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🚨 Kotlin Exception Handling – try, catch, finally, throw & Custom Errors

🧲 Introduction – Handle Errors Gracefully with Kotlin Exception Handling

In real-world applications, errors and unexpected conditions are inevitable. Kotlin provides a robust and expressive exception handling system that lets you gracefully manage runtime problems such as invalid input, failed network calls, or null values.

With constructs like try, catch, finally, and throw, Kotlin’s error handling is clean and developer-friendly. You can also define custom exceptions to handle domain-specific logic effectively.

🎯 In this guide, you’ll learn:

• How to handle runtime exceptions using try-catch-finally
• How and when to use throw and throws
• How to create and use custom exceptions in Kotlin
• Best practices for safe, clean error management

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📘 Topics Covered

🧱 Topic	📖 Description
🛠️ Kotlin – Exception Handling	Using try, catch, and finally blocks to handle exceptions.
🚫 Kotlin – Throw & Throws	Manually throwing exceptions and declaring throwable functions.
✍️ Kotlin – Custom Exceptions	Creating user-defined exception classes for app-specific error scenarios.

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🔍 Detailed Sections & Examples

🛠️ Kotlin – Exception Handling (try-catch-finally)

fun divide(a: Int, b: Int): Int {
    return try {
        a / b
    } catch (e: ArithmeticException) {
        println("Error: ${e.message}")
        0
    } finally {
        println("Execution completed.")
    }
}

🧠 Explanation:

• try block wraps risky code.
• catch handles specific exceptions.
• finally executes always (for cleanup or logging).

✅ Output Example:

Error: / by zero
Execution completed.

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🚫 Kotlin – Throw & Throws

fun validateAge(age: Int) {
    if (age < 18) {
        throw IllegalArgumentException("Age must be 18 or above")
    }
}

📌 throw is used to explicitly throw an exception.

💡 Unlike Java, Kotlin doesn’t require throws declarations—but it supports interoperability with Java’s @Throws annotation:

@Throws(IOException::class)
fun readFile() {
    // might throw IOException
}

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✍️ Kotlin – Custom Exceptions

class InvalidInputException(message: String) : Exception(message)

fun process(input: String) {
    if (input.isEmpty()) throw InvalidInputException("Input cannot be empty")
}

try {
    process("")
} catch (e: InvalidInputException) {
    println("Custom Exception: ${e.message}")
}

✅ Custom exceptions let you create meaningful, domain-specific error classes for better readability and structure.

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📌 Summary – Recap & Next Steps

Exception handling in Kotlin allows developers to write safe, crash-resistant code using clean and expressive syntax. From built-in handling to custom exceptions, Kotlin gives you powerful tools to catch, log, and respond to problems in real time.

🔍 Key Takeaways:

• Use try-catch-finally to handle and recover from runtime errors.
• Throw exceptions manually with throw for validation and control flow.
• Define custom exception classes to represent specific failure conditions.
• Kotlin doesn’t enforce throws but supports it for Java interop.

⚙️ Practical Use Cases:

• Handling null pointer and arithmetic errors
• Validating user input and throwing exceptions
• Building clean error interfaces for APIs or services

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❓ Frequently Asked Questions

❓ Is finally always executed in Kotlin?
✅ Yes. The finally block runs regardless of exception—ideal for resource cleanup.

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❓ What’s the Kotlin equivalent of Java’s throws keyword?
✅ Kotlin doesn’t require checked exceptions but supports @Throws for Java interop.

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❓ Can I throw multiple exception types in Kotlin?
✅ Yes. Use multiple catch blocks:

try {
    // risky code
} catch (e: IOException) {
} catch (e: NullPointerException) {
}

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❓ Are all exceptions in Kotlin unchecked?
✅ Yes. Kotlin treats all exceptions like unchecked—no compile-time enforcement of try/catch.

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❓ When should I create a custom exception?
✅ When you need to represent a specific business rule or domain error in your application logic.

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