⚠️ Exception Handling

📖 What are Exceptions?

Exceptions are errors that occur during program execution. When you handle exceptions well, your program runs stably without stopping!

💡 Basic try-catch

Basic Usage

fun main() {
    try {
        val result = 10 / 0  // Error occurs!
        println(result)
    } catch (e: Exception) {
        println("Error occurred: ${e.message}")
    }
    // Error occurred: / by zero

    println("Program continues running!")
}

Handling Specific Exceptions

fun main() {
    try {
        val text = "abc"
        val number = text.toInt()  // NumberFormatException
    } catch (e: NumberFormatException) {
        println("Cannot convert to number: ${e.message}")
    } catch (e: Exception) {
        println("Unknown error: ${e.message}")
    }
}

finally

fun main() {
    try {
        println("Starting task")
        // Perform task
    } catch (e: Exception) {
        println("Error occurred")
    } finally {
        println("Cleanup (always executed)")
    }
}

🎯 Practical Examples

Safe Number Conversion

fun safeToInt(text: String): Int? {
    return try {
        text.toInt()
    } catch (e: NumberFormatException) {
        null
    }
}

fun main() {
    println(safeToInt("123"))   // 123
    println(safeToInt("abc"))   // null

    val age = safeToInt("25") ?: 0
    println("Age: $age")
}

File Reading Simulation

class FileReader {
    fun readFile(filename: String): String {
        if (filename.isEmpty()) {
            throw IllegalArgumentException("Filename is empty")
        }
        if (!filename.endsWith(".txt")) {
            throw IllegalArgumentException("Only text files are supported")
        }
        return "File content: $filename"
    }
}

fun main() {
    val reader = FileReader()

    try {
        val content = reader.readFile("data.txt")
        println(content)
    } catch (e: IllegalArgumentException) {
        println("Error: ${e.message}")
    }

    try {
        reader.readFile("")
    } catch (e: IllegalArgumentException) {
        println("Error: ${e.message}")
        // Error: Filename is empty
    }
}

User Input Validation

data class User(val name: String, val age: Int)

fun createUser(name: String, age: Int): User {
    require(name.isNotBlank()) { "Name is required" }
    require(age > 0) { "Age must be positive" }
    require(age < 150) { "Age is too high" }

    return User(name, age)
}

fun main() {
    try {
        val user1 = createUser("Hong Gildong", 25)
        println(user1)

        val user2 = createUser("", 25)  // Error!
    } catch (e: IllegalArgumentException) {
        println("Validation failed: ${e.message}")
        // Validation failed: Name is required
    }
}

API Response Handling

sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val exception: Exception) : Result<Nothing>()
}

fun divide(a: Int, b: Int): Result<Int> {
    return try {
        Result.Success(a / b)
    } catch (e: ArithmeticException) {
        Result.Error(e)
    }
}

fun main() {
    val result1 = divide(10, 2)
    when (result1) {
        is Result.Success -> println("Result: ${result1.data}")
        is Result.Error -> println("Error: ${result1.exception.message}")
    }

    val result2 = divide(10, 0)
    when (result2) {
        is Result.Success -> println("Result: ${result2.data}")
        is Result.Error -> println("Error: ${result2.exception.message}")
    }
}

🔍 Using as an Expression

try as a Value

fun main() {
    val number = try {
        "123".toInt()
    } catch (e: NumberFormatException) {
        0  // Default value
    }

    println(number)  // 123

    val invalid = try {
        "abc".toInt()
    } catch (e: NumberFormatException) {
        -1
    }

    println(invalid)  // -1
}

Chaining

fun safeDivide(a: Int, b: Int): Int? {
    return try {
        a / b
    } catch (e: ArithmeticException) {
        null
    }
}

fun main() {
    val result = safeDivide(10, 2)
        ?.let { it * 2 }
        ?.let { it + 5 }
        ?: 0

    println(result)  // 15 (10/2 * 2 + 5)
}

🛡️ Writing Safe Code

require vs check

fun processOrder(quantity: Int, price: Int) {
    // Input validation
    require(quantity > 0) { "Quantity must be positive" }
    require(price > 0) { "Price must be positive" }

    val total = quantity * price

    // State validation
    check(total < 1000000) { "Total amount is too large" }

    println("Order completed: $total won")
}

fun main() {
    try {
        processOrder(10, 5000)  // Success
        processOrder(-1, 5000)  // require fails
    } catch (e: IllegalArgumentException) {
        println("Input error: ${e.message}")
    } catch (e: IllegalStateException) {
        println("State error: ${e.message}")
    }
}

checkNotNull

fun processName(name: String?) {
    val validName = checkNotNull(name) { "Name is null" }
    println("Processing: $validName")
}

fun main() {
    try {
        processName("Hong Gildong")  // Success
        processName(null)            // Fails
    } catch (e: IllegalStateException) {
        println("Error: ${e.message}")
    }
}

runCatching

A convenient method available since Kotlin 1.3!

fun main() {
    // Using runCatching
    val result = runCatching {
        "123".toInt()
    }

    result
        .onSuccess { println("Success: $it") }
        .onFailure { println("Failure: ${it.message}") }

    // getOrNull
    val number = runCatching { "abc".toInt() }.getOrNull()
    println(number)  // null

    // getOrDefault
    val safe = runCatching { "abc".toInt() }.getOrDefault(0)
    println(safe)  // 0
}

🎯 Practical Patterns

Retry Logic

fun <T> retry(times: Int, block: () -> T): T? {
    repeat(times) { attempt ->
        try {
            return block()
        } catch (e: Exception) {
            println("Attempt ${attempt + 1} failed: ${e.message}")
            if (attempt == times - 1) throw e
        }
    }
    return null
}

fun main() {
    var count = 0

    try {
        val result = retry(3) {
            count++
            if (count < 3) {
                throw Exception("Not yet ready")
            }
            "Success!"
        }
        println(result)
    } catch (e: Exception) {
        println("Final failure")
    }
}

Resource Cleanup

class Resource : AutoCloseable {
    init {
        println("Resource created")
    }

    fun use() {
        println("Resource used")
    }

    override fun close() {
        println("Resource cleaned up")
    }
}

fun main() {
    try {
        Resource().use { resource ->
            resource.use()
            // Automatically calls close()
        }
    } catch (e: Exception) {
        println("Error occurred")
    }
}

🤔 Frequently Asked Questions

Q1. When should I use throw?

A: When you need to explicitly throw an exception!

fun validateAge(age: Int) {
    if (age < 0) {
        throw IllegalArgumentException("Age cannot be negative")
    }
    if (age > 150) {
        throw IllegalArgumentException("Age is too high")
    }
}

Q2. Exception vs RuntimeException?

A: Kotlin doesn't distinguish!

// Java distinguishes between checked/unchecked
// Kotlin treats all as unchecked (no throws declaration needed)

fun divide(a: Int, b: Int): Int {
    return a / b  // No throws declaration needed!
}

Q3. What about custom exceptions?

A: Inherit from Exception!

class InvalidEmailException(message: String) : Exception(message)

fun validateEmail(email: String) {
    if (!email.contains("@")) {
        throw InvalidEmailException("Invalid email: $email")
    }
}

fun main() {
    try {
        validateEmail("invalid")
    } catch (e: InvalidEmailException) {
        println(e.message)
    }
}

🎬 Conclusion

Build stable programs with exception handling!

Key Takeaways:
✅ Handle exceptions with try-catch
✅ Validate with require/check
✅ Simplify with runCatching
✅ Return values as expressions
✅ Cleanup with finally

Next Step: Try working with files in File I/O!