Java集合（七）、Map集合 HashMap(常用)方法

 

 

HashMap的存值

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(1,"Jeo");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map);
    }
}

注：添加重復的鍵會被最新添加的覆蓋

 

HashMap的取值

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.get(3));
        System.out.println(map.get("Sezzy"));
    }
}

 

HashMap的判斷為空

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        System.out.println(map.isEmpty());
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.isEmpty());
    }
}

 

HashMap判斷是否含有key

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.containsKey(4));
        System.out.println(map.containsKey(2));
    }
}

 

 HashMap判斷是否含有value

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.containsValue("wang"));
        System.out.println(map.containsValue("Sezzy"));
    }
}

 

HashMap刪除這個key下的value

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.remove(2));
        System.out.println(map);
    }
}

 

HashMap顯示所有的value值

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.values());
        System.out.println(map);
    }
}

 

HashMap的元素個數

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        System.out.println(map.size());
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.size());
    }
}

 

HashMap取出全部的key

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        System.out.println(map.keySet());
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        System.out.println(map.keySet());
        map.put(3,"Pit");
        System.out.println(map.keySet());
        System.out.println(map);
    }
}

 

HashMap替換key的value

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map);
        System.out.println(map.replace(3, "Jack"));
        System.out.println(map);
    }
}

 

清空這個HashMap

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map);
        map.clear();
        System.out.println(map);
    }
}

 

HashMap的克隆

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map.clone());
        Object clone = map.clone();
        System.out.println(clone);
    }
}

 

如果當前 Map 不存在鍵 key 或者該 key 關聯的值為 null，那么就執行 put(key, value)；否則，便不執行 put 操作：（java8新增方法）

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(maps);
        System.out.println(map.putIfAbsent(1, "wang"));
        System.out.println(map.putIfAbsent(4, "Lee"));
        System.out.println(map);
    }
}

 

如果當前 Map 的value為xx時則值為xx否則為xx：（java8新增方法）compute 方法更適用於更新 key 關聯的 value 時，新值依賴於舊值的情況 

import java.util.*;

public class test{
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"leslie");
        map.put(2,"Sezzy");
        map.put(3,"Pit");
        System.out.println(map);
        map.compute(2, (k, v) -> v == null ? "Suzan" : "Faker");
        System.out.println(map);
    }
}