遍历map的四方方法

 public static void main(String[] args) {

   Map<String, String> map = new HashMap<String, String>();
   map.put("1", "value1");
   map.put("2", "value2");
   map.put("3", "value3");

   //第一种：普遍使用，二次取值
   System.out.println("通过Map.keySet遍历key和value：");
   for (String key : map.keySet()) {
     System.out.println("key= "+ key + " and value= " + map.get(key));
   }

   //第二种
   System.out.println("通过Map.entrySet使用iterator遍历key和value：");
   Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
   while (it.hasNext()) {
     Map.Entry<String, String> entry = it.next();
     System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
   }

   //第三种：推荐，尤其是容量大时
   System.out.println("通过Map.entrySet遍历key和value");
   for (Map.Entry<String, String> entry : map.entrySet()) {
     System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
   }

   //第四种
   System.out.println("通过Map.values()遍历所有的value，但不能遍历key");
   for (String v : map.values()) {
     System.out.println("value= " + v);
   }
 }