/**
     * Adds the specified thread to this thread group.
     *
     * <p> Note: This method is called from both library code
     * and the Virtual Machine. It is called from VM to add
     * certain system threads to the system thread group.
     *
     * @param  t
     *         the Thread to be added
     *
     * @throws  IllegalThreadStateException
     *          if the Thread group has been destroyed
     */

    void add(Thread t) {

  {- -------------------------------------------
  (1) 引数で指定された Thread オブジェクトを, threads フィールドに格納する.

      (ついでに, nthreads フィールドの値をインクリメントと nUnstartedThreads フィールドの値をデクリメントも行っている)
      (ただし, この ThreadGroup が既に破棄されている場合は, IllegalThreadStateException を出す)
      (また, threads フィールドが未初期化もしくは一杯の場合には, 新しい配列の確保も行っている)
      ---------------------------------------- -}

            synchronized (this) {
                if (destroyed) {
                    throw new IllegalThreadStateException();
                }
                if (threads == null) {
                    threads = new Thread[4];
                } else if (nthreads == threads.length) {
                    threads = Arrays.copyOf(threads, nthreads * 2);
                }
                threads[nthreads] = t;

                // This is done last so it doesn't matter in case the
                // thread is killed
                nthreads++;

                // The thread is now a fully fledged member of the group, even
                // though it may, or may not, have been started yet. It will prevent
                // the group from being destroyed so the unstarted Threads count is
                // decremented.
                nUnstartedThreads--;
            }
        }