catalogue

1: Introduction to Map interface

  2: Common methods in Map

3: HashMap_ Add element

code

4: HashMap_ Get element

The first method

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  The second method

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  The third method

code

  5: Map container union operation

code

  6: HashMap_ Delete element

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  7: HashMap_ Determine whether key or value exists  

code

1: Introduction to Map interface

The Map interface defines the storage characteristics of the two column set. It is not a sub interface of the Collection. The two column set is stored with the characteristics of Key and Value (similar to the combination in Mathematics)

● the elements in the Map exist in pairs, similar to couples in the society, while the elements in the Collection exist individually, similar to single dogs in the society

● commonly used containers in Map include HashMap and TreeMap

  2: Common methods in Map

3: HashMap_ Add element

HashMap is the implementation class of Map interface, which is implemented by hash algorithm. Duplicate keys are not allowed. If there are duplicate new values, they will replace the old values. HashMap is very efficient in finding, deleting and adding

V put(K,Key ,V value)

If the same Key appears, the new Key will replace the old Key and return the Value corresponding to the old Key

If the Key appears for the first time, null will be returned

code

import java.util.HashMap;
import java.util.Map;

public class HashMapTest {
    public static void main(String[] args) {
        //Instantiate HashMap
        Map<String,String> map=new HashMap<>();
        //Add element
        String str=map.put("Chen Shuchang","Greater Songbi");
        System.out.println(str);//Null will be returned because if there is no corresponding Key in the Map, null will be returned. If there is, the same Value will be returned
        String str2=map.put("Chen Shuchang","Little counsellor");
        System.out.println(str2);

    }
}

4: HashMap_ Get element

The first method

V get(K key) obtains the corresponding Value through the corresponding Key. If the Key does not exist, it will return null

code

 //Get element
        System.out.println("-------Get element-------------");
        //Get the corresponding Value through the Key
        System.out.println(map.get("Chen Shuchang"));
        System.out.println(map.get("ha-ha"));

  The second method

Set keySet() is often used with the get method

Return the Key in the Map container in the form of Set set (return a Set container)

code

 System.out.println("--------------keySet--------");
        //Key Set gets all the keys in the Map Set and puts them into the Set set
        //Get all the elements in the HashMap, and the KeySet gets the elements together with get
        //Instantiate a Set container
        Set<String> set=map.keySet();
        map.put("Lu Xinyu","Handsome force!");
        map.put("Lu Xiaoyu","Handsome force!!");
        map.put("Lu Dayu","Handsome!!!");
        for(String str1:set){
            String str3=map.get(str1);
            System.out.println(str1+"------->"+str3);
        }

  The third method

Set < Map.Entry < K, V > > entryset() returns a set based on all the Value values in the Map set of type Map.Entry

code

  System.out.println("----------entrySet----------");
        Set<Map.Entry<String,String>> entries=map.entrySet();
        //The Entry interface is defined inside the Map interface. Call the Entry interface, Map.Entry
        //There are getKey and getValue methods in the Entry type
        for(Map.Entry<String,String> entrys:entries){
            String key= entrys.getKey();
            String val=entrys.getValue();
            System.out.println(key+"----->"+val) ;
        }

  5: Map container union operation

 void  putAll(Map M);

Merge elements from another container into this container. If the merged Key is the same as the Key value of the original container, it will replace the Key of the original container

code

        System.out.println("----------Map Consolidation of containers----------");
        Map<String ,String> map2=new HashMap<>();
        map2.put("Lu Xinyu","Dashuaibi");
        map2.putAll(map);
        Set<String> set3=map2.keySet();
        for(String str4:set3){
            System.out.println("key:"+str4+"value:"+map2.get(str4));

        }

  6: HashMap_ Delete element

V remove(Object   key) delete the corresponding element through the key and return the Value of the deleted element

code

  7: HashMap_ Determine whether key or value exists  

boolean  containsKey(Object Key)

boolean  containsValue(Object Value)

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