Java -- Map

HashMap

• 继承关系

• 遍历

  // initialize a HashMap 
  HashMap<String, Integer> map = new HashMap<>(); 
  // Iterate the map using 
  // for-each loop 
  for (Map.Entry<String, Integer> e : map.entrySet()) 
      System.out.println("Key: " + e.getKey() 
                         + " Value: " + e.getValue()); 
  

• 如何随机获取一个键值对？

  Map<String,Integer> mapTest = new HashMap<>();
  mapTest.put("one",1);
  mapTest.put("two",2);
  mapTest.put("three",3);
  Random r = new Random();
  int randNum = r.nextInt(3);
  Set keys = mapTest.keySet();
  System.out.println(keys.toArray()[randNum]);
  

• 有时需要使用有序的HashMap 即 LinkedHashMap

  • 使用LinkedHashMap更改一个键的值不会更改顺序，键的顺序在初始映射时已被固定。

    "key1" -> "value1"
    "key2" -> "value2"
    "key3" -> "value3"
    "key4" -> "value4"
    "key5" -> "value5"
       change to
    "key1" -> "value1"
    "key2" -> "value8"
    "key3" -> "value3"
    "key4" -> "value6"
    "key5" -> "value5"
        //方法
    map.put("key2","value8");
    map.put("key4","value6");
    

• 注意需要存储相同键不同值时不要使用map