Java之集合类【HashMap】【入门版，实例解析】

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我是靠谱客的博主曾经橘子，最近开发中收集的这篇文章主要介绍Java之集合类【HashMap】【入门版，实例解析】，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

例子一：


import java.util.Map;
import java.util.Set;
import java.util.HashMap;
public class HashMapDemo1 {
public	static void main(String args[]){
User user1 = new User("001","Jack",12);
User user2 = new User("002","Marry",13);
User user3 = new User("003","Xiaoming",14);
User user4 = new User("004","Xiaofang",15);
HashMap one= new HashMap();
one.put(user1.getId(),user1);
one.put(user2.getId(),user2);
one.put(user3.getId(),user3);
one.put(user4.getId(),user4);
//键值对的个数
System.out.println(one.size());
//直接打印Map对象会打印Key的toString= value的toString
System.out.println(one);
//Key和value都是Object
User user = (User)one.get("002");
System.out.println(user);
//是否包含某个key
System.out.println(one.containsKey("122"));
//遍历
//遍历方法一 把Map集合中所有的Key都放入set集合中
/*然后遍历set集合，得到Key,通过key获取value
* */
Set set = one.keySet();
for(Object object: set){
String key = (String)object;
User u = (User)one.get(key);
System.out.println(key+" = "+u);
}
System.out.println("=====================");
//放入HashMap集合中的key,value其实都会被包装成
/*Map.Entry这个内部色属性
* 有一个键值对就存在一个Map.Entry的实例对象
* 通过entrySet()方法就可以把这些实例对象都放在Set集合中
* 遍历Set获取每个对象
* */
Set set1 = one.entrySet();
for(Object object:set1){
Map.Entry me = (Map.Entry)object;
System.out.println(me.getKey()+" = "+me.getValue());
}
}
}

例子二:

package corelesson;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
public class HashMapDemo2 {
public static void main(String[] args){
User u1 = new User("001","zhang",20);
User u2 = new User("002","xiaoming",21);
User u3 = new User("003","nihao",23);
HashMap<String ,User>users = new HashMap<String,User>();
users.put(u1.getId(),u1);
users.put(u2.getId(),u2);
users.put(u3.getId(),u3);
Set<String> set = users.keySet();
for(String key:set){
System.out.println(key+"=="+users.get(key));
}
System.out.println("+++++++++++++++++++++++++++");
Set<Map.Entry<String, User>> set1 = users.entrySet();
for(Map.Entry<String, User> me : set1){
System.out.println(me.getKey()+" = "+me.getValue());
}
}
}

例子三：

package corelesson;
import java.util.HashMap;
public class HashMapDemo3 {
public static void main(String[] args){
HashMap<String,Integer> hm = new HashMap<String,Integer>();
hm.put("x", 10);
hm.put("y", 20);
hm.put("x", 30);
//key相同 会覆盖之前的value key如何才能相同
/*注意equals和方法的实现和hashCode方法的实现
*
* */
System.out.println(hm);
System.out.println("=====================");
//计算每个字符出现的次数
String s = "abcdafdfdfdasfdfgrgfrgsafgrgtads";
/*
* 思路：拿到每个字符，作为key放入HashMap如果
* 如果map中不存在 那么put(key,1);
* 如果已经存在 那么put(key,value+1)
*
* */
HashMap<String,Integer> hm1 = new HashMap<String,Integer>();
for(int i = 0;i<s.length();i++){
String str = s.substring(i,i+1);
if(hm1.containsKey(str)){
hm1.put(str,hm1.get(str)+1);
}
else{
hm1.put(str, 1);
}
}
System.out.println(hm1);
}
}