Can we access another application's class or fire an intent to another application's class from our application?

Question

I want to fire an intent to another application's activity . What i am thinking is to use intent filter of the other's. Or can i access the other application package in some other way?

Suppose My app is 'A' and another app also exist in phone which is 'B'

Q1. Can I access or call classes of B and its method by any means? Q2. And If it can be done by Implicit Intents then What happens in that case where I cant have access of B's Source code, means I cannot find the B's class Intent filter.