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<br /><b>Section:</b> Nested Classes
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<div id="PageTitle"><h1>Anonymous Classes</h1></div>
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<p>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.</p>
<p>This section covers the following topics:</p>
<ul>
<li><a href="#declaring-anonymous-classes">Declaring Anonymous Classes</a></li>
<li><a href="#syntax-of-anonymous-classes">Syntax of Anonymous Classes</a></li>
<li><a href="#accessing">Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class</a></li>
<li><a href="#examples-of-anonymous-classes">Examples of Anonymous Classes</a></li>
</ul>
<h2><a name="declaring-anonymous-classes">Declaring Anonymous Classes</a></h2>
<p>While local classes are class declarations, anonymous classes
are expressions, which means that you define the class in another
expression. The following example,
<a class="SourceLink" target="_blank" href="examples/HelloWorldAnonymousClasses.java" onclick="showCode('../../displayCode.html', 'examples/HelloWorldAnonymousClasses.java'); return false;"><code>HelloWorldAnonymousClasses</code></a>, uses anonymous classes in the initialization statements of the
local variables <code>frenchGreeting</code> and <code>spanishGreeting</code>, but uses a local class for the
initialization of the variable <code>englishGreeting</code><code>:</code></p>
<pre class="codeblock">
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();
}
}
</pre>
<h2><a name="syntax-of-anonymous-classes">Syntax of Anonymous Classes</a></h2>
<p>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.</p>
<p>Consider the instantiation of the <code>frenchGreeting</code> object:</p>
<pre> 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);
}
};</pre>
<p>The anonymous class expression consists of the following:</p>
<ul>
<li><p>The <code>new</code> operator</p></li>
<li><p>The name of an interface to
implement or a class to extend. In this example, the anonymous
class is implementing the interface <code>HelloWorld</code>.</p></li>
<li><p>Parentheses that contain the
arguments to a constructor, just like a normal class instance
creation expression. <strong>Note</strong>: When you implement an
interface, there is no constructor, so you use an empty pair of
parentheses, as in this example.</p></li>
<li><p>A body, which is a class
declaration body. More specifically, in the body, method
declarations are allowed but statements are not.</p></li>
</ul>
<p>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 <code>frenchGreeting</code> object. (This
explains why there is a semicolon after the closing brace.)</p>
<h2><a name="accessing">Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class</a></h2>
<p>Like local classes, anonymous classes can
<a class="TutorialLink" target="_top" href="localclasses.html#accessing-members-of-an-enclosing-class">capture variables</a>; they have the same access to local variables of
the enclosing scope:</p>
<ul>
<li><p>An anonymous class has access to the members of its enclosing
class.</p></li>
<li><p>An anonymous class cannot access local variables in its
enclosing scope that are not declared as <code>final</code> or effectively final.</p></li>
<li><p>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
<a class="TutorialLink" target="_top" href="../../java/javaOO/nested.html#shadowing">Shadowing</a> for more information.</p></li>
</ul>
<p>Anonymous classes also have the same restrictions as local
classes with respect to their members:</p>
<ul>
<li><p>You cannot declare static
initializers or member interfaces in an anonymous
class.</p></li>
<li><p>An anonymous class can
have static members provided that they are constant
variables.</p></li>
</ul>
<p>Note that you can declare the following in anonymous classes:</p>
<ul>
<li><p>Fields</p></li>
<li><p>Extra methods (even if they do not implement any methods of the supertype)</p></li>
<li><p>Instance initializers</p></li>
<li><p>Local classes</p></li>
</ul>
<p>However, you cannot declare constructors in an anonymous class.</p>
<h2><a name="examples-of-anonymous-classes">Examples of Anonymous Classes</a></h2>
<p>Anonymous classes are often used in graphical user
interface (GUI) applications.</p>
<p>Consider the JavaFX example <a href=
"http://docs.oracle.com/javafx/2/get_started/hello_world.htm">
<code>HelloWorld.java</code></a> (from the section <a href=
"http://docs.oracle.com/javafx/2/get_started/hello_world.htm">
Hello World, JavaFX Style</a> from <a href=
"http://docs.oracle.com/javafx/2/get_started/jfxpub-get_started.htm">
Getting Started with
JavaFX</a>). This
sample creates a frame that contains a <strong>Say 'Hello World'</strong>
button. The anonymous class
expression is highlighted:</p>
<pre>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(<b>new EventHandler<ActionEvent>() {</b>
<b>@Override</b>
<b>public void handle(ActionEvent event) {</b>
<b>System.out.println("Hello World!");</b>
<b>}</b>
<b>}</b>);
StackPane root = new StackPane();
root.getChildren().add(btn);
primaryStage.setScene(new Scene(root, 300, 250));
primaryStage.show();
}
}</pre>
<p>In this example, the method invocation <code>btn.setOnAction</code> specifies what happens when you select the <b>Say 'Hello World'</b> button. This method requires an object of type <code>EventHandler<ActionEvent></code>. The <code>EventHandler<ActionEvent></code> 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 <code>btn.setOnAction</code> method.</p>
<p>Because the
<code>EventHandler<ActionEvent></code>
interface contains only one method, you can use a lambda
expression instead of an anonymous class expression. See the
section
<a class="TutorialLink" target="_top" href="lambdaexpressions.html">Lambda Expressions</a> for more
information.</p>
<p>Anonymous classes are ideal for implementing an interface that contains two or more methods. The following JavaFX example is from the section <a href="http://docs.oracle.com/javafx/2/ui_controls/custom.htm"> Customization of UI Controls</a>. The highlighted code creates a text field that only accepts numeric values. It redefines the default implementation of the <code>TextField</code> class with an anonymous class by overriding the <code>replaceText</code> and <code>replaceSelection</code> methods inherited from the <code>TextInputControl</code> class.</p>
<pre class="codeblock">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 = <b>new TextField() {</b>
<b>@Override</b>
<b>public void replaceText(int start, int end, String text) {</b>
<b>if (!text.matches("[a-z, A-Z]")) {</b>
<b>super.replaceText(start, end, text); </b>
<b>}</b>
<b>label.setText("Enter a numeric value");</b>
<b>}</b>
<b>@Override</b>
<b>public void replaceSelection(String text) {</b>
<b>if (!text.matches("[a-z, A-Z]")) {</b>
<b>super.replaceSelection(text);</b>
<b>}</b>
<b>}</b>
<b>};</b>
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);
}
}</pre>
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