android - how to find the name of the main activity of an application?

This can be found in the application's manifest.

The main activity is the activity with the intent-filter whose name is android.intent.action.MAIN.

You can plug your phone into the computer and look at the DDMS log, application launches are printed there, e.g:

05-11 09:19:15.725: INFO/ActivityManager(96): Starting: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x2000000 cmp=com.google.android.gm/.ConversationListActivity bnds=[125,410][235,540] } from pid 2457

So, com.google.android.gm/.ConversationListActivity, would seem like the right choice, at least, that's what the icon seems to launch.

Step1: Start "adb logcat" in command prompt.

Step2: Open the app (either in emulator or real device)

You don't need to know it's name, instead you should use implicit intent and specify action along with type and some extras, for example

            final Intent intent = new Intent();
            intent.setType("message/rfc822");
            intent.setAction(Intent.ACTION_SEND);
            intent.putExtra(Intent.EXTRA_SUBJECT, "Some subject");

System will search for components available to run this intent.