public String toString() {
        ThreadGroup group = getThreadGroup();
    if (group != null) {
        return "Thread[" + getName() + "," + getPriority() + "," + 
                   group.getName() + "]";
    } else {
        return "Thread[" + getName() + "," + getPriority() + "," + 
                    "" + "]";
    }
    }

------解决方案--------------------
看API，Thread实现了Runnable接口。currentThread是一个静态方法。
在main方法中， Thread.currentThread ().getName，得到的是主线程，名字叫Thread-0。
Thread.currentThread () 得到main方法中的主线程：Thread[Thread-0,5,main]

------解决方案--------------------
Thread.currentThread ().getName 运行得到：Thread-0
Thread-0就是当前线程（主线程）的名字。
Thread.currentThread () 运行得到：Thread[Thread-0,5,main]
Thread.currentThread ()这个的值就是当前线程（主线程）的对象。
Thread[Thread-0,5,main]Thread-0不就是上面的线程名嘛，5表示线程优先级，main是线程组的名称。（API中就说明了）System.out.printn(Thread.currentThread ())；会自动调用对象的toString（）方法，这是Java的特性。也就是说，Thread[Thread-0,5,main]是Thread.currentThread ()的toString（）的返回值。

------解决方案--------------------
Thread[Thread-0,5,main]是自动调用toString方法打印的
Java code

 public String toString() {
        ThreadGroup group = getThreadGroup();
    if (group != null) {
        return "Thread[" + getName() + "," + getPriority() + "," + 
                   group.getName() + "]";
    } else {
        return "Thread[" + getName() + "," + getPriority() + "," + 
                    "" + "]";
    }
    }