Why we need inner classes in java

Inner classes are defined inside a class.

Why you need inner classes?

• Consider you have class B which uses the members of class A and class B is only used by class A (no other classes use Class b), then you can embed class B inside class A. You can create class B as an inner class of class A.
• By writing Class B as an inner class of class A, class B can access the class A’s private members also. Incase If you are not using the inner class, then the class A members cannot be declared as private, as it needs to be accessed by other class B. So, it increases encapsulation.
• By grouping the classes, code maintenance becomes easy.

Types of inner classes:

Non-static Inner class Example:

package explore.innerclass;
import java.util.Iterator;
public class InnerClassDemo {
 private int[] arrayOfInts = {1,2,3,4,5,6,7,8,9};
 //Creating array with values.
public void printEven()
//this method get the array values ,check if it is even and print the even numbers.
{
private class EvenIterator implements Iterator<Object>
    {
        private int nextIndex = 0;
     @Override
     //over riding the  Iterator interface method to check if the next the value is available
        public boolean hasNext() {
        return (nextIndex <= arrayOfInts.length - 1);
                }
            @Override
        public Integer next() {
        //Get the array value by the index
      Integer arrayValue = Integer.valueOf(arrayOfInts[nextIndex]);
        nextIndex =nextIndex+1;	 //increasing the index value by one.
    return arrayValue;
         }
    }
 public static void main(String args[]) {
      InnerClassDemo ds =new InnerClassDemo();
    ds.printEven();
     }

• The EvenIterator class is only used by the InnerClassDemo class. So EvenIterator class is defined as the inner class for the InnerClassDemo.
• If you don’t want to use inner class, then you have to implement an Iterator interface with InnerClassDemo and also you have to implement the methods of the Iterator interface. So more number of lines will be added with your own logic and its difficult to maintain.
• Also if you didn’t create EvenIterator as an inner class, you cannot access the private member of the InnerClassDemo.

Points to be Remembered :

• Inner classes are broadly classified as a static inner class and Non-static inner class.
• Non-static inner class intern classified as a local class and anonymous class. A non-static inner class can access all the members of the outer class. But the static inner class can access only the static members of the class.