它只是帮助你添加一个标志好在之后使用。

It just helps you add a mark to use later.

youdao

在当前播放的位置添加一个标志时间并加到下表，看23,24。

Add a mark of the current playing position to the list below, see 23, 24.

youdao

如果我们已经向 Leaf 类添加了 getValue 方法，我们要么是不得不返回一些类型的标志值表明此方法应用是无意义的，要么是实际抛出一个异常。

If we had added a getValue method to class Leaf, we would either have to return some sort of flag value indicating that the method application is non-sensical or actually throw an exception.

youdao