Map<K,V> HashMap

Map<K,V>  HashMap

        Map是一个双列集合，一个元素包含两个值（一个key，一个value）

        Map集合中的元素，key和value的数据类型可以相同，也可以不同

        Map中的元素，key不允许重复，value可以重复

        key一样的，会覆盖

import java.util.Collection;
import java.util.HashMap;
import java.util.Set;

public class HashMap1 {
    public static void main(String[] args) {
        HashMap<String,String> hm = new HashMap<>();
        //hm是否为空
        System.out.println(hm.isEmpty());//true // Empty空的
        hm.put("s1","java");
        hm.put("s23","mysql");
        hm.put("s2"," php");
        hm.put("s1","1000");
        System.out.println(hm);//{s23=mysql, s1=1000, s2= php}

        System.out.println(hm.isEmpty());
        System.out.println(hm.size());//3
        System.out.println("----------------");
        System.out.println(hm.containsKey("s3"));// false  //判断key中有没有s3
        System.out.println(hm.containsValue("java"));//判断value中有没有java
        //删除
       // hm.clear();
        hm.remove("s1");
        System.out.println(hm);//{s23=mysql, s2= php}  //根据key来删除元素
        hm.remove("php");
        System.out.println(hm);//{s23=mysql, s2= php}//根据value不能删除
        hm.remove("s2","mysql");
        System.out.println(hm);//{s23=mysql, s2= php}

        String str = hm.replace("s23","javascript");
        //修改key的value，将旧的value返回，如果输入的key没有，则返回null，不修改
        System.out.println(str);//mysql
        System.out.println(hm);//{s23=javascript, s2= php}

        System.out.println(hm.get("s23"));//javascript
        //获取s7的value，若存在返回对象，若不存在使用默认值html
        System.out.println(hm.getOrDefault("s7","html"));
        System.out.println(hm.get("s5"));//返回不存在的key，value为null


//方法一遍历value值
       Collection<String> col = hm.values();
//        System.out.println(col);//遍历value值
        //方法二
        for(Object o : col){
            System.out.println(o);
        }
        System.out.println("---------------");
        //遍历key值
        Set<String> keys = hm.keySet();
        System.out.println(keys);
        for(String k :keys){
            System.out.printf("%s = %s %n" , k , hm.get(k));
        }

        System.out.println("================");
//s23 = javascript  s2 =  php
        hm.forEach((k,v) -> System.out.printf("%s = %s %n" , k , v));


    }
}

import java.util.*;

public class HashMap2 {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("k5", "go");
        map.put("k1", "ruby");
        map.put("k3", "php");
        map.put("k4", "c");
        map.put("k2", "java");
        map.put("k6", "python");
        map.put("k7", "javascript");
        map.put("k8", "vs");
        Collection<String> v = map.values();
        System.out.println(v);//[ruby, java, php, c, go, python, javascript, vs]

        //将map的key 集合排序  升序
        Set<String> keys = map.keySet();
        List<String> likey = new ArrayList<>(keys);
        Collections.sort(likey);
        System.out.println(likey);//[k1, k2, k3, k4, k5, k6, k7, k8]
//降序
        Collections.sort(likey, Comparator.reverseOrder());
        System.out.println(likey);//[k8, k7, k6, k5, k4, k3, k2, k1]

        for (String k : likey) {
            System.out.println(k);//k8 k7 k6 k5 k4 k3 k2 k1
//        }
        }
    }
}

import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;

public class HashMap3 {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("k523","go");
        map.put("k12","ruby");
        map.put("k3","php");
        map.put("k4244","c");
        map.put("k253454","java");
        map.put("k623","python");
        map.put("k73","javascript");
        map.put("k8","vs");
        //按字符串key的长度排序
        var k =  new ArrayList<>(map.keySet());
        System.out.println(k);
        //升序
        k.sort(Comparator.comparingInt(String::length));
        System.out.println(k);
//降序
        k.sort((a,b)->b.length()-a.length());
        System.out.println(k);
    }
}