/**
     * Increments the count of unstarted threads in the thread group.
     * Unstarted threads are not added to the thread group so that they
     * can be collected if they are never started, but they must be
     * counted so that daemon thread groups with unstarted threads in
     * them are not destroyed.
     */

    void addUnstarted() {

  {- -------------------------------------------
  (1) nUnstartedThreads フィールドの値をインクリメントするだけ.
      (ただし, この ThreadGroup が既に破棄されている場合は, IllegalThreadStateException を出す)
      ---------------------------------------- -}

            synchronized(this) {
                if (destroyed) {
                    throw new IllegalThreadStateException();
                }
                nUnstartedThreads++;
            }
        }