What is Inner Class in Java?

The concept of inner class in Java is fundamental to Java programming. In Java, a nested class and inner class are two very similar yet slightly distinct ideas of class designing and often go hand-in-hand.

A nested class is defined within another class and has two types – static nested class and non-static nested class. On the other hand, an inner class is a type of non-static nested class and is further of three types – member inner class, anonymous inner class, and local inner class.  

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So, what do these three types of inner classes mean, and what is their use? Read on to find out in this detailed guide on Java inner class.

Meaning of Inner Class in Java

A non-static nested class or inner class is defined within another class and has access to members of the outer class (enclosing class). On the contrary, a static nested class is also a class defined within another class, but unlike a non-static nested class, it cannot access the member variables of its outer class. In the case of a non-static nested class, we are not required to create an instance of the outer class. But in the case of an inner class, we must first instantiate the outer class to be able to instantiate the inner class.

Source

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Types of Inner Class in Java 

In Java, inner classes are of the following three types:

• Member inner class
• Anonymous inner class
• Local inner class

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Now, let us understand each type of inner class with examples. 

1. Member Inner Class

A member inner class in Java is a non-static nested class created inside a class but outside a method. Also referred to as a regular inner class, a member inner class can be declared with access modifiers like private, public, default, and protected.

The syntax for a member inner class is as follows:

class Outer{  

 //code  

 class Inner{  

  //code  

 }  

}  

Member inner class in Java example:

Given below is a program to demonstrate how to create an inner class and access it. We will make the inner class private and use the method display_Inner()to access the class.

class DemoOuter {

   int num;

 // inner class

   private class DemoInner {

      public void print() {

         System.out.println(“This is the inner class”);

      }

   }

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   // Accessing the inner class from the method within

   void display_Inner() {

      DemoInner inner = new DemoInner();

      inner.print();

   }

}

   public class MyClass {

   public static void main(String args[]) {

      // Instantiating the outer class 

      DemoOuter outer = new DemoOuter();

      

      // Accessing the display_Inner() method

      outer.display_Inner();

   }

}

Output: This is the inner class

In the above example, OuterDemo is the outer class, InnerDemo is the inner class, we are instantiating the inner class inside the method display_Inner(), which is invoked from the main method.

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2. Anonymous Inner Class

An anonymous inner class in Java is an inner class declared without any class name and for which only a single object is created. Anonymous inner classes in Java are declared and instantiated at the same time. They are most valuable when we want to override the method of a class or interface. 

The syntax for an anonymous inner class in Java is as follows:

AnonymousInner an_inner = new AnonymousInner() {

   public void my_method() {

      

   //code

   }   

};

 An anonymous inner class in Java can be created in two ways:

1. Using class
2. Using interface

Anonymous inner class in Java example (using class):

abstract class Meal{  

  abstract void eat();  

}  

class TestAnonymousInner{  

 public static void main(String args[]){  

  Meal m=new Meal(){  

  void eat(){System.out.println(“Bon appetit!”);}  

  };  

  m.eat();  

 }  

}  

Output: Bon appetit!

Anonymous inner class in Java example (using interface):

interface Game{  

 void play();  

}  

class TestAnnonymousInner1{  

 public static void main(String args[]){  

 Game g=new Game(){  

  public void play(){System.out.println(“Outdoor games”);}  

 };  

 g.play();  

 }  

}  

Output: Outdoor games

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3. Local Inner Class

A local inner class in Java is a class created inside a method. As in the case of local variables, the scope of the local inner class remains restricted within the method. 

Local inner classes are defined inside a block: a method body, an if clause, or a for loop. A local inner class is not a member of any enclosing class, but instead, it belongs to the block within which it is defined. Thus, even though a local inner class in Java cannot have any access modifiers associated with it, you can mark it abstract or final. If we want to invoke the methods of a local inner class, we have to instantiate the class inside the method.

Local inner class in Java example:

Below is an example program to demonstrate how to instantiate the object of a local inner class declared inside the method of an outer class.

package innerClass; 

public class Outer_Class 

{ 

// An instance method of class Outer_Class. 

   public void display()

  { 

// Declaration of a method local inner class. 

    class Inner_Class 

    { 

       public void msg(){ 

        System.out.println(“This is a local inner class!”); 

       } 

     } 

// Create an instance of a method local inner class and call the msg() method using object reference variable i. 

     Inner_Class i = new Inner_Class(); 

       i.msg(); 

  } 

public static void main(String[] args) 

{ 

// Create an object of the outer class Outer_Class. 

     Outer_Class o = new Outer_Class(); 

       o.display(); 

   } 

}

Output: This is a local inner class!

With the fundamentals of Java inner class in mind, let us understand why we need them in the first place.

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Advantages of Java Inner Class

An inner class in Java brings three significant utilities to the table. These are as follows:

1. Since a Java inner class can access the data members and methods of the outer/main class, including private, it represents a specific type of relationship.

2. A Java inner class significantly reduces coding since it requires less to write.

3. An inner class in Java logically groups classes and interfaces in a single place, thereby enhancing the readability of the code and making it easier to maintain.

Summing Up

The concept of nested classes simplifies and adds flexibility to creating classes anywhere in Java programming. In this regard, a non-static nested class known as inner class is particularly useful, as we have already seen in our discussion so far. 

We would like to wrap up by stating two instances where the use of inner class seems most appropriate. First, a Java inner class is ideal when two classes have a close relationship and are not accessible by any other class. And second, inner classes are commonplace in Java GUI programming event handling. Therefore, Java inner classes can be safely used where listeners and their GUI components are involved.

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