Custom Exception in Kotlin

Introduction

Custom Exception is a fundamental concept every Kotlin developer should understand. Exceptions signal that something went wrong at runtime. Kotlin treats checked exceptions differently from Java, encouraging explicit error handling where it matters.

You can define custom exceptions by extending Exception class. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• You can define custom exceptions by extending Exception class.
• Custom exceptions improve clarity for domain-specific errors.
• They are thrown and caught like built-in exceptions.

Syntax

class MyException(message: String) : Exception(message)

Custom Exception in Kotlin Example Program in Kotlin

class InvalidMarksException(message: String) : Exception(message)

fun checkMarks(marks: Int) {
    if (marks > 100) throw InvalidMarksException("Marks cannot exceed 100")
    println("Marks accepted: $marks")
}

fun main(args: Array<String>) {
    try {
        checkMarks(150)
    } catch (e: InvalidMarksException) {
        println(e.message)
    }
}

Sample Output

Marks cannot exceed 100

When to use

Use try/catch when calling code that can fail — file access, network calls, parsing — and you need a controlled recovery path.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. The println("Marks accepted: $marks") statement writes a line to the console — this produces part of the sample output below.

3. The println(e.message) statement writes a line to the console — this produces part of the sample output below.

4. You can define custom exceptions by extending Exception class.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: you can define custom exceptions by extending Exception class.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• You can define custom exceptions by extending Exception class.
• Custom exceptions improve clarity for domain-specific errors.
• They are thrown and caught like built-in exceptions.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.