作者回复:
下面是Thread里start方法的注释。
注意到“The result is that two threads are running concurrently”
所以start方法返回之后，线程是保证已经启动运行了的。

所以调用start方法，返回之后就可以调用join了。sleep1ms只是一个程序员的多疑。

    /**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>
     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * <code>start</code> method) and the other thread (which executes its
     * <code>run</code> method).
     * <p>
     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception IllegalThreadStateException if the thread was already
     * started.
     * @see #run()
     * @see #stop()
     */
    public synchronized void start() {