多线程操作----找错误

找找下面错误

class ListHepler{
	public List<E> list = Collections.synchronizedList(new ArrayList<E>());
	public synchronized boolean putIfAbsent(E x){
		boolean absent = !list.contains(x);
		if(absent)
			list.add(x);
		return absent;
		
	}
}

这里putIfAbsent方法和list使用的不是同一个锁，所以有隐患

class ListHepler{
	public List<E> list = Collections.synchronizedList(new ArrayList<E>());
	public  boolean putIfAbset(E x){
                synchronized (list) {
		      boolean absent = !list.contains(x);
		
			if(absent)
				list.add(x);
		}
		return absent;
	}
}

上面的方法都很傻逼

class ListHepler{
	public List<E> list = new ArrayList<E>();
	public synchronized boolean putIfAbset(E x){
		    boolean absent = !list.contains(x);
			if(absent)
				list.add(x);
		}
		return absent;
	}
}