这道题我搞不懂,

题目

这道题我搞不懂,
20.public class SychTest{
private int x;
private int y;
public void setX(int i){ x=i;}
public void setY(int i){y=i;}
public Synchronized void setXY(int i){
setX(i);
setY(i);
}
public Synchronized boolean check(){
return =y;
}
}
Under which conditions will check() return true when called from a different class?
A.check() can never return true.
B.check() can return true when setXY is callled by multiple threads.
C.check() can return true when multiple threads call setX and setY separately.
D.check() can only return true if SychTest is changed allow x and y to be set separately.

答案

答案是C.
好吧,补充一下.
这个setXY方法用了synchronized（线程同步）,所以会防止多个线程同时访问这个方法.但是setX()和setY()方法没有synchronized保护,而且setX()和setY()是public公开的,因此C提到的情况会出现.
A.check() can never return true.（错,因为可以不通过setXY()来更改x和y的值）
B.check() can return true when setXY is callled by multiple threads.(错,setXY()是被保护的）
C.check() can return true when multiple threads call setX and setY separately.(正确答案.)
D.check() can only return true if SychTest is changed allow x and y to be set separately.（错.这个其实原则上是对的.但是这个答案用了only一词（只有在这情况下）,所以它错了.因为C也是可以造成x!=y.