Friday, July 30, 2010

inner Classes in java

To see an inner class in use, let's first consider an array. In the following example, we will 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 below 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 = 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();
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.

Inner classes can be used to implement helper classes like the one shown in the example above. If you plan on handling user-interface events, you will need to know how to use inner classes because the event-handling mechanism makes extensive use of them.

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