confuse on the answer

[Kelvin Mak]

1. public class SyncTest {
2. private int x;
3. private int y;
4. public synchronized void setX (int i) {x=1;}
5. public synchronized void setY (int i) {y=1;}
6. public synchronized void setXY(int i) {set X(i); setY(i);}
7. public synchronized boolean check() {return x !=y ;}
8. }
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 called by multiple threads.
C. Check() can return true when multiple threads call setX and setY separately.
D. Check() can only return true if SyncTest is changed to allow x and y to be set separately.
The answer from the mock paper is A, but I regards it should C. Anyone would explain for it if you are agree A?
[ Jess disabled smilies ]
[ Kelvin changed all the illegal syntax error ( ) --> } )]
[ November 16, 2002: Message edited by: Jessica Sant ]
[ November 17, 2002: Message edited by: Kelvin Michael ]

[Jamal Hasanov]

Hi, Kelvin
Yes, it's a very tricky question:-)) At first, it confused me,as well... In setX(int) and setY(int) methods value of X and Y set to 1,not to i!!! That's why X and Y are always equal !
I suggest you to read questions carefully and don't hurry ro answer.
Regards,
Jamal Hasanov
www.j-think.com

[Kelvin Mak]

thx,Jamal. I found two version about this thread at mock exam, but they are difference.

1. public class SyncTest {
2. private int x;
3. private int y;
4. public synchronized void setX (int i) {x=1 ;} 
5. public synchronized void setY (int i) {y=1 ;} 
6. public synchronized void setXY(int i) {set X(i); setY(i) ;} 
7. public synchronized Boolean check() {return x !=y ;} 
8. }
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 called by multiple threads.
C. Check() can return true when multiple threads call setX and setY separately.
D. Check() can only return true if SyncTest is changed to allow x and y to be set separately.
Answer: A

1. public class SyncTest {
2. private int x;
3. private int y;
4. private synchronized void setX {int i} (x=1  ;}  
5. private synchronized void setY {int i} (y=1  ;}  
6. public void setXY(int i){set X(i); setY(i) ;}   // note!
7. public synchronized Boolean check() {return x !=y ;} 
8. }
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 called by multiple threads.
C. Check() can return true when multiple threads call setX and setY separately.
D. Check() can only return true if SyncTest is changed to allow x and y to be set separately.
Answer: B

[ November 16, 2002: Message edited by: Kelvin Michael ]
[ November 16, 2002: Message edited by: Kelvin Michael ]

[Kelvin Mak]

Whthout/with the synchronized keyword will very different answer! Becareful if anyone meet this question again.
haha...

[Abu Yoosuf]

 
1. public class SyncTest {
2.   private int x;
3.   private int y;
4.   public synchronized void setX (int i) {x=1 ;}
5.   public synchronized void setY (int i) {y=1 ;}
6.   public synchronized void setXY(int i) {set X(i); setY(i) ;} 
7.   public synchronized Boolean check() {return x !=y ;} 
8. }

What happens if we come across a scenario where by:
SyncTest is new'ed - SyncTest test = new SyncTest();
this results in x = 0; y = 0;
Thread A calls setX(i);
x = 1; y = 0;
Thread B calls check();
shouldn't it return true for the evaluation of x!=y ???
[ Jess adjusted the [code] so its not all on one line ]
[ November 16, 2002: Message edited by: Jessica Sant ]

[Abu Yoosuf]

Sorry Jessica,
Had to go out soon after posting the message and didn't wait to see what I posted.
Thanks for formatting the msg.
[ From Jess: no problem -- that's why I'm here ]
[ November 16, 2002: Message edited by: Jessica Sant ]

[sun par]

Consider the code below

  
class SyncTest {
private int x;
private int y;
public synchronized void setX (int i) {x=1;}
public synchronized void setY (int i) {y=1;  }
public synchronized void setXY(int j){setX(j); setY(j) ;}
public synchronized boolean check() {return x !=y;}
}
public class Sync
{
  public static void main(String args[])
  {
   SyncTest s1=new SyncTest();
   s1.setX(1);
   System.out.println(s1.check());
  }
}

When it runs it returns true..
So how come the answer is check() never returns true?
Thanks

[Kelvin Mak]

Jessica,
Pls note the setX, setY method, no matter when you call it, it always set the x = 1, y = 1. so that is reason why cannot return true.
Thanks for disable the smilies!
[ November 17, 2002: Message edited by: Kelvin Michael ]