Inner Class Example

To see an inner class which is in use, let's first consider an array. In the following example, it will create an array, fill it with interger values and then output only values of even indices of the array in accending order.

The DataStructure class consists of:

• The DataStructure outer class, which includes methods to add an integer onto the array and print out values of even indices of the array.
• The InnerEvenIterator inner class, which is similar to a standard Java iterator. Iterators are used to step through a data structure and typically have methods to test for the last element, retrieve the current element, and move to the next element.
• A main method that instantiates a DataStructure object (ds) and uses it to fill the arrayOfInts array with integer values (0, 1, 2, 3, etc.), then calls a printEven method to print out values of even indices of arrayOfInts.

   public class DataStructure {
   // create an array
   private final static int SIZE = 15;
   private int[] arrayOfInts = new int[SIZE];
   
   public DataStructure() {
       // fill the array with ascending integer values
       for (int i = 0; i < SIZE; i++) {
           arrayOfInts[i] = i;
       }
   }
   
   public void printEven() {
       // print out values of even indices of the array
       InnerEvenIterator iterator = this.new InnerEvenIterator();
       while (iterator.hasNext()) {
       System.out.println(iterator.getNext() + " ");
       }
   }
       // inner class implements the Iterator pattern
   private class InnerEvenIterator {
       // start stepping through the array from the beginning
       private int next = 0;
       public boolean hasNext() {
           // check if a current element is the last in the array
           return (next <= SIZE - 1);
       }
       public int getNext() {
           // record a value of an even index of the array
           int retValue = arrayOfInts[next];
           //get the next even element
           next += 2;
           return retValue;
       }
   }