Inner Classes Example

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.java example that follows consists of:

• The 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.
• The 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.
• A main method that instantiates a DataStructure object (ds), then invokes the printEven method to print elements of the arrayarrayOfInts 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.

本文来自javase8的官网tutorial,这个小程序主要展示使用Inner Classes的好处。如果不使用Inner Classes，你需要定义一个接口类，然后再另外新建一个外部类，这样就会有三个类了，显然对于实现这样的一个小功能来说，这样做显得麻烦。(根据文档的个人理解，如有错误，欢迎指出)