Inner Class in Java

Introduction to Inner Class in Java

An inner class in a Java program is nothing but the class that is declared and used in an already functioning class to use all the functions and members accessible to the outer class. This is typically used when the program’s coding pattern needs to be more organized while reducing the length of the code. There are three types of inner classes: member inner class, anonymous inner class, and local inner class. The major factors seen as advantages of using the inner class in a Java program snippet are that the inner class enables the code to be more optimized and organized, increases readability, and maintenance for this kind of classes is pretty low.

Types of Inner Class in Java

Inner class in java is basically those classes that are not static and are declared inside another class. So the inner class is a member of the outer class. There are 3 types of inner classes in Java:

• Member, inner class
• Local inner classes
• Anonymous inner classes

Kindly note that Inner Class is a type of nested class that is not static. Let us discuss different types of Inner classes in java one by one.

1. Member Inner Class

In this case, you just require to declare a class inside its outer class. But the class must not be inside the method of the outer class. It needs to be defined outside the method of the outer class. Inner class needs to be a non-static type. An object of an inner class type can access the variable of the outer class.

2. Local Inner Classes

In this case, it also you just require to declare a class inside its outer class. But the class must be inside the method of the outer class. It needs to be defined inside the method of the outer class. Inner class needs to be a non-static type. The object of the inner class type can access the variable of the outer class. As this class is declared inside a method of the outer class, its access is also restricted, similar to local variables. If you require to invoke the Inner class’s function, you have to instantiate that inside the function.

Points to be noted for local inner class:

• Specifier of Local variable can’t be private, public or protected.
• One cannot invoke the Local inner class outside the function.
• Till JDK version 1.7, Non-final local variables cannot be accessed by the local inner class. But, a non-final local variable in the local inner class can be accessed since JDK version 1.8.

3. Anonymous Inner Classes

Anonymous, as the name suggests, is a call without a name. But how it is possible? Actually, it is a type of inner class where we do both declaration and instantiation (means, object creation) simultaneously. Whenever you want to override a method of a class, you might need to use an anonymous inner class. As an anonymous inner class does not have a name, we cannot create an anonymous inner class constructor. Also, note that you can only access anonymous inner classes at the point of its definition.

Anonymous inner classes can be are created in two ways.

• As a subclass of a particular type.
• As an implementer of the particular interface.

Examples of Inner Class in Java

Below we will discuss some code examples of Java Inner Class:

Example #1: Member, Inner Class

In this example, we will demonstrate an example of a Member inner class. In this simple example where we declare a class inside an outer class. Inside “OuterClass”, we have a private data member called “num1” and an inner class “InnerClass”. We can access the private variable “num1” of outer calls inside the inner class by an object of the “InnerClass” type. In this way, we are taking advantage of using the inner class where a private member of outer calls can be accessed by a method of Inner class.

Code:

class OuterClass{
private int num1=36;
class InnerClass{
void shw(){
System.out.println("Member Inner Class");
}
}
}
public class DemoOfMemberInnerClass{
public static void main(String args[]){
OuterClass oc=new OuterClass();
OuterClass.InnerClass ic=oc.new InnerClass();
ic.shw();
}
}

Output:

Example #2: Anonymous Inner Class

In this example, we will demonstrate an example of the Local inner class. Here our primary objective is to override the functionality of a method of the outer class. As the name suggests, the class name is explicitly not known and hence called an anonymous inner class. Here, both declaration and instantiation of the inner class are done at some point in time, and hence it is called as Anonymous Inner Class.

Code:

abstract class Cat{
abstract void drink();
}
public class AnonymousInnerClassDemo{
public static void main(String args[]){
Cat ct=new Cat(){ //anonymous inner class
void drink(){
System.out.println("kitty loves milk");
}
};
ct.drink();
}
}

Output:

Example #3: Method Local Inner Class

In this example, we will demonstrate an example of a method local inner class. In this simple example where we declare a class inside the method “display()” of the outer class “OuterClassDemo”. After that, we can access the method by an object of the outer class.

Code:

public class OuterClassDemo{
private int num1=36;//instance variable
void display(){
class LocalInnerClass{ // local inner class inside the method of outer class
void shw(){
System.out.println(num1);
}
}
LocalInnerClass li=new LocalInnerClass();
li.shw();
}
public static void main(String args[]){
OuterClassDemo obj=new OuterClassDemo();
obj.display();
}
}

Output:

Advantages

Here are some of the advantages of the java inner class, which are explained below:

• The advantage of an inner class is that it can access all of the members that means functions, and variables of the outer class.
• This feature in Java facilitates us to logically organize our codes based on functionality, increasing code readability and maintenance. It also facilitates the optimization of your code.
• An inner class can access all of the private members (which means both data and methods) of its outer class.

Conclusion

This concludes our learning of the topic “Inner Class in Java”. An inner class is a very interesting feature in java where you can use a member of the outer class by the object of its inner class. Write yourself the codes mentioned in the above examples in the java compiler and verify the output. Learning of codes will be incomplete if you will not write code by yourself.

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