示例：

  public class Test {

    public static void main(String[] args) {

        Map<Integer, String> map = new <Integer, String>();

        map.put(10, "北京");

        map.put(20, "广州");

        map.put(23, "重庆");

        map.put(25, "南京");

        map.put(21, "上海");

        System.out.println(map.size());

        System.out.println("======================");

        for (Integer in : map.keySet()) {

            //map.keySet()返回的是所有key的值

            String str = map.get(in);//得到每个key多对用value的值

            System.out.println(in + "     " + str);

        }

        System.out.println("======================");

        Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();

        while (it.hasNext()) {

             Map.Entry<Integer, String> entry = it.next();

               System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());

        }

        System.out.println("================================");

        for (Map.Entry<Integer, String> entry : map.entrySet()) {

            System.out.println("key= " + entry.getKey() + " and value= "

                    + entry.getValue());

        }

    }

}