Question

I am reading Java Thread Programming a book by Paul Hyde.I am into intial chapter. I am running various sample programs in Eclipse IDE.

public class TwoThreadAlive extends Thread {
public void run() {
    for (int i = 0; i < 10; i++) {
        printMsg();
    }
}

public void printMsg() {
    // get a reference to the thread running this
    Thread t = Thread.currentThread();
    String name = t.getName();
    System.out.println("name=" + name);
}

public static void main(String[] args) {
    TwoThreadAlive tt = new TwoThreadAlive();
    tt.setName("my worker thread");

    System.out.println("before start(), tt.isAlive()=" + tt.isAlive());
    tt.start();
    System.out.println("just after start(),tt.isAlive()=" + tt.isAlive());

    for (int i = 0; i < 10; i++) {
        tt.printMsg();
    }

    System.out
            .println("at the end of main(), tt.isAlive()=" + tt.isAlive());
}}

This program is giving SAME output every time I run it.But it has mentioned that output will differ as JVM will be busy in context switching.Can you please tell me that why it is giving same output?

And it is giving

at the end of main(), tt.isAlive()=true

This should have been

at the end of main(), tt.isAlive()=false

Please help me , this is making me mad.

Answer 1

Nope, the output is correct because by the time the main method executes the last println there is no guarantee that also your thread has finished executing. Add a tt.join(); right before the last print:

tt.join();

System.out
        .println("at the end of main(), tt.isAlive()=" + tt.isAlive());

Now the main thread will be blocked until tt finishes, and so the println will execute only after tt is dead.

As for your output being always the same, 10 is a too fine grain value and most likely each thread finishes printing everything by the time it gets context switched. Try with a much larger value.