Question

Hello i am having a problem with resuming a thread my code is

 public boolean Wait(String Reply){     
        if (Reply.equalsIgnoreCase("Y")){
            try {
                t.resume();
            }
            catch (Exception e){
              System.out.println("\n" + "The exception in resume thread method:::: " + e);                
            }
            System.out.println("\n" + "In the Wait Function of Sender");
            return true;

        }
        JOptionPane.showMessageDialog(j ,
                    "Please Wait While The User Accpets the Trasmission ",
                    "",
                    JOptionPane.INFORMATION_MESSAGE);


       try{
         t = new Thread(this);
         t.sleep(100000);  
        }
       catch (InterruptedException ie){
         System.out.println(ie.getMessage());
       }
    return false;
}

I might explain you how it works as it will help u determine the problem.
First the thread is put to sleep……Then i call this public boolean Wait() function from another function named ReplyYes which passes the value “Y” and i then try to resume the thread but the t.resume() function call, instead of resuming the thread gives me a Java.Lang.Null.PointerException and the thread isn’t resumed resulting in returning a FALSE value. Plus because of this thread i can’t even Stop my Service i have to wait for the thread to timeOut.

Can anyone explain how to make it work correctly!!
Thank you

Answer 1

I think you misunderstand how Thread.sleep works. It is a static method.

The line t.sleep(100000); puts the current thread to sleep, not the thread t.

From the documentation:

Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds

Emphasis mine.

You should start the thread and call sleep from that thread. See the following article for two different ways to start a thread:

• Defining and Starting a Thread

Furthermore, resume is only for use with suspend and they have both been deprecated. From the documentation:

Deprecated. This method exists solely for use with suspend(), which has been deprecated because it is deadlock-prone. For more information, see Why Are Thread.stop, Thread.suspend,
Thread.resume and Runtime.runFinalizersOnExit Deprecated?

The reason you get a NullPointerException is probably because you try to create the new Thread object after you call t.resume(). So at that point, t still has the value null. Basically, your code needs to be completely rewritten from scratch. I would suggest following the tutorial I linked to above, then once you understand how to create threads move to the next chapters:

• Pausing Execution with Sleep
• Interrupts