java ConcurrentHashSet

Java并没有提供ConcurrentHashSet类，但是可以通过ConcurrentHashMap来实现类似的功能。ConcurrentHashMap是线程安全的哈希表，可以在多线程环境下使用。可以通过将值设置为一个固定的Object对象来实现ConcurrentHashSet的功能。具体实现如下：

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashSet<E> extends Collections {
    private final Set<E> set;
    private final Object lock = new Object();

    public ConcurrentHashSet() {
        set = Collections.newSetFromMap(new ConcurrentHashMap<E, Boolean>());
    }

    public boolean add(E e) {
        synchronized (lock) {
            return set.add(e);
        }
    }

    public boolean remove(Object o) {
        synchronized (lock) {
            return set.remove(o);
        }
    }

    public boolean contains(Object o) {
        return set.contains(o);
    }

    public boolean isEmpty() {
        return set.isEmpty();
    }

    public int size() {
        return set.size();
    }
}

使用示例：

ConcurrentHashSet<String> set = new ConcurrentHashSet<>();
set.add("hello");
set.add("world");
System.out.println(set.contains("hello")); // 输出：true
System.out.println(set.contains("java")); // 输出：false