Custom exceptions

Learn to write custom exception classes in Kotlin.

Custom exceptions: your own error class

You can define your own class for application-specific errors. It inherits from a generic exception type; you pass the message to the constructor.

class TaskError(message: String) : Exception(message)

throw TaskError("Task not found")

The class name explains the error clearly. You can also choose RuntimeException instead of Exception. A custom type makes both catching and debugging easier.

Throwing and catching custom exceptions

You throw your class with throw and catch it with catch. The type name shows which error is caught.

try {
    throw TaskError("Not found")
} catch (e: TaskError) {
    println(e.message)
}

catch (e: TaskError) only catches this type. You can set up different catch blocks for different error types. This keeps error handling tidy and meaningful.

Exercise Define the TaskError class

Define the class TaskError(message: String) : Exception(message). Write fun find(id: Int) that throws TaskError("Task not found: $id"). In main call find(99); catch TaskError and print e.message.

Show example solution

class TaskError(message: String) : Exception(message)

fun find(id: Int) {
    throw TaskError("Task not found: $id")
}

fun main() {
    try {
        find(99)
    } catch (e: TaskError) {
        println(e.message)
    }
}

Test yourself

Fill in the blank: superclass

class TaskError(message: String) : ____(message)

Show answer

class TaskError(message: String) : Exception(message)

Why define a custom exception?
- For less memory
- To write less code
- For speed
- For more meaningful error types
Show answer

For more meaningful error types — Application-specific error types are more meaningful.
Match part to role

class TaskError

Class definition
: Exception

Superclass
throw TaskError

Throwing
catch (e: TaskError)

Catching

Next lesson: Summary - Exceptions

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies