Android simulated programs are not displayed

Android has been put down for a long time. Today I think of it again. I need to call the above hello-class mini programs to run them. Look for the feeling. Who knows that when it is running, it is not displayed on the simulator. It cannot be seen from the output information, and the simulator can also be started. Everything is normal, that is, there is no picture that should be run. Depressed. Later, after clicking Hu on the simulator, we found that the menu key can be used to call up the program. If a simulator runs several programs without being shut down, the last menu will appear at each time, click the host key to return to the main screen, and click the return key to return to the previous program one by one. I do not know whether it is because I do not understand the running principle of the android program or the simulator bug.