关于java: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<String, String> map = new HashMap<>();        map.put("zhang", "31");//寄存键值对        System.out.println(map.containsKey("zhang"));//键中是否蕴含这个数据        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("=========================");    }}