Java之集合类【HashMap】【入门版，实例解析】

标签：java   面向对象   

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());
		}
		
	}
}

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());
		}
	}
}

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);
	}
}

Java之集合类【HashMap】【入门版，实例解析】

标签：java   面向对象   

原文地址：http://blog.csdn.net/wei_chong_chong/article/details/47205649