Thread

[Abhi vijay]

    class Q010 implements Runnable
{
    String s="hello";
    String name = new String("");

    public Q010(String n)
    {
        name = n;

    }

    public void run()
    {
        synchronized(s)
        {
            for ( int i = 0; i < 10; i++ )
            {
                System.out.println("Message from " + name + ".");
            }
        }
    }
}

public class Q010a
{
    private static String s = new String("g");

    public static void main(String[] args)
    {
        Q010 q1 = new Q010("q1");
        Q010 q2 = new Q010("q2");

        Thread t1 = new Thread(q1);
        Thread t2 = new Thread(q2);

        t1.start();
        t2.start();
    }


}

Here the output is fixed, right? As we it is synchronized with a String reference s??

But If I replace synchronized(new String("s")), then Output becomes unpredictable, right?

[Treimin Clark]

String s="hello"; creates a String instance in the String pool, which can be reused.

But new String("hello") always create new instances.

[Vinayak Bhat]

Abhi vijay wrote:Here the output is fixed, right? As we it is synchronized with a String reference s??

But If I replace synchronized(new String("s")), then Output becomes unpredictable, right?

Output is not fixed. It differs from one JVM to another. The thread t1 might go to runnable, but unless the thread scheduler pulls out the thread t1 from runnable pool to be thread of execution, the output is not fixed. It might be possible that the thread t2 might gain the access to be in running state before t1. So, my opinion. No the output is not fixed.

[Vijitha Kumara]

Can't we assume the fixed output, considering the String constant pool concept here. As the block in the run method synchronizes on a String object which will be in the constant pool (two instances of Q010, q1 & q2 shares it)?

[Ruben Soto]

You are both right . Both threads are synchronizing their execution on a single String object. Add this to the end of main():
System.out.println("q1.s and q2.s are " + (q1.s == q2.s ? "the same object" : "different"));
So that means there are 2 possible outputs, depending on which thread starts running first.

[Himalay Majumdar]

Treimin Clark wrote:String s="hello"; creates a String instance in the String pool, which can be reused.

But new String("hello") always create new instances.

String constant pool is not a collection of String objects, it's a collection of references to String objects.
For further reference visit String Constant Pool