Java Map common usage examples

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

public  class BasicHashMap {

    public static void main(String[] args) {

        // 新建map
        Map<String, Integer> fstMap =  new HashMap<>();

        // 存值
        fstMap.put("a", 1);
        fstMap.put("b", 2);
        fstMap.put("c", 3);
        fstMap.put("d", 4);

        System.out.println( "=== fstMap output===" );
        System.out.println(fstMap);

        Map<String, Integer> sndMap = new HashMap<>();

        sndMap.put("ab", 1);
        sndMap.put("cd", 2);

        System.out.println("=== aMap.putAll(bMap) ===");
        sndMap.putAll (fstMap);
        System.out.println( "The merged sndMap" );
        System.out.println(sndMap);
        System.out.println( "The merged fstMap remains" );
        System.out.println(fstMap);

        System.out.println( "=== map.getOrDefault(aKey, dV), if there is akey, return the corresponding value, otherwise return dV ===" );
        System.out.println(sndMap.getOrDefault("ab", 3));
        System.out.println(sndMap.getOrDefault("m", 3));
        System.out.println(sndMap);

        System.out.println("=== map.put(aKey) 重复key===");
        System.out.println( "map put duplicate value, will do get first, return the corresponding value, and then put; no duplicate value, put directly, return null" );
        System.out.println(sndMap.put("a", 5));
        System.out.println(sndMap.put("z", 10086));
        System.out.println(sndMap);

        System.out.println("=== map.remove(aKey) VS map.remove(aKey, aValue) ===");
        System.out.println( "map remove(akey) operation: if there is akey, first get and return the corresponding value, and then delete; otherwise, return null" );
        System.out.println(sndMap.remove("a"));
        System.out.println(sndMap.remove("m"));
        System.out.println( "map remove(aKey, aValue) operation, return true or false to indicate whether the remove is successful" );
        System.out.println(sndMap.remove("b", 2));
        System.out.println(sndMap.remove("m", 8));
        System.out.println(sndMap);

        System.out.println("=== map.clear() ===");
        sndMap.clear();
        System.out.println(sndMap);

    }
}

out：

=== fstMap output === 
{a =1, b=2, c=3, d=4 }
 === aMap.putAll(bMap) ===
Merged sndMap
{ab = 1, cd = 2, a = 1, b = 2, c = 3, d = 4 }
Merged fstMap remaining
{a =1, b=2, c=3, d=4 }
 === map.getOrDefault(aKey, dV), if there is akey, return the corresponding value, otherwise return dV ===
1
3 
{ab =1, cd=2, a=1, b=2, c=3, d=4 }
 === map.put(aKey) repeat key===
map put duplicate value, will do get first, return the corresponding value, and then put; no duplicate value, put directly, return null
1
null
{ab=1, cd=2, a=5, b=2, c=3, d=4, z=10086}
=== map.remove(aKey) VS map.remove(aKey, aValue) ===
map remove(akey) operation: if there is an akey, first get and return the corresponding value, and then delete; otherwise, return null
5
 null 
map remove(aKey, aValue) operation, return true or false to indicate whether the remove is successful
 true 
false 
{ab =1, cd=2, c=3, d=4, z=10086 }
 === map.clear () === 
{}