Inner classes | Local Inner Classes | Anonymous Inner Classes | When to Use an Anonymous Class |When to Use an Inner Class

Inner Classes

• Inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields.
• It’s allowed only final variable.
• Also, because an inner class is associated with an instance, it cannot define any static members itself.
• Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {

...

class InnerClass {

...

}

}

• To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass.InnerClass innerObject = outerObject.new InnerClass()

Local Inner Classes

• We can declare an inner class within the body of a method. Such a class is known as a local inner class.
• A local class is a nested class that is not a member of any class and that has a name.
• Every local class declaration statement is contained by a block.
• Local class defined in a method has access to final method variables and also to the outer class's member variables.

Anonymous Inner Classes

• We can declare an inner class within the body of a method without naming its called anonymous inner classes.
• An anonymous class is an inner class with the usual
• class body, but
• No class name
• No access specifier (i.e., no public/private/protected)
• No constructor
• No explicit use of extends or implements
• It either extends one class or implements one interface
• An anonymous class is never abstract. An anonymous class is always an inner class; it is never static. An anonymous class is always implicitly final.
• Are commonly used in Swing code to implement listeners or adapters.

Syntax for Anonymous Classes

new class-name ( [ argument-list ] ) { class-body }

or:

new interface-name () { class-body }

When to Use an Anonymous Class

• The class has a very short body.
• Only one instance of the class is needed.
• The class is used right after it is defined.
• The name of the class does not make your code any easier to understand.