Java中HashMap的用法

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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