public class T {

    private final StampedLock stampedLock = new StampedLock();

    private double x;

    private double y;

    public void move(double deltaX, double deltaY) {
        // 获取写锁
        long stamp = stampedLock.writeLock();
        try {
            x += deltaX;
            y += deltaY;
        } finally {
            // 释放写锁
            stampedLock.unlockWrite(stamp);
        }
    }

    public double distanceFromOrigin() {
        long stamp = stampedLock. ; // 获得一个乐观读锁
        // 注意下面两行代码不是原子操作
        // 假设x,y = (100,200)
        double currentX = x;
        // 此处已读取到x=100，但x,y可能被写线程修改为(300,400)
        double currentY = y;
        // 此处已读取到y，如果没有写入，读取是正确的(100,200)
        // 如果有写入，读取是错误的(100,400)
        if (!stampedLock.validate(stamp)) { // 检查乐观读锁后是否有其他写锁发生
            stamp = stampedLock.readLock(); // 获取一个悲观读锁
            try {
                currentX = x;
                currentY = y;
            } finally {
                stampedLock.unlockRead(stamp); // 释放悲观读锁
            }
        }
        return Math.sqrt(currentX * currentX + currentY * currentY);
    }
}