Learn Anonymous Classes in JAVA

Learn Anonymous Classes in JAVA

Java Anonymous Inner Class

• Anonymous inner classes of Java are called anonymous because they have no name.
• They are anonymous and inline. 
• Anonymous classes are essentially inner classes and defined within some other classes. 
• However, the way anonymous classes are written in Java code may look weird but anonymous inner classes facilitate programmers to declare and instantiate the class at the same time. 
• Another important point to note about anonymous inner classes is that they can be used only once on the place they are coded. 
• In other words, if you want to create only one sub-classed object of a class, then you need not to give the class a name and you can use anonymous inner class in such a case. 

Anonymous Classes

• Anonymous inner classes can be defined not just within a method, but even within an argument to a method.
• Anonymous inner classes cannot have explicit constructors declared because they have no name to give the constructor.

How to Declare Java Anonymous Inner Classes

1. Anonymous inner classes are defined at the same time they are instantiated with new.
2. They are not declared as local classes are done rather anonymous inner classes are defined in the new expression itself, as part of a statement.

How to Declare Java Anonymous Inner Classes

• An anonymous inner class declaration expression looks like a constructor invocation, except that there is a class definition contained in a block of code.
• Before going into further details of anonymous inner classes we must understand that an anonymous inner class is not an independent inner class rather it is a sub-class of either a class type or an anonymous implementer of the specified interface type.
• So, when anonymous inner classes are in picture polymorphism must be there. 
• And when polymorphism is there you can only call methods from parent class reference those are defined in the reference variable type. 
• Java's anonymous inner classes being sub-classes or implementer do strictly adhere to the polymorphism rules.

Java Anonymous Inner Class of a Class Type

/* AnonymousClassDemo.java */  

 public class AnonymousClassDemo {

 public static void main(String[] args) {

 Dog dog = new Dog() { public void someDog () { System.out.println("Anonymous Dog"); } }; // anonymous class body closes here //dog contains an object of anonymous subclass of Dog. dog.someDog(); } }   class Dog { public void someDog() { System.out.println("Classic Dog"); } }

• Remember, anonymous inner classes are inherited ones, and we always use a superclass reference variable to refer to an anonymous subclass object.
• And, we can only call methods on an anonymous inner class object that are defined in the superclass.
• Though, we can introduce new methods in anonymous inner class, but we cannot access them through the reference variable of super-class because super-class does not know anything about new methods or data members introduced in subclass.

Java Anonymous Implementer of an Interface Type

• an anonymous inner class can implement an interface anonymously. 

/* AnonymousInterfaceDemo.java */ 

interface Manageable 

{ public void manage(); }   

public class AnonymousInterfaceDemo

 { public static void main(String[] args) { 

Manageable m = new Manageable() { 

public void manage() { 

System.out.println("It is manageable"); } }; // anonymous interface implementer closes here //m contains an object of anonymous interface implementer of Manageable. m.manage(); } }

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