Java中HashMap的用法

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HashMap

学习java基础的时候对map不熟悉，再加上图算法经常用到这个结构来存储，特此加一篇文章来介绍Map

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map.Entry;

public class HashMapTest {

	public static void main(String[] args) {
		HashMap<String, String> map = new HashMap<>();
		map.put("zhang", "31");//存放键值对

		System.out.println(map.containsKey("zhang"));//键中是否包含这个数据
		System.out.println(map.containsKey("daniu"));
		System.out.println("=========================");

		System.out.println(map.get("zhang"));//通过键拿值
		System.out.println(map.get("daniu"));
		System.out.println("=========================");

		System.out.println(map.isEmpty());//判空
		System.out.println(map.size());
		System.out.println("=========================");

		System.out.println(map.remove("zhang"));//从键值中删除
		System.out.println(map.containsKey("zhang"));
		System.out.println(map.get("zhang"));//获取值
		System.out.println(map.isEmpty());
		System.out.println(map.size());
		System.out.println("=========================");

		map.put("zhang", "31");
		System.out.println(map.get("zhang"));
		map.put("zhang", "32");
		System.out.println(map.get("zhang"));
		System.out.println("=========================");

		map.put("zhang", "31");
		map.put("cheng", "32");
		map.put("yun", "33");

		for (String key : map.keySet()) {
			System.out.println(key);
		}
		System.out.println("=========================");

		for (String values : map.values()) {
			System.out.println(values);
		}
		System.out.println("=========================");

		map.clear();
		map.put("A", "1");
		map.put("B", "2");
		map.put("C", "3");
		map.put("D", "1");
		map.put("E", "2");
		map.put("F", "3");
		map.put("G", "1");
		map.put("H", "2");
		map.put("I", "3");
		for (Entry<String, String> entry : map.entrySet()) {
			String key = entry.getKey();
			String value = entry.getValue();
			System.out.println(key + "," + value);
		}
		System.out.println("=========================");

		// you can not remove item in map when you use the iterator of map
		// for(Entry<String,String> entry : map.entrySet()){
		// if(!entry.getValue().equals("1")){
		// map.remove(entry.getKey());
		// }
		// }

		// if you want to remove items, collect them first, then remove them by
		// this way.
		List<String> removeKeys = new ArrayList<String>();
		for (Entry<String, String> entry : map.entrySet()) {
			if (!entry.getValue().equals("1")) {
				removeKeys.add(entry.getKey());
			}
		}
		for (String removeKey : removeKeys) {
			map.remove(removeKey);
		}
		for (Entry<String, String> entry : map.entrySet()) {
			String key = entry.getKey();
			String value = entry.getValue();
			System.out.println(key + "," + value);
		}
		System.out.println("=========================");

	}

}

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