Concept of Inner Class in Java

• Nested Classes
• The Types of Classes:
• Top level classes
• Nested classes
• Static classes
• Inner classes
• Anonymous classes
• Local classes
• Nested Interface
• Multi Nesting Access
• GUI Example without Inner Class
• GUI Example with Inner Class
• GUI Example with Anonymous Inner Class
• Using extends in inner class and nested static class

Nested classes are a feature of Java version 1.1 or later where nested classes are allowed to be defined inside enclosing classes.

Such a class is called a nested class and is can define as under here:

class OuterClass1 
{
    ...
    class NestedClass1 {
        ...
    }
}

Note : Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.

class OuterClass1 
{
    ...
    static class StaticNestedClass1 {
        ...
    }
    class InnerClass1 {
        ...
    }
}

• Top level classes: It is also called TLC and it can’t be private, protected and static.
• Nested classes: either static or inner.
• Static classes: no associated outer class instance.
• Inner classes: with an associated outer class instance.
• Anonymous classes: unnamed inner class defined in the middle of a method
• Local classes: named inner class defined in the middle of a method.

Note:- Its practical usefulness is about zero.

Rules for Anonymous class :

• Anonymous classes are defined where they are constructed.
• Anonymous classes cannot have explicit constructors.
• Anonymous classes can implement an interface or explicitly extend a class. Cannot do both.

  Syntax: new interface name() { } or new class name() { }
  

• Keywords “implements” and “extends” are not used in anonymous classes.
• Abstract classes can be specified in the creation of an anonymous class.
• The new class is a concrete class, which automatically extends the abstract class.