Thread Synchronization Q

[Win jones]

From K&B book - Chapter 9 on Threads Pg 522 under synchronization and Locks

If two methods are synchronized in a class, only one thread can be accessing one of the two methods. In other words, once a thread acquires the lock on an object, no other thread can enter any of the synchronized methods in that class (for that object).

I wrote a small example to try this out thinking I would get the same final value for the thread T1 , T2 and T3 i.e 10 but that does not seems to be the case from the above statement and the code I was expecting this ?? I am new to threading so help me out ppl.

class B{
int i = 10;
public static void main(String [] atgs){
Threadclass tc = new Threadclass();
Thread t1 = new Thread(tc);
Thread t2 = new Thread(tc);
Thread t3 = new Thread(tc);
t1.setName("T1");
t2.setName("T2");
t3.setName("T3");
t1.start();
t2.start();
t3.start();
}
}
 
class Threadclass implements Runnable{
objectcheck ob;
Threadclass(){
ob = new objectcheck();
}

public void run(){
System.out.println("Run "+Thread.currentThread().getName());
for (int j = 0; j < 3; j++){
ob.increment();
ob.decrement();
}
System.out.println("Final value of i from "+Thread.currentThread().getName()+ " " +ob.display()+" #######" );
}


}
 
class objectcheck{

private int i = 10;
public synchronized void increment(){
System.out.println("Before increment value of I: " +i+ " " +Thread.currentThread().getName());
i = ++i;
System.out.println("After increment value of I: " +i+ " " +Thread.currentThread().getName());
}


public synchronized void decrement(){
System.out.println("Before decrement value of I: " +i+ " " +Thread.currentThread().getName());
i = --i;
System.out.println("After decrement value of I: " +i+ " " +Thread.currentThread().getName());
}

public int display(){
return i;
}
}

Thanks in advance

[Barry Gaunt]

A question for you: What could happen between the calls to the methods increment and decrement?

[Win jones]

Thanks for pointing it out Barry, so I should probably do something like this

synchronized(this)
{
ob.increment();
ob.decrement();
}

then when would one use synchronised for methods the way I have used ?