Question on java.lang.Thread "starting problems"

Question

hi everybody
i've just little trouble running a tiny program which uses Threads.
maybe there's a misunderstanding from my side:
About my code:
01  public class MyThread extends Thread {
02
03    String any_string = null;
04
05    public MyThread(String param) {
06      any_string = param;
07      this.start();
08    }
09
10    public void run() {
11      while(!isInterrupted()) {
12        System.out.println(" > This Thread is running!");
13        // any other operation...
14        try {
15        sleep(1000);
16      }
17      catch(Exception e) {}
18    }
19  }This is a Thread's code similar to those you can find it in the java.lang.Thread Documentation or in the java Tutorial.
In my example I got a IllegalThreadStateException in codeline 07 which means (compare to java.lang.Thread Documentation) that the code tries to start the Thread while the Thread is still running.
I always thought a Thread will not start itself.
I thought (and that's the way, i red it in the documentation) that the constructor of a Thread only allocates the new Thead-Object.
But by constructing MyThread I got the descripted exception (i'm beginning to repeat myself...). The "funny" thing on this exception is that the Thread at least IS running but there could be no Object allocated
01  ...
02  /**
03   * construction of a new Object of the class MyThread.
04   * test will be auto-started by the MyThread constructor
05   */
06  MyThread test = new MyThread("any-string");
07  ...
08  test.anyMethodOfClassMyThread();This little code generates a java.lang.NullPointerException in line 08 (when trying to access the object of class MyThread). Even after the application exits (in case of this exception), the run()-Method of MyThread generates its Text-Output " > This Thread is running!". argh
Does anyone see my mistake / misunderstanding??
It's really frustrating if you are sitting in front of your pc and can't get the reason why your prog is generating that "bullshit" (ok... i know - it's only generatting the bullshit I told it to generate :( )
Any suggestions or ideas
thanks,
Thof

Answer 1

Well now, I don't get a NullPointerException, when I try to call a method of MyThread. Here's your code, with slight modifications by me:
public class MyThread extends Thread {
    String any_string = null;
    public MyThread(String param) {
        any_string = param;
        this.start();
    public String getAnyString(){
        return any_string;
    public void run() {
        while(!isInterrupted()) {
            System.out.println(" > This Thread is running!");
            try {
                sleep(1000);
            catch(Exception e) {}
}To test the app I wrote this class:
public class TestMyThread {
    public static void main(String[] args) {
        MyThread temp = new MyThread("Hello World");
        System.out.println(temp.getAnyString());
}Now it does print out the correct "Hello World".
Your other problem of the thread not shutting down is because it is not a daemon thread. To solve this you will need to add this Thread to a ThreadGroup and set the ThreadGroup as a Daemon group. (see ThreadGroup::setDaemon() )
Or you can put a little boolean in your while loop and break out when you change the value of the boolean. Then when you want to shut down you simply change the boolean
Hope this helps
-Philip