[kaffe] Bug in ThreadGroup

Dalibor Topic robilad at yahoo.com
Wed Apr 9 08:47:01 PDT 2003

Hi Hervé,

--- Hervé_Roussain <Herve.Roussain at univ-ubs.fr> wrote:
> Hi!
> The 'parentOf' method doesn't work

thanks for the bug report and the fix.

> The method is
> final public boolean parentOf(ThreadGroup g) {
>          return ((parent == g) ||
> (parentOf(g.getParent())));
> }
> but should be something like this:
> final public boolean parentOf(ThreadGroup g) {
>          return (g != null && this == g.getParent())
>                  || parentOf(g.getParent());
> }

I've expanded your test class a bit to cover two
fringe cases as well: null, and self.

class ThreadGroupTest {
  public static void main(String[] args) {
    ThreadGroup daddy = new ThreadGroup("daddy");
    ThreadGroup child = new ThreadGroup(daddy,
    System.out.println("daddy.parentOf(child)=" +
    System.out.println("child.parentOf(daddy)=" +
    System.out.println("child.parentOf(child)=" +
    System.out.println("child.parentOf(null)=" +

I think that your fix fails the null test and the self

A better fix is:

final public boolean parentOf(ThreadGroup g) {
	return (g != null)
	  && ((this == g) ||(parentOf(g.getParent())));

which passes all four tests.

By the way, Classpath developers didn't implement the
method in a recursive fashion. In my opinion their
approach to walk the parent group chain in a loop is
better, as it does all the work within a single stack
frame. But that's a matter of taste, I guess.

dalibor topic

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