How to manage memory and threads?

Question

Let's imagine a scenario like the following:
public class Test
   Test()
   public void execute()
    for(int i=0; i<100; i++)
         new MyThread().start();
public class MyThread
  MyThread(){}
  public void run()
    //Do some work which doesn't take a lot of "computing power" but lasts for 1-2s.
}In this example, using multiple threads should help to work quicker because you haven't to wait for one process to wait until the next can work. This works fine indeed, but it will take more memory than the non-threaded model. This is all perfect, except for one thing:
Why doesn't the VM free up the memory the threads are allocating. Imagine you call the execute method one time after another (let's say you pass different parameters so it makes sense). This would cause your VM to eventually crash because memory would get used up. (try this with a w2k or wxp task-manager).
I've read that the threads will belong to a thread-group (which one or how can I access it?) and will therefore still be referenced even though they have run out. What can I do about it?

Answer 1

Just in case, I forgot something:
public class Test
   Test()
    public void execute()
    for(int i=0; i<100; i++)
          new MyThread().start();
public class MyThread extends Thread
  MyThread(){}
  public void run()
    //Do some work which doesn't take a lot of "computing power" but lasts for 1-2s.
}