Day 2/100: The 4 Android Components — What Senior Engineer Get Wrong

This is Day 2 of my **100 Days to Senior Android Engineer * series. Each post follows a consistent format: what I thought I knew → what I actually learned → interview implications.* 🔍 The concept Android has exactly four fundamental application components: Activity — a single screen with a UI Service — background work with no UI BroadcastReceiver — responds to system-wide or app-wide events ContentProvider — structured data sharing between apps Every Android developer learns these in week one. What separates junior from senior isn't knowing the list — it's understanding the boundaries and the cost of each one. 💡 What I thought I knew After years of building apps, my mental model was roughly: Need a screen? → Activity (or Fragment ) Need background work? → Service Need to react to a system event? → BroadcastReceiver Need to share data with other apps? → ContentProvider Simple enough. And for most day-to-day work, this is fine. The problem is that this mental model is when to use them ,