K&B Book, Thread question

[Dep Joy]

The code below is from K&B's book, it will print out 100 As, then 100 Bs, finally 100 Cs. My question is at line 23-25, three anonymous objects are created and they are different objects, and since "letter" is an instance variable rather than an class variable, there should be three "letter" variables on three separated stack, the operation in run() method should not affect the "letter" in others' stacks, therefor it should print 300 As. But they do print 100 As, then 100Bs, then 100Cs, could anybody tell me why? Thanks in advance.
1. class InSync extends Thread {
2. StringBuffer letter;
3.
4. public InSync(StringBuffer letter) {
5. this.letter = letter;
6. }
7.
8. public void run() {
9. synchronized(letter) {
10. for(int i = 1;i<=100;++i) {
11. System.out.print(letter);
12. }
13. System.out.println();
14. // Increment the letter in StringBuffer:
15. char temp = letter.charAt(0);
16. ++temp;
17. letter.setCharAt(0, temp);
18. }
19. }
20.
21. public static void main(String [] args) {
22. StringBuffer sb = new StringBuffer("A");
23. new InSync(sb).start();
24. new InSync(sb).start();
25. new InSync(sb).start();
26. }
27. }
[ August 31, 2003: Message edited by: Dep Joy ]
[ August 31, 2003: Message edited by: Dep Joy ]

[Alton Hernandez]

Hi Dep,
Yes, there are 3 instances of InSync, so as you've said, there will be 3 instances of the member variable letter. But all the instances of this letter is pointing to only one object (the StringBuffer sb), which is passed to the constructor of InSync at lines 23, 24 & 25. In short, all 3 threads are working on 1 object only.
Hope this helps.
[ August 31, 2003: Message edited by: Alton Hernandez ]

[Dep Joy]

Thank you very much! It seems I need to do more work on the by value and by reference section, I thought I had grasped it