您的activity_main.xml文件的FrameLayout。这是一个占位符，另一种观点认为，和therfore不显示任何内容。通常你会在你使用的片段使用了这一观点。您但是没有使用的片段。

You have a framelayout in your activity_main.xml file. This is a placeholder for another view, and therfore does not show anything. Normally you would use this view when you are using fragments. You are however not using fragments.

既然你是一个初学者复制从fragment_main的全部内容到activity_main.xml文件。

Since you are a beginner copy the entire content from fragment_main into the activity_main.xml file.