Container-Map interface, HashMap (15)

Container-Map interface, HashMap (15)

1. Map interface: Defines the storage characteristics of the double-case set pair , it is not a sub-interface of the Collection interface. The storage characteristics of the double-case set are stored in key and value structures , which embodies the concept of y=f(x) in mathematics.

2. The difference between Map and Collection:

   • In the container in the Collection, the elements exist in isolation (understood as a singleton), and the elements stored in the collection are stored in the form of one element.
   • The container in the Map, the elements exist in pairs (understood as a couple in modern society), each element is composed of two parts, a key and a value, and the corresponding value can be found through the key.
   • The container in the Collection is called a singleton set, and the container in the Map is called a doubleton set
   • The collection in the Map cannot contain duplicate keys, and the values ​​can be duplicated : each key can only correspond to one value.
   • The commonly used containers in Map are HashMap, TreeMap, etc.

3. Some common methods of HashMap :
   

4. The concrete realization of some common methods of HashMap

   • Add element

     import java.util.HashMap;
     import java.util.Map;
     
     public class HashMapTest {
             
             
         public static void main(String[] args) {
             
             
             //实例化HashMap容器
             Map<String,String> map=new HashMap<>();
             //添加容器
             map.put("a","A");//用put（K，V）添加元素，并且返回V的值，V为空时，返回的是null,
             String value=map.put("a","B");//K（键）相同时，新的V值会覆盖旧的V，返回的是被覆盖的旧的V
             System.out.println(value);
     
     
         }
     }
     

   • The first way to get the element is to get the element through the get method .

      //通过K获取value的方法
             String val = map.get("a");//缺点：1.当HashMap有很多元素时，我们需要调用多次get方法
             System.out.println(val);//       2.假如我们不知道有哪些K，通过get方法就无法获取
                                     //优点：1.通过已知的Key来获取Value，get方法是很方便的
     

   • The second method of obtaining elements, obtaining elements through the KeySet method , can solve the two shortcomings of the get method.

         map.put("b","B");
         map.put("c","D");
         map.put("d","D");
         map.put("e","E");
         //获取HashMap容器中有的元素，可以使用KeySet方法与get方法一并完成
         Set<String> keys=map.keySet();//KeySet方法获取所有的Key,放在一个Set集合里
         for (String key:keys){
             
             //遍历所有的Key
             String v1=map.get(key);//通过get方法是通过每个Key，去获取Value
             System.out.println(key+"----"+v1);//在Map接口中，Key一定要不同，Value可以相同
         }
     
     

   • The third way to get elements is to get the Map.Entry type to get the element through the entrySet method .

     //这个entrySet方法可以返回一个Set集合，Set所存放的类型是Map.Entry类型
     //Map.Entry类型是：Entry接口继承Map，所以Entry是内部接口，Entry内部接口提供了两个方法，分别是getKey()、getValue() 获取K和V的值。Map外部接口调用Entry是内部接口的方法就要用Map.Entry
     //Map本身就是K-V的结构，而Entry类型就是一个K-V，entrySet方法返回的Set集合里就是存取多个K-V（键值对）
             Set<Map.Entry<String,String>> entrySet=map.entrySet();
             for (Map.Entry<String,String> entry:entrySet){
             
             
                 String key=entry.getKey();
                 String v=entry.getValue();
                 System.out.println(key+"------------------"+v);
             }
     

   • Union operation

       //并集操作
             System.out.println("------------------------------");
             Map<String,String> map2=new HashMap<>();
             map2.put("f","F");
             map2.put("c","cc");//当Key相同，value不同时，被并集的集合中的V，会覆盖并集的集合中的V
             map2.putAll(map);//map2去并集map
             //通过第二种keySet()方法获取元素
             Set<String> keys2=map2.keySet();
             for (String key:keys2){
             
             
                 System.out.println("Key: "+key+" Value: "+map2.get(key));
            }
     

   • Delete element

         //删除元素
         System.out.println("-------------------------------");
         String v=map.remove("e");//remove()通过Key删除Value值
         System.out.println(v);//返回删除的Value值
         Set<String> keys3=map.keySet();
         for (String key:keys3){
             
             
             System.out.println("Key: "+key+" Value "+map.get(key));
         }
     

   • Determine whether the key exists

      boolean flag = map.containsKey("a");
     System.out.println(flag);
     

   • Determine whether Value exists

     boolean flag2 = map.containsValue("BB");
     System.out.println(flag2);