ANONYMOUS CLASSES

Anonymous classes can be declared to extend another class or to implement a single interface. If you declare a class that implements a single explicit interface, then it is a direct subclass of java.lang.Object.

Anonymous classes give you a convenient way to avoid having to think up trivial names for classes. They should be small and easy to understand as they do not contain descriptive names.

You cannot define any specific constructor for an anonymous inner class. This is a direct consequence of the fact that you do not specify a name for the class, and therefore you cannot use that name to specify a constructor. Anonymous Class Declarations

new Identifier() { /* class body */ }

Identifier is a class or interface name. The expression by itself isn't of much use, it returns a reference to an object that you usually assign to a reference variable of the same type of the identifier:

Identifier = new Identifier() { /* class body */ };

Of course you can also pass the resulting reference to a method:

Method(new Identifier() { /* class body *} );

Example:

class Test {

public static void main(String args[]) {

Base o = new Base() {

public void Hi() {

System.out.println("Hi!");

}

public void Bye() {

System.out.println("Bye");

}

};

o.Hi();

}

}

interface Base {

public void Hi();

}

Result:

Hi!

Anonymous classes should provide methods defined by the base class or the interface but not brand new methods although they can be declared without compilation errors.

Passing arguments

You can define a constructor inside a inner class, but if you provide arguments, the matching constructor of base class will be invoked:

class Test {

public static void main(String args[]) {

Base o = new Base(15) {

public void Hi() {

System.out.println("Hi!");

}

public void Bye() {

System.out.println("Bye");

}

};

o.Hi();

// o.Bye(); // ERROR.

}

}

class Base {

Base (int i) {

System.out.println("I've got "+i);

}

public void Hi() { }

}

Result:

I've got 15

Hi!

Initialising Anonymous classes

You can't define constructors for an anonymous class, but you can declare an initialisation block:

class Test {

public static void main(String args[]) {

Object o = new Object() {

{

System.out.println("Init");

}

};

}

}

Result:

Init

This feature is available to all classes, not only anonymous ones.