Java之集合类【HashMap】【入门版，实例解析】

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toStringSystem.out.println(one);//Key和value都是ObjectUser user = (User)one.get("002");System.out.println(user);//是否包含某个keySystem.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);}}

吃水不忘挖井人。

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