Collection and map thread safety problem solving

1.ArrayList Thread unsafe （ There is no reentrant lock ）
 resolvent  Vector(synchronized)  Collections(synchronized) CopyOnWriteArrayList(ReentrantLock)

2.HashSet——》CopyOnWriteArraySet

3.HashMapt——》ConcurrentHashMap

package juc.two;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CopyOnWriteArraySet;

/**
 * ArrayList Thread unsafe （ There is no reentrant lock ）
 *  resolvent  Vector(synchronized)  Collections(synchronized) CopyOnWriteArrayList(ReentrantLock)
 */

public class ThreadDemo4 {
    public static void main(String[] args) {
        // establish ArrayList aggregate 
        //List<String> list = new ArrayList<>();
        //Vector solve 
        //List<String> list = new Vector<>();
        //Collections solve 
        //List<String> list = Collections.synchronizedList(new ArrayList<>());
        //CopyOnWriteArrayList solve 
        //List<String> list = new CopyOnWriteArrayList<>();

        /*for(int i = 0; i<30; i++){
            new Thread(()->{
                // Add content to collection 
                list.add(UUID.randomUUID().toString().substring(0,8));
                // Get content from collection 
                System.out.println(list);
            },String.valueOf(i)).start();
        }*/

        // demonstration HashSet
        //Set<String> set = new HashSet<>();

        //Set<String> set = new CopyOnWriteArraySet<>();

        /*for(int i = 0; i<30; i++){
            new Thread(()->{
                // Add content to collection 
                set.add(UUID.randomUUID().toString().substring(0,8));
                // Get content from collection 
                System.out.println(set);
            },String.valueOf(i)).start();
        }*/

        // demonstration HashMap
        //Map<String,String> map = new HashMap<>();

        Map<String,String> map = new ConcurrentHashMap<>();

        for(int i = 0; i<30; i++){
            String key = String.valueOf(i);
            new Thread(()->{
                // Add content to collection 
                map.put(key,UUID.randomUUID().toString().substring(0,8));
                // Get content from collection 
                System.out.println(map);
            },String.valueOf(i)).start();
        }
    }
}