Synchronization - K&B question

[Marx Villegas]

Hi, I have a question about synchronization of statics. I'm a bit confused. The problem is the following (from K&B)

public class Letters extends Thread {
private String name;
public Letters(String name) { this.name = name; }
public void write()
System.out.print(name);
System.out.print(name);
}

public static void main(String[] args) {
new Letters("X").start();
new Letters("Y").start();
}
}

And the options are:
A. public void run() { write(); }
B. public synchronized void run() { write(); }
C. public static synchronized void run() { write(); }
D. public void run() { synchronized(this) { write(); } }
E. public void run() { synchronized(Letters.class) { write(); } }
F. public void run() { synchronized(System.out) { write(); } }
G. public void run() { synchronized(System.out.class) { write(); } }

The answers in the book are E & F, which I could corroborate by running the code... But I'm confused about option E.
As far as I know, the expression synchronized(Letters.class) synchronizes the static methods of the class. But since the methoid write is not static, the way is it forbidden that it could be run by two different threads simultaneously?
Thanks in advance
XM

[sanjeev mehra]

I have a question about synchronization of statics. I'm a bit confused. The problem is the following (from K&B)

And the options are:

What is the question? Sorry if I am missing something.

Thanks.

[Marx Villegas]

The question is the question #2 from the self test section of the chapter 9.
Here it is:

Given:
public class Letters extends Thread {
private String name;
public Letters(String name) {
this.name = name;
}

public void write() {
System.out.print(name);
System.out.print(name);
}

public static void main(String[] args) {
new Letters("X").start();
new Letters("Y").start();
}
}
We want to guarantee that the output can be either XXYY or YYXX, but never XYXY or any other combination. Which of the following method definitions could be added to the Letters class to make this guarantee? (Choose all that apply.)
A. public void run() { write(); }
B. public synchronized void run() { write(); }
C. public static synchronized void run() { write(); }
D. public void run() { synchronized(this) { write(); } }
E. public void run() { synchronized(Letters.class) { write(); } }
F. public void run() { synchronized(System.out) { write(); } }
G. public void run() { synchronized(System.out.class) { write(); } }

XM

[magesh sripathy]

syncronized(classname.class) ---

if one thread enters the lock of the object of the class..no other thread can enter any other object of the same class..

------------------------------------------------------------------
Magesh.s

[Henry Wong]

As far as I know, the expression synchronized(Letters.class) synchronizes the static methods of the class. But since the methoid write is not static, the way is it forbidden that it could be run by two different threads simultaneously?

Static methods of a class, uses the class object. This means that the static methods of the Letter class uses the Letter.class object to synchronize. Just because static methods uses a particular object, it doesn't mean that no other method can use the same object.

So... it is perfectly valid to use the Letter.class object.

Henry

[Shaliey Gowtham]

Block of code Synchronized at Class level ensures that only one thread can execute the block of code to completion.
So E is correct
Further F is also correct
[ April 18, 2006: Message edited by: Shaliey G ]

[Nilesh Deshpande]

Can some one please explain me how the lock on System.out works here? Does that means for all the objects of that class, it will not be possible for 2 objects to call println method?

Thanks in advance

[Shaliey Gowtham]

out is static and final reference variable in the System class.
So its at class level, hence there can be only one thread executing the code locked at class level to completion until the lock is released.

[Phani Kumar]

Synchronizing System.out doesn't guarantee that the two System.out statements are executed atomically. We can have a combination where

1. Thread 1 acquires the lock on System.out, prints 'X'.
2. Thread 2 acquires the lock on System.out, prints 'Y'.
3. Thread 1 acquires the lock on System.out, prints 'X'.
2. Thread 2 again acquires the lock on System.out, prints 'Y'.

Which, clearly is not the desired combination for the output.

Any comments?
[ April 19, 2006: Message edited by: Phani Kumar ]

[d jones]

Why isn't option D valid here?

I thought that a lock on any object could be used in a synchronized block of code??

Thanks