JAVA单排日记-2020/1/6-Map集合

1.特点

2.子类

3.常用方法

1.集合中放入元素 对象.put(k,v)

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

public class DemoMap {
    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value1"); value值可以重复
        map.put("key3","value3"); key值不可以重复

        System.out.println(map);
    }
}

2.集合中去除元素 对象.remove(k)

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

public class DemoMap {
    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value1");
        map.put("key3","value3");

        System.out.println(map);

        map.remove("key2");
        System.out.println(map);
    }
}

3.获取集合中的元素 对象.get(k)

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

public class DemoMap {
    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value1");
        map.put("key3","value3");

        System.out.println(map);

        map.remove("key2");
        System.out.println(map);
        
        String c = map.get("key1");
        System.out.println(c);
    }
}

4.判断集合中是否含有某元素 对象.containKey(k)

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

public class DemoMap {
    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value1");
        map.put("key3","value3");

        System.out.println(map);

        map.remove("key2");
        System.out.println(map);
        
        String c = map.get("key1");
        System.out.println(c);
        
        boolean b1 = map.containsKey("key3");
        System.out.println(b1);
        boolean b2 = map.containsKey("key4");
        System.out.println(b2);
    }
}

5.遍历Map集合

法一：键(key)找值(value)

import java.util.*;

public class DemokeySet {
    public static void main(String[] args) {
        Set<Integer> KEY = new HashSet<>();
        Map<Integer,String> map = new HashMap<>();
        map.put(2,"one");
        map.put(5,"two");
        map.put(23,"three");
        map.put(7,"four");

        KEY = map.keySet();

        for (int i : KEY){
            System.out.println(map.get(i));
        }
    }
}

法二：键值对

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

public class DemoEntrySet {
    public static void main(String[] args) {
        Map<Integer,String> map = new HashMap<>();
        map.put(23,"a");
        map.put(21,"b");
        map.put(4,"c");
        map.put(2,"d");
        map.put(7,"e");

        Set<Map.Entry<Integer,String>> set = map.entrySet();

        for(Map.Entry<Integer,String> i:set){
            System.out.println(i.getKey()+i.getValue());
        }
    }
}