Map用法

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()) {