集合类不安全

ArryList集合

        多线程下不安全；可能会报错：java.util.ConcurrentModificationException（并发修改异常）

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

//java.util.ConcurrentModificationException 并发修改异常
public class ListTest {
    public static void main(String[] args) {
        //并发下ArrayList是不安全的
        List<Object> list = new ArrayList<>();

        //CopyOnWrite 写入时复制 COW 计算机程序应用领域的一种优化策略
        //多个线程调用的时候，list，读取的时候，固定的，写入（覆盖）
        //在写入的时候避免覆盖，造成数据问题
        for (int i = 1; i <= 10; i++) {
            new Thread(()->{
                list.add(UUID.randomUUID().toString().substring(0,5));
                System.out.println(list);
            }).start();
        }
    }
}

解决方案：

1. List<String> list = new Vector<>();
2. List<String> strings = Collections.synchronizedList(new ArrayList<>());
3. List<String> strings = new CopyOnWriteArrayList<>();

        概念：CopyOnWrite写入时复制，计算机程序设计语言的一种优化策略。(保证效率和性能问题)

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

//java.util.ConcurrentModificationException 并发修改异常
public class ListTest {
    public static void main(String[] args) {
        //并发下ArrayList是不安全的
        /**
         * 解决方案：
         * 1.List<Object> list = new Vector<>();
         * 2.List<Object> list = new Vector<>();
         * 3.List<Object> list = Collections.synchronizedList(new ArrayList<>());
         * 4.List<Object> list = new CopyOnWriteArrayList<>();
         */
//        List<Object> list = new ArrayList<>();
//        List<Object> list = new Vector<>();
//        List<Object> list = Collections.synchronizedList(new ArrayList<>());
        //CopyOnWrite 写入时复制 COW 计算机程序应用领域的一种优化策略
        //多个线程调用的时候，list，读取的时候，固定的，写入（覆盖）
        //在写入的时候避免覆盖，造成数据问题

        List<Object> list = new CopyOnWriteArrayList<>();
        for (int i = 1; i <= 10; i++) {
            new Thread(()->{
                list.add(UUID.randomUUID().toString().substring(0,5));
                System.out.println(list);
            }).start();
        }
    }
}

HashSet集合

        多线程下不安全；可能会报错：java.util.ConcurrentModificationException（并发修改异常）

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArraySet;

//java.util.ConcurrentModificationException

public class SetList {
    public static void main(String[] args) {
        HashSet<Object> set = new HashSet<>();
        for (int i = 1; i <=10 ; i++) {
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,5));
                System.out.println(set);
            },String.valueOf(i)).start();

        }
    }
}

解决方案：

1. Set<String> strings = Collections.synchronizedSet(new HashSet<>());
2. Set<String> strings = new CopyOnWriteArraySet<>();

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArraySet;

//java.util.ConcurrentModificationException
/**
 *  解决方案
 *  1.Set<Object> set = Collections.synchronizedSet(new HashSet<>());
 *  2.Set<Object> set = new CopyOnWriteArraySet<>();
 */
public class SetList {
    public static void main(String[] args) {
//        HashSet<Object> set = new HashSet<>();
//        Set<Object> set = Collections.synchronizedSet(new HashSet<>());
        Set<Object> set = new CopyOnWriteArraySet<>();
        for (int i = 1; i <=13 ; i++) {
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,5));
                System.out.println(set);
            },String.valueOf(i)).start();

        }
    }
}

        hashset集合的底层是hashmap的key

HashMap集合

        多线程下不安全；可能会报错：java.util.ConcurrentModificationException（并发修改异常）

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;

public class MapTest {
    public static void main(String[] args) {
        //hashmap默认值 new Hashmap<>(16,0.75);
        Map<String, String> map = new HashMap<>();
        for (int i = 1; i <= 10; i++) {
            new Thread(()->{
                map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0,5));
                System.out.println(map);
            },String.valueOf(i)).start();

        }
    }
}

解决方案：

1. Map<String, String> concurrentHashMap = new ConcurrentHashMap<>();
2. Map<String, String> map = Collections.synchronizedMap(new HashMap<>());

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;

public class MapTest {
    public static void main(String[] args) {
        //hashmap默认值 new Hashmap<>(16,0.75);
//        Map<String, String> map = new HashMap<>();
//        Map<String, String> map = Collections.synchronizedMap(new HashMap<>());
        ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
        for (int i = 1; i <= 10; i++) {
            new Thread(()->{
                map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0,5));
                System.out.println(map);
            },String.valueOf(i)).start();

        }
    }
}