Cannot start new Intent by setClassName with different package in Android

Question

I want to start a new Intent dynamically. Therefore setClassName seems the best choice.

First, I define 3 activity in Manifest

Answer 1

intent.setClassName(packageName, className);

where
packageName - The name of the package implementing the desired component, i.e. the package where the caller belongs to.
className - fully qualified name of the class [from different package]

Calling from com.example.pkg2.Act:

intent.setClassName("com.example.pkg2", "com.example.pkg1.Act1");