The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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.java
example that follows consists of:
DataStructure
outer class, which includes a constructor to create an instance of DataStructure
containing an array filled with consecutive integer values (0, 1, 2, 3, and so on) and a method that prints elements of the array that have an even index value.EvenIterator
inner class, which implements the DataStructureIterator
interface, which extends the
Iterator
<
Integer
>
interface. 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.main
method that instantiates a DataStructure
object (ds
), then invokes the printEven
method to print elements of the array arrayOfInts
that have an even index value.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 DataStructureIterator iterator = this.new EvenIterator(); while (iterator.hasNext()) { System.out.print(iterator.next() + " "); } System.out.println(); } interface DataStructureIterator extends java.util.Iterator<Integer> { } // Inner class implements the DataStructureIterator interface, // which extends the Iterator<Integer> interface private class EvenIterator implements DataStructureIterator { // Start stepping through the array from the beginning private int nextIndex = 0; public boolean hasNext() { // Check if the current element is the last in the array return (nextIndex <= SIZE - 1); } public Integer next() { // Record a value of an even index of the array Integer retValue = Integer.valueOf(arrayOfInts[nextIndex]); // Get the next even element nextIndex += 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 EvenIterator
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 this example. To handle user interface events, you must know how to use inner classes, because the event-handling mechanism makes extensive use of them.
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.
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.