Question on exercise about Thread

[brian]

Do you know the answer

public class SyncTest { 
  private int x; 
  private int y; 
  private synchronized void set X (int i){ x=i; } 
  private synchronized void set Y (int i){ y=i; } 
  public void setXY (int i) {set X(i); set Y(i); }
  public synchronized boolean check() {return X != Y; } 
} 

Under which conditions will check() return when called from a different class. Choose one

A. check() can never return true
B. check() can return true when set XY is called by multiple threads
C. check() can true when multiple threads call set X and set Y separately
D. check() can only return true if synchTest is changed to allow x and y to be set separately
[ January 25, 2003: Message edited by: Jessica Sant ]

[hiti hitox]

A) isnt right, because of B)
Answer B) is right.
A call to setXY makes calls to the synchronized methods setX / setY.
But the method setXY isn't synchronized. So it can be called by multible threads.
Ex:
When thread1 call setXY(8), method setX(8) called.
Here comes thread2 with call setXY(9).
Thread1 leaves setX(8) and call setY(8).
Now Thread2 can enter setX(9)
Thread1 call check() and ups! x==9, y==8!
C) Can't happen, cause setX/setY are private
D) completely beside it

[Valentin Crettaz]

brian,
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