Exceptions & Error Handling

What is an exception?

An exception is Kotlin’s way of signalling that something went wrong at runtime — a network failed, a number couldn’t be parsed, a list index was out of range. If you don’t handle it, the app crashes. Good apps anticipate failures and respond gracefully.

try / catch / finally

try {
    val number = "abc".toInt()        // throws NumberFormatException
} catch (e: NumberFormatException) {
    println("Not a valid number")
} finally {
    println("This always runs (cleanup)")
}

try as an expression

val number = try {
    input.toInt()
} catch (e: NumberFormatException) {
    0          // default on failure
}

Throwing your own

fun withdraw(amount: Double) {
    if (amount <= 0) throw IllegalArgumentException("Amount must be positive")
    // ...
}

// handy precondition helpers
require(amount > 0) { "Amount must be positive" }   // throws IllegalArgumentException
checkNotNull(user) { "User must be loaded first" }

The Result type (no exceptions)

Instead of throwing, you can return a Result that is either success or failure — clean for functions that can fail:

fun parseAge(text: String): Result<Int> = runCatching { text.toInt() }

parseAge("25")
    .onSuccess { println("Age is $it") }
    .onFailure { println("Invalid: ${it.message}") }

Handling errors in coroutines

viewModelScope.launch {
    try {
        val data = api.getData()
        _state.value = UiState.Success(data)
    } catch (e: IOException) {
        _state.value = UiState.Error("Check your connection")
    } catch (e: Exception) {
        _state.value = UiState.Error("Something went wrong")
    }
}

Best practices

• Catch specific exceptions first, then a general one.
• Never swallow an exception silently — at least log it.
• Show the user a friendly message; keep the technical detail in logs.
• Use require/check to validate inputs early.

Summary: Wrap risky code in try/catch, validate inputs with require/check, and consider Result/runCatching for failable functions. Always fail gracefully with a clear message.