— Scott McNealy, Sun Microsystems Chairman, President and ceo

increment();
System.out.println(“balance is “ + balance);
}
}
public void increment() {
int i = balance;
balance = i + 1;
}
 }
public class TestSyncTest {
public static void main (String[] args) {
TestSync job = new TestSync();
Thread a = new Thread(job);
Thread b = new Thread(job);
a.start();
b.start();
}
}
each thread runs 50 times, 
incrementing the balance on 
each iteration
Here’s the crucial part
! We increment the balance by 
adding 1 to whatever the value of balance was AT THE 
TIME WE READ IT (rather than adding 1 to whatever 
the CURRENTvalue is)
synchronization 
matters

---

networking 
and
threads
you are here
4
513
Let’s run this code...
A
B
1
Thread A runs for awhile
Put the value of balance into variable i.
Balance is 0, so i is now 0.
Set the value of balance to the result of i + 1.
Now balance is 1.
Put the value of balance into variable i.
Balance is 1, so i is now 1.
Set the value of balance to the result of i + 1.
Now balance is 2.
B
2
Thread B runs for awhile
Put the value of balance into variable i.
Balance is 2, so i is now 2.
Set the value of balance to the result of i + 1.
Now balance is 3.
Put the value of balance into variable i.
Balance is 3, so i is now 3.
[now thread B is sent back to runnable, 
before it sets the value of balance to 4]
A
3
Thread A runs again, picking up where it left off
Put the value of balance into variable i.
Balance is 3, so i is now 3.
Set the value of balance to the result of i + 1.
Now balance is 4.
Put the value of balance into variable i.
Balance is 4, so i is now 4.
Set the value of balance to the result of i + 1.
Now balance is 5.
B