HashMap的三种遍历方式

import java.util.Map;

import java.util.Random;

import java.util.Iterator;

import java.util.HashMap;

import java.util.HashSet;

import java.util.Map.Entry;

import java.util.Collection;

/*

 * @desc 遍历HashMap的测试程序。

 *   (01) 通过entrySet()去遍历key、value，参考实现函数：

 *        iteratorHashMapByEntryset()

 *   (02) 通过keySet()去遍历key、value，参考实现函数：

 *        iteratorHashMapByKeyset()

 *   (03) 通过values()去遍历value，参考实现函数：

 *        iteratorHashMapJustValues()

 *

 * @author skywang

 */

public class HashMapIteratorTest {

    public static void main(String[] args) {

        int val = 0;

        String key = null;

        Integer value = null;

        Random r = new Random();

        HashMap map = new HashMap();

        for (int i=0; i<12; i++) {

            // 随机获取一个[0,100)之间的数字

            val = r.nextInt(100);

            key = String.valueOf(val);

            value = r.nextInt(5);

            // 添加到HashMap中

            map.put(key, value);

            System.out.println(" key:"+key+" value:"+value);

        }

        // 通过entrySet()遍历HashMap的key-value

        iteratorHashMapByEntryset(map) ;

        // 通过keySet()遍历HashMap的key-value

        iteratorHashMapByKeyset(map) ;

        // 单单遍历HashMap的value

        iteratorHashMapJustValues(map);       

    }

    /*

     * 通过entry set遍历HashMap

     * 效率高!

     */

    private static void iteratorHashMapByEntryset(HashMap map) {

        if (map == null)

            return ;

        System.out.println("\niterator HashMap By entryset");

        String key = null;

        Integer integ = null;

        Iterator iter = map.entrySet().iterator();

        while(iter.hasNext()) {

            Map.Entry entry = (Map.Entry)iter.next();

            key = (String)entry.getKey();

            integ = (Integer)entry.getValue();

            System.out.println(key+" -- "+integ.intValue());

        }

    }

    /*

     * 通过keyset来遍历HashMap

     * 效率低!

     */

    private static void iteratorHashMapByKeyset(HashMap map) {

        if (map == null)

            return ;

        System.out.println("\niterator HashMap By keyset");

        String key = null;

        Integer integ = null;

        Iterator iter = map.keySet().iterator();

        while (iter.hasNext()) {

            key = (String)iter.next();

            integ = (Integer)map.get(key);

            System.out.println(key+" -- "+integ.intValue());

        }

    }

    /*

     * 遍历HashMap的values

     */

    private static void iteratorHashMapJustValues(HashMap map) {

        if (map == null)

            return ;

        Collection c = map.values();

        Iterator iter= c.iterator();

        while (iter.hasNext()) {

            System.out.println(iter.next());

       }

    }

}