How do i dynamically choose which activity to launch when opening an app

There is another way to do that using activity-alias.

1. In the Manifest :

   <activity
       android:name=".LoginActivity"
       android:icon="@drawable/ic_launcher_main"
       android:label="Login" >
       <intent-filter>
           <action android:name="android.intent.action.MAIN" />
           <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
   </activity>
   
   <activity
       android:name=".MainActivity"
       android:icon="@drawable/ic_launcher_main"
       android:label="MainActivity" >
   </activity>
   
   <activity-alias
       android:name=".AliasActivity"
       android:label="AliasActivity"
       android:enabled="false"
       android:targetActivity=".MainActivity" >
       <intent-filter>
           <action android:name="android.intent.action.MAIN" />
           <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
   </activity-alias>
   

   2.Somewhere in the Login Activity:

   String s = getApplicationContext().getPackageName();
   ComponentName cm = new ComponentName(s, s+".AliasActivity");
   ComponentName cm2 = new ComponentName(s, s+".Login");
   PackageManager pm = this.getPackageManager();
   pm.setComponentEnabledSetting(cm, PackageManager.COMPONENT_ENABLED_STATE_ENABLED, 1);
   pm.setComponentEnabledSetting(cm2, PackageManager.COMPONENT_ENABLED_STATE_DISABLED, 0);
   

after that, the app will be killed once and next time you launch app, the MainActivity would be the launcher.

The Android Framewowrk provides the method
public Intent setClassName (String packageName, String className)

of the Intent class that you can use to dynamically choose which activity to invoke with just the name of the class in String.

Here's an example

    String packageName = getPackageName(), className=packageName+"subFolder.myActivity";
    Intent i = new Intent();
    i.setClassName(packageName, className);
    startActivity(i); 

https://developer.android.com/reference/android/content/Intent.html#setClassName(java.lang.String,%20java.lang.String)

No, since you have to run some code, there's no way to declaratively (in manifest) to say this. You have to launch an activity (set in manifest), then have this activity decide based on if the user is logged on or not what second activity to launch via Intent:

final Class<? extends Activity> activityClass;
if(userIsLoggedOn())
    activityClass = LoggedOnActivity.class;
else
    activityClass = LogInActivity.class;

Intent newActivity = new Intent(context, activityClass);
context.startActivity(newActivity);