The procedure you use (clicking on the project and selecting Debug as => Android Application in order for the first debugging session ( Run as , non debug, respectively).
If you want to debug the application again (no matter what is selected in the left side navigator), you just have to click on the circular Green Run icon (or rather on the small sign > on the right) if you want to choose a different launch configuration) or a small error left (for debugging).
If you click on one of > , you will choose from the last 8 runs / debugged applications. If you are sure that the last one is the one you want to start again, just click one of these two icons (error for debugging, green arrow to start) instead of arrows.
When you click on a project and select Debug as , what you are actually doing is configuring the default launch configuration for this application or class, which can be changed later with Run/debug Configurations... to better suit your customization needs. If you click again, eclipse will detect that it has already created the default configuration (and will not create a new identical one). Alternatively, you can directly create a new Run / Debug configuration directly through the Run/debug Configurations... submenu.
Since the ADT plugin sent by Google does not include the Run as... Android Application submenu when you click on the java class in an android project, the default launch setting is either a java application or a server application (there is no static main method).
Update
To clarify the situation a bit. What do you ask (your Android application is running, while the java class selected in the package explorer is possible). What happens when you click the “small error” icon does not depend on what is selected in any navigator.
The "default startup configuration" is poorly worded.
Let me describe what happens in more detail. When you right-click on the java class, say, in the package explorer (or in the project explorer for the Java EE version), eclipse (navigator) calls up all the contributions (plugins) that the handler registered for this event and lists the possible candidates in the submenu ,
In a standard Java EE eclipse installation, you will see at least the run on server parameter, and if the class has a basic static method with string array arguments, you will also have the runs as ... java application option. In both cases, eclipse will create a “launch configuration” for you. These startup configurations (both debugging and debugging) are available in the startup dialogs (available through the icons described above). They have many different configuration parameters depending on their nature (for example, whether you are debugging on a server or a simple java application or applet). What I called the "default configuration" is that for each of these categories of launch configuration, eclipse will leave these options empty.
So, the first time you select an Android project and select the Run as... Android Application option. This will create a default configuration. In the case of ADT, the default configuration is also likely to select the default ADV and run it (if it is not already running). And in subsequent cases, just click the start or debug icon to redeploy the application back to ADV, and this will happen regardless of what is selected in the navigator.