Java集合之一HashMap

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 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 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 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 removeKeys = new ArrayList();
        for (Entry entry : map.entrySet()) {
            if (!entry.getValue().equals("1")) {
                removeKeys.add(entry.getKey());
            }
        }
        for (String removeKey : removeKeys) {
            map.remove(removeKey);
        }
        for (Entry entry : map.entrySet()) {
            String key = entry.getKey();
            String value = entry.getValue();
            System.out.println(key + "," + value);
        }
        System.out.println("=========================");

    }

}