Inner Class Example

To see an inner class in use, first consider an array. In the following example, you create an array, fill it with integer values, and then output only values of even indices of the array in ascending order.

The DataStructure class that follows 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, and so on), 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;
        }
    }
    
    public static void main(String s[]) {
        // fill the array with integer values and print out only
        // values of even indices
        DataStructure ds = new DataStructure();
        ds.printEven();
    }
}

The output is:

0 2 4 6 8 10 12 14 

Note that the InnerEvenIterator class refers directly to the arrayOfInts instance variable of the DataStructure object.

You can use inner classes to implement helper classes such as the one shown in the preceeding example. To handle user-interface events, you must know how to use inner classes, because the event-handling mechanism makes extensive use of them.

Local and Anonymous Classes

There are two additional types of inner classes. You can declare an inner class within the body of a method. These classes are known as local classes. You can also declare an inner class within the body of a method without naming the class. These classes are known as anonymous classes.

Modifiers

You can use the same modifiers for inner classes that you use for other members of the outer class. For example, you can use the access specifiers private, public, and protected to restrict access to inner classes, just as you use them to restrict access do to other class members.