【Java基础18_2】HashMap类

た入场券 2022-06-07 14:15 140阅读 0赞

> **概述**

*  底层数据结构是哈希表，是Map集合类的一个子类，它的数据结构只针对键有效。
 *  没有特有的方法，使用参考[Map集合][Map]。

> **HashMapDemo**

import java.util.HashMap;
    import java.util.Map.Entry;
    import java.util.Set;
    
    /* * HashMapDemo */
    public class HashMapDemo {
    
        public static void main(String[] args) {
    
            //定义一个HashMap并放入元素
            HashMap<String,String> hm =new HashMap<>();
            hm.put("001", "张三");
            hm.put("002", "李四");
            hm.put("003", "王五");
            hm.put("004", "赵六");
    
            //============遍历方法一================
            //获取key的集合
            Set<String> keySet = hm.keySet();
    
            System.out.println("\t\t编号\t\t姓名");
    
            //遍历key集合
            for (String key : keySet) {
                //用key获取值
                System.out.println("\t\t"+key+"\t\t"+hm.get(key));
            }
    
            System.out.println("---------------------------------------------");
    
            //============遍历方法二================
    
            System.out.println("\t\t编号\t\t姓名");
    
            //获取Entry对象
            Set<Entry<String, String>> entrySet = hm.entrySet();
    
            for (Entry<String, String> entry : entrySet) {
    
                //用Enrty分别获取键和值
                System.out.println("\t\t"+entry.getKey()+"\t\t"+entry.getValue());
            }
    
    
        }
    
    }

> **HashMap集合键是String值是自定义类Singer的Demo**

![image][]

import java.util.HashMap;
    import java.util.Map.Entry;
    import java.util.Set;
    
    import com.itbull.map.traversal.Singer;
    
    /* * HashMap集合键是String值是自定义类Singer的Demo */
    public class HashMapDemo1 { 
    
        public static void main(String[] args) {
    
            //定义Singer对象
            Singer s1 = new Singer("邓紫棋", "泡沫");
            Singer s2 = new Singer("银 临", "牵丝戏");
            Singer s3 = new Singer("谢春花", "只道寻常");
            Singer s4 = new Singer("Taylor", "TikTok");
    
            //定义HashMap集合
            HashMap<String,Singer> hm = new HashMap<>();
    
            //将Singer对象放入HashMap
            hm.put("001", s1);
            hm.put("002", s2);
            hm.put("003", s3);
            hm.put("004", s4);
    
            //获取Entry集合
            Set<Entry<String, Singer>> entrySet = hm.entrySet();
    
            //打印标题
            System.out.println("\t\t"+"编号"+"\t\t"+"歌手"+"\t\t"+"歌曲");
    
            //遍历Entry对象
            for (Entry<String, Singer> entry : entrySet) {
    
                //分别用Entry获取键和值
                System.out.println("\t\t"+entry.getKey()+"\t\t"+entry.getValue().getName()+"\t\t"+entry.getValue().getSongName());
    
            }
        }
    
    }

> **HashMap集合键是自定义类Singer值是String的Demo**

![image][]

*  注意：自定义类需要**重写hashCode\*\***和equals方法**才能**保证元素唯一\*\*

import java.util.HashMap;
    import java.util.Set;
    import java.util.Map.Entry;
    
    import com.itbull.map.traversal.Singer;
    
    /* * HashMap集合键是自定义类Singer值是String的Demo */
    public class HashMapDemo2 { 
    
        public static void main(String[] args) {
    
            //定义Singer对象
            Singer s1 = new Singer("邓紫棋", "泡沫");
            Singer s2 = new Singer("银 临", "牵丝戏");
            Singer s3 = new Singer("谢春花", "只道寻常");
            Singer s4 = new Singer("Taylor", "TikTok");
    
            //定义HashMap集合
            HashMap<Singer,String> hm = new HashMap<>();
    
            //将Singer对象放入HashMap
            hm.put(s1,"001");
            hm.put(s2,"002");
            hm.put(s3,"003");
            hm.put(s4,"004");
    
            //获取Entry集合
            Set<Entry<Singer, String>> entrySet = hm.entrySet();
    
            //打印标题
            System.out.println("\t\t"+"编号"+"\t\t"+"歌手"+"\t\t"+"歌曲");
    
            //遍历Entry对象
            for (Entry<Singer, String> entry : entrySet) {
    
                //分别用Entry获取键和值
                System.out.println("\t\t"+entry.getValue()+"\t\t"+entry.getKey().getName()+"\t\t"+entry.getKey().getSongName());
    
            }
    
    
        }
    
    }

> HashMap嵌套HashMap（理解）

[该Demo源码传送门][Demo]

> HashMap嵌套ArrayList（理解）

[该Demo源码传送门][Demo 1]

> HashMap嵌套ArrayList（理解）

[该Demo源码传送门][Demo 2]

> **HashMap与Hashtable的区别**

*  通过查看API可以知道：  
    
    
     *  HashMap: 线程**不安全** , 效率高；**允许null**值和**null**键
     *  Hashtable: 线程**安全** , 效率低；**不允许null**值和**null**键

[Map]: http://note.youdao.com/noteshare?id=9574091584d28ebb298b7ca7b8c355c1&sub=94C3FF86E3BD4278A10D17879BC13C3C
[image]: /images/20220607/a50826d52195447d9f78630f5df8b33f.png
[Demo]: http://note.youdao.com/noteshare?id=edaf4adc649d2a20a491de50801830af&sub=99B1016C400742D38236E24F763860BB
[Demo 1]: http://note.youdao.com/noteshare?id=bf8711af5280ac9b0d05bcb7ff6d5a07&sub=F8DF787B1EA54482A4657B66F2462F41
[Demo 2]: http://note.youdao.com/noteshare?id=65ec31b653105e1fa98d38ceac043116&sub=0A6EB8DC186A4B0186554683279456E0