• An anonymous class has access to the members of its enclosing
  class.

• An anonymous class cannot access local variables in its
  enclosing scope that are not declared as final or effectively final.

• Like a nested class, a declaration of a type (such as a variable) in an anonymous class shadows any other declarations in the enclosing scope that have the same name. See
  Shadowing for more information.

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

• You cannot declare static
  initializers or member interfaces in an anonymous
  class.

• An anonymous class can
  have static members provided that they are constant
  variables.

Note that you can declare the following in anonymous classes:

• Fields

• Extra methods (even if they do not implement any methods of the supertype)

• Instance initializers

• Local classes

However, you cannot declare constructors in an anonymous class.

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);
    }
}