例子一：

import java.util.Map;

import java.util.Set;

import java.util.HashMap;

public class HashMapDemo1 {

publicstatic 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);

HashMapusers = new HashMap();

users.put(u1.getId(),u1);

users.put(u2.getId(),u2);

users.put(u3.getId(),u3);

Set set = users.keySet();

for(String key:set){

System.out.println(key+"=="+users.get(key));

}

System.out.println("+++++++++++++++++++++++++++");

Set> set1 = users.entrySet();

for(Map.Entry 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 hm = new HashMap();

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 hm1 = new HashMap();

for(int i = 0;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);

}

}