Debugging Crashes with Logcat

"The app stopped" tells the user nothing and you even less. Logcat, Android's system log, records exactly which exception was thrown, where, and why. Learning to read it is one of the highest-leverage debugging skills you can build.

Find the crash in Logcat

Android Studio's Logcat supports query filters. Useful ones:

package:mine level:error          # only your app, errors and above
package:mine tag:OrderRepo        # one tag from your app
FATAL EXCEPTION                   # search for crash headers
is:crash                          # crashes only (newer Logcat versions)

Anatomy of a stack trace

FATAL EXCEPTION: main
Process: com.example.shop, PID: 12345
java.lang.IllegalStateException: Product price missing for id=42
    at com.example.shop.cart.CartMapper.toUi(CartMapper.kt:18)
    at com.example.shop.cart.CartViewModel$load$1.invokeSuspend(CartViewModel.kt:34)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(...)
    at kotlinx.coroutines.DispatchedTask.run(...)
    ...
Caused by: kotlinx.serialization.MissingFieldException: Field 'price' is required ...
    at com.example.shop.api.ProductDto$$serializer.deserialize(...)

Common exceptions and their usual fixes

From crash to fix: using the debugger

1. Click the file link in the stack trace (for example CartMapper.kt:18) to jump to the line.
2. Set a breakpoint there, or an exception breakpoint for that exception type (Run → View Breakpoints).
3. Run in debug and reproduce. Inspect variables and the call stack when it stops.
4. Write a unit test that reproduces the failing input, fix the code, and watch the test pass.

@Test
fun `product without price is shown as unavailable instead of crashing`() {
    val ui = CartMapper.toUi(ProductDto(id = "42", name = "Mug", price = null))
    assertEquals("Unavailable", ui.priceText)
}

Logging well

private const val TAG = "OrderRepo"

suspend fun placeOrder(order: Order) {
    Log.d(TAG, "placeOrder items=${order.items.size} total=${order.totalPaise}")   // no personal data
    try {
        api.place(order.toDto())
    } catch (e: IOException) {
        Log.w(TAG, "placeOrder failed: network", e)       // include the throwable for its stack trace
        throw e
    }
}

• Use levels meaningfully: d for debug detail, i for milestones, w for recoverable problems, e for failures.
• Never log tokens, passwords or personal data.
• Strip verbose logs from release builds (for example with a logging wrapper or Timber's debug tree).

Crashes in production

Logcat only shows your device. For users' crashes, use Android vitals in the Play Console and a crash reporter such as Firebase Crashlytics. Upload R8 mapping files so stack traces are readable (see R8), and add breadcrumbs and custom keys (screen name, feature flag) to reproduce context.

Debugging scenarios

Frequently asked questions

Conclusion

To debug a crash: filter Logcat, read the exception and message, jump to the first frame in your code, check "Caused by", reproduce with breakpoints, and lock in the fix with a regression test. Use Android vitals and Crashlytics for production crashes.

Next step: automate your quality checks with CI/CD for Android with GitHub Actions.