Re: Why can't I start a thread twice?

Lew <>
Sat, 02 May 2009 16:57:43 -0400
Chuan C. wrote:

When I try to start the run() method of a subclass of Thread for
a second time I get this runtime error:

  Exception in thread "AWT-EventQueue-1"
  at java.lang.Thread.start(

Peter answered your primary question, so I'm going to point out some things
unrelated to that question.

This is what I do:

class X extends Thread {

It is usually better to implement 'Runnable' and pass it to a 'Thread' than to
extend 'Thread'.

Not much better, but better.

  Boolean go;

Why did you choose 'Boolean' instead of 'boolean'?

If the variable were 'boolean' it would be initialized to 'false' for you

You can also initialize it directly in the declaration, so you wouldn't need
to do it in the constructor body (or even declare a constructor at all, perhaps).

   boolean go = false;

Also, why did you choose package-private (i.e., default) access for the
variable instead of private access?

  public X() {
    go = false;

  public void run() {
    go = true;
    for (int n = 0; n < 100 && go; n++) {
      // various activities

The calling program does:
  b = new X();
  b.go = false; -> works OK

Perhaps this is why 'go' is package-private. This line would fail if the
calling program were in a different package from the 'X' class.

A better idiom is to declare the variable 'private' and use public accessor
and mutator methods (a "getter" and a "setter") to manipulate the variable.

  b.start(); <- the runtime error above

Am I not supposed to repeat a thread this way?

There are a number of excellent books and articles on threaded programming in
Java. Look for authors Brian Goetz, Doug Lea and Joshua Bloch. /Java
Concurrency in Practice/, by Brian Goetz, Messrs. Lea and Bloch and others, is
an excellent book on the subject. Bloch's /Effective Java/ touches on the
matter as well, and is essential reading for all Java programmers. Brian
Goetz also has multiple articles published on IBM's DeveloperWorks site that
deal with multi-threaded programming. The Sun Java tutorial touches on it as



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