Synchronized block of code

[Paulo Aquino]

I got this code from K & B:

class InSync extends Thread {
StringBuffer letter;
 
public InSync(StringBuffer letter) {
 this.letter = letter;
}
 
 public void run() {
 synchronized(letter) {
 for(int i = 1;i<=100;++i) {
 System.out.print(letter);
 }
 
 
 System.out.println();
 // Increment the letter in StringBuffer:
 char temp = letter.charAt(0);
 ++temp;
 letter.setCharAt(0, temp);
 }
 }
 
 
 public static void main(String [] args) {
 StringBuffer sb = new StringBuffer("A");
 new InSync(sb).start();
 new InSync(sb).start();
 new InSync(sb).start();
 }
 }  

What is the logic behind the synchronization on the letter object? Can you put just any object inside the synchronization parentheses just to satisfy the requirement of synchronizing a block of code? I'm trying to learn the synchronization concepts of threads and I'm looking for a clear cut explanation on this concept.

[Henry Wong]

Yes, you can practically use any object as the synchronization lock... but there is a reason why the stringbuffer was chosen. It is the same instance with all three threads, and synchronization was done to prevent the threads from stepping on each other.

So if you use any other object, you have to make sure that the three threads are using the same instance / object.

Henry
[ April 24, 2005: Message edited by: Henry Wong ]