Set和Map--线程不安全

1、线程不安全写法

public class ContainerNotSafeDemo {

    public static void main(String[] args){
        Set set = new HashSet<>();//线程不安全，报java.util.ConcurrentModificationException
        for (int i=1;i<30;i++){
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);

            },String.valueOf(i)).start();
        }
    }
}

2、线程安全写法

public class ContainerNotSafeDemo {

    public static void main(String[] args){
//        Set set = new HashSet<>();//线程不安全，报java.util.ConcurrentModificationException
        Set set = new CopyOnWriteArraySet<>();//线程安全
        for (int i=1;i<30;i++){
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(set);

            },String.valueOf(i)).start();
        }
    }
}

HashSet底层是HashMap，key是e，value是PRESENT，key是唯一且无序的

private static final Object PRESENT = new Object();

public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

3、Map

        HashMap map = new HashMap<>();//线程不安全
        Map map1 = new ConcurrentHashMap<>();//线程安全
        Map map2 = Collections.synchronizedMap(new HashMap<>());//线程安全