Robolectric: Shadows, Setup & Activities

Robolectric is a unit testing framework for Android that lets Android-dependent code (Activities, Views, SDK classes) run and be tested directly on a regular JVM, without an actual device or emulator -- providing a simulated Android environment for fast, standard unit tests.

Basic Test Setup

@RunWith(RobolectricTestRunner::class)
@Config(sdk = [33])  // simulate a specific Android API level --
                     // list multiple to catch version-specific bugs
class MainActivityTest {

    @Test
    fun `clicking button updates text`() {
        val activity = Robolectric.buildActivity(MainActivity::class.java)
            .create().resume().get()  // explicit control over lifecycle stages

        val button = activity.findViewById<Button>(R.id.submitButton)
        button.performClick()

        val label = activity.findViewById<TextView>(R.id.resultLabel)
        assertThat(label.text.toString()).isEqualTo("Submitted!")
    }
}

Shadow Classes: How the Simulation Works

When test code calls a method on a real Android TextView, Robolectric intercepts the call and routes it to a ShadowTextView implementation that can actually run on the JVM -- the real Android framework classes have no meaningful implementation outside an actual Android runtime.

Real Behavior, Not Just Mocked Calls

// Robolectric's shadow SharedPreferences is a genuinely functioning
// in-memory store -- write, then read back to verify REAL round-trip
// persistence, not just "was putString() called with these args"
val prefs = context.getSharedPreferences("settings", Context.MODE_PRIVATE)
prefs.edit().putString("username", "alice").apply()

assertThat(prefs.getString("username", null)).isEqualTo("alice")