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
• Syntax of Anonymous Classes
• Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class
• Examples of Anonymous Classes

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:

• 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.)

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:

• 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.