Nested Class in Java

Properties of Nested classes in Java

• In Java, a nested class is a class that is defined inside some other class.
• Nested classes are used to group certain classes to improve the readability of the code.
• The scope of a nested class is the same as its outer class.
• Nested classes can access any member of the outer class, even if the outer class is private.

Types of Nested classes in Java

A nested class can either be defined as a static type or a non-static type. Types in detail:-

• Static Nested Classes
• Non-static Nested Classes (or Inner Classes) We can further divide the Non-static Classes into -
  • Member Inner Classes
  • Local Inner Classes
  • Anonymous Inner Classes

Static Nested Classes

A static class is a class that is created inside a class. A static variable is common to all the instances of that particular class. Similarly, a static class can access all the instance functions of the outer class.

To do this, we will have to make objects of child classes and refer to them. We can access all the static functions of the outer class without object creation.

Syntax:

We can create an object of this class like this:

Let us see an example of using the static nested class and accessing its method from other functions.

Output:

Explanation: Here, we have created a nested class. The parent_class is the outer class, and the static_child_class is the inner class. The main() function of the parent_class access the inner class methods with the object.

Non-static Nested Classes (Inner Classes)

A non-static nested class or inner class is a class within a class. We do not define it as static so that it can directly use all the functions and variables of the outer class.

From the inner class, if we want to access any static method of the outer class, we do not need any object; we can call it directly.

1. Member Inner Classes

Syntax:

To access the non-static inner class from the static main method or any other static method, we have to create objects of the parent_class as well as the child_class. The syntax for these objects:

Let us see an example of using the non-Static nested class and accessing its method from other functions:-

Output:

Explanation: Here, we are creating an object of the parent class and, with the help of it, creating the object of the child class. We are accessing the methods of the child_class using the childObj. We had to do this because the child_class is non-static, while the main method is static.

2. Local Inner Classes

A Local Inner class is a class that is defined inside any block, i.e., for block, if block, methods, etc. Similar to local variables, the scope of the Local Inner Class is restricted to the block where it is defined.

Syntax:

Let us see an example of using the Local inner class and accessing its method:-

Output:

Explanation: Here, we are defining a class inside an if block. And we are creating an object of child_class and calling it from the same block. The child_class is only accessible from the if block and can not be called from outside.

3. Anonymous Inner Classes

Anonymous Inner class is an inner class but without a name. It has only a single object. It is used to override a method. It is only accessible in the block where it is defined.

We can use an abstract class to define the anonymous inner class. It has access to all the members of the parent class. It is useful to shorten over code. Basically, it merges the step of creating an object and defining the class.

Syntax:

Let us see an example of Anonymous inner class and accessing its methods:-

Output:

Explanation: The Printer object is used to define the anonymous inner class. Then we are calling the print() method of the anonymous inner class using the object we just defined.