Is it not possible to set another activity as the main activity in Android Studio?

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It is possible to set another activity as the main activity in Android Studio. You can do this by editing the "AndroidManifest.xml" file in your project and changing the "android:name" attribute of the activity you want to be the main activity to "android.intent.action.MAIN" and "android.intent.category.LAUNCHER". This will make the chosen activity the default activity that is launched when the application is started.