在JAVA程序中一次只有1个线程

Question

Here's my code (Class):

package table2thread;

import java.util.logging.Level;

import java.util.logging.Logger;

public class Table2Thread extends Thread {

    private int j;
    private boolean flag;

    Table2Thread(int j0) {
        j = j0;
    }

    public void run() {

        if (Thread.currentThread().getName().equals("Thread1")) {
            for (int i = 1; i < 11; i++) {
                System.out.println(j + "*" + i + "=" + j * i);
                flag = true;
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ex) {
                    Logger.getLogger(Table2Thread.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        } else if (Thread.currentThread().getName().equals("Thread1") && flag == true) {
            for (int i = 1; i < 11; i++) {
                System.out.println(j + "*" + i + "=" + j * i);
                flag = false;
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ex) {
                    Logger.getLogger(Table2Thread.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }
    }
}

Here is my main class:

public class Main {
    public static void main(String[] args) {

        Table2Thread a1 =new Table2Thread(3);
        a1.setName("Thread1");
        a1.start();
        Table2Thread a2 =new Table2Thread(4);
        a2.setName("Thread2");
        a2.start();
    }
}

I need only 1 thread at a time and only unique one. First a1 then a2 then again a1 and a2, but right now when i run my code i get the duplication like first a1 starts then a2 then after 3-4 increments a1 starts and again a1 starts then a2 starts and again a2 starts. I tried many different things but as i am newbie i am unable to get it to work.

PLEASE HELP

Answer 1

If you want to use a boolean toggle-flag you have to check it more than once ... like this:

(Pseudo-Code)

public static volatile boolean toggle = true;

...

if( isThread1 )
{
    for( ... )
    {
        while( !toggle )
        {
            Thread.sleep(100);
        }
        // Do your stuff here
        toggle = false;
    }
}
else
{
    for( ... )
    {
        while( toggle )
        {
            Thread.sleep(100);
        }
        // Do you stuff here
        toggle = true;
    }
}

You may have to check for additional conditions. For example if a1 has a different number of iterations than a2.

I know this not complete working code. But since this is an assignment, I just wanted to give you a hint to work it out yourself.

Good luck!

Edit: I do not claim that this is the best solution and it sure isn't the only one.

Answer 2

Why would you want to use threading in such case then?

Simply put the logic of your thread in a method (or a plain class), and in your main, just loop to invoke that method.

If you really want to use thread like what you do here, there are lots of thing you can do:

Method 1: First if you want a1 and a2 to keep running, you should have some kind of loop to keep them running again and again. First way is have a loop in your main, and after starting a1 or a2, add a a1.join()/a2.join(), so that it will start a thread, wait until it finish, and then start the next one.

Method 2: You may have the loop in the thread instead, and both thread share a monitor/semaphore. Before running the logic in the loop, acquire the monitor, like this:

public void run() {
   while (running) {
      synchronized(sharedMonitor) {
          while (flag != thisThread) {
              sharedMonitor.wait();
          }
          //your actual logic here
          flag = nextThread;
          sharedMonitor.notifyAll();
      }
   }
}

Method 3: Instead of managing the thread yourselves, create a Executor. Consider using a 2 element block queue for the executor, and you may use a 1-thread Thread Pool Executor. So your main loop will keep creating Runnable which represent the logic of your a1/a2, and submit to the executor to execute the Runnable.

Answer 3

I suggest to use Thread::Join method.

public class Main {
    public static void main(String[] args) {

        Table2Thread a1 =new Table2Thread(3);
        a1.setName("Thread1");
        a1.start();
        a1.join();
        Table2Thread a2 =new Table2Thread(4);
        a2.setName("Thread2");
        a2.start();
    }
}

Thanks Niraj Rathi