Why inner classes in java

Static nested classes do not have access to other members of the enclosing class. As a member of the OuterClass , a nested class can be declared private , public , protected , or package private. Recall that outer classes can only be declared public or package private. It is a way of logically grouping classes that are only used in one place : If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together.

Nesting such "helper classes" makes their package more streamlined. It increases encapsulation : Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared private. By hiding class B within class A, A's members can be declared private and B can access them.

In addition, B itself can be hidden from the outside world. It can lead to more readable and maintainable code : Nesting small classes within top-level classes places the code closer to where it is used. As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. 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:. An inner class is a part of a nested class. Non-static nested classes are known as inner classes. There are two types of nested classes non-static and static nested classes. The non-static nested classes are also known as inner classes. JavaTpoint offers too many high quality services.

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The following program shows how to use a method-local inner class. An inner class declared without a class name is known as an anonymous inner class.

In case of anonymous inner classes, we declare and instantiate them at the same time. Generally, they are used whenever you need to override the method of a class or an interface.

In the same way, you can override the methods of the concrete class as well as the interface using an anonymous inner class. Generally, if a method accepts an object of an interface, an abstract class, or a concrete class, then we can implement the interface, extend the abstract class, and pass the object to the method. If it is a class, then we can directly pass it to the method. But in all the three cases, you can pass an anonymous inner class to the method.

A static inner class is a nested class which is a static member of the outer class. It can be accessed without instantiating the outer class, using other static members. Use it if you need a simple instance of a functional interface and none of the preceding criteria apply for example, you do not need a constructor, a named type, fields, or additional methods. Nested class : Use it if your requirements are similar to those of a local class, you want to make the type more widely available, and you don't require access to local variables or method parameters.

Use a non-static nested class or inner class if you require access to an enclosing instance's non-public fields and methods. Use a static nested class if you don't require this access.