question on Synchronized

[meena latha]

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

Under which condition 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 return true only if SyncTest is changed to allow x and y to be set separately.

Answer: B. check can return true when setXY is called by multiple threads.

I am not 100% sure about this concept and why B is correct, is it because setXY is not synchronized.

Regards
JP

[Lakshmanan Arunachalam]

Reason:

You may or may not get true for check method even if you use multiple threads to call setXY() method. The reason is, no thread is willing to give the CPU cycles to other threads to do their work (i.e) sleep/yield should be there so that other threads can also do their job. If the underlying OS uses time slicing mechanism, then you may get true for check method. \

I modified your code, sothat check can return true.

Note: Only SetX and SetY are synchronized methods, not SetXY. Hence it can be called by more than one thread at a time.

 
import static java.lang.System.out;
 
public class SyncTest {
private int x;
private int y;
private synchronized void setX( int i ) { x = i; }
private synchronized void setY( int i ) { y = i; }
public void setXY( int i )
{
setX(i);
try
{
Thread.sleep(1000); //if you comment this line, then you may not get true for check method
}
catch (InterruptedException ie)
{
}
setY(i);
}
public synchronized boolean check() { return x != y; }
public static void main(String[] a)
{
int num_of_threads = Integer.parseInt(a[0]);
SyncTest test = new SyncTest();
ThreadA[] threads = new ThreadA[num_of_threads];
for (ThreadA aThread : threads)
{
aThread = new ThreadA(test);
aThread.start();
}
}
}
 
class ThreadA extends Thread
{
SyncTest sync = null;
static int counter = 0;
int myCounter = 0;
 
public ThreadA(SyncTest anObj)
{
super();
sync = anObj;
myCounter = ++counter;
 
}
public void run()
{
sync.setXY(myCounter);
checkValues();
}
 
private void checkValues()
{
if (sync.check())
{

out.println("Thread Name: " + myCounter + ", the values are different");
}
}
}
 

[meena latha]

I see your point, so bottom line it is OS dependent as to whether the code I listed initially returns true. Now my question is can we expect such questions on the exam and if should what should be our answer ?

Regards,
Remi

[Lakshmanan Arunachalam]

check can return true when setXY is called by multiple threads.

You may have to note the word here, it is "can return" not "will return". So the
right answer is B.

Now my question is can we expect such questions on the exam and if should what should be our answer ?

I am not a SCJP, ) Just started like you. But the ans should be B