Custom exception classes in Kotlin - Mini Project: Build & Apply

Custom Exception Classes in Kotlin

📖 Scenario: You are building a simple banking app. You want to handle errors clearly when someone tries to withdraw more money than they have.

🎯 Goal: Create a custom exception class called InsufficientFundsException and use it to show a clear error message when a withdrawal is too large.

📋 What You'll Learn

Create a custom exception class named InsufficientFundsException that inherits from Exception

Create a variable balance with the value 1000

Create a variable withdrawAmount with the value 1500

Write a function withdraw that throws InsufficientFundsException if withdrawAmount is greater than balance

Use a try-catch block to call withdraw and print the exception message

💡 Why This Matters

🌍 Real World

Custom exceptions help apps handle errors clearly, like showing a message when a bank withdrawal fails.

💼 Career

Understanding custom exceptions is important for writing robust Kotlin applications in jobs like Android development or backend services.

1

Create the initial variables

Create a variable called balance and set it to 1000. Then create a variable called withdrawAmount and set it to 1500.

Kotlin

# Create variables balance and withdrawAmount
// Your code here

Hint

Use val to create variables and assign the exact numbers.

2

Create the custom exception class

Create a custom exception class called InsufficientFundsException that inherits from Exception. It should accept a message parameter of type String and pass it to the superclass.

Kotlin

val balance = 1000
val withdrawAmount = 1500

// Create InsufficientFundsException class here
// Your code here

Hint

Define a class with class InsufficientFundsException(message: String) : Exception(message).

3

Write the withdraw function with exception

Write a function called withdraw that takes no parameters. Inside, check if withdrawAmount is greater than balance. If yes, throw InsufficientFundsException with the message "Not enough balance". Otherwise, print "Withdrawal successful".

Kotlin

val balance = 1000
val withdrawAmount = 1500

class InsufficientFundsException(message: String) : Exception(message)

// Write withdraw function here
// Your code here

Hint

Use throw to raise the exception inside the if condition.

4

Call withdraw with try-catch and print error

Use a try-catch block to call the withdraw() function. Catch InsufficientFundsException and print its message.

Kotlin

val balance = 1000
val withdrawAmount = 1500

class InsufficientFundsException(message: String) : Exception(message)

fun withdraw() {
    if (withdrawAmount > balance) {
        throw InsufficientFundsException("Not enough balance")
    } else {
        println("Withdrawal successful")
    }
}

// Use try-catch to call withdraw and print exception message
// Your code here

Hint

Use try { withdraw() } and catch (e: InsufficientFundsException) { println(e.message) }.

Practice

1. What is the main purpose of creating a custom exception class in Kotlin?

easy

A. To speed up program execution

B. To define a specific type of error with a clear name

C. To automatically fix errors in code

D. To replace all built-in exceptions

Solution

1. Step 1: Understand what exceptions represent

   Exceptions represent errors or unexpected events in a program.

2. Step 2: Purpose of custom exceptions

   Custom exceptions let you name specific error types clearly, making code easier to read and debug.

3. Final Answer:

   To define a specific type of error with a clear name -> Option B

4. Quick Check:

   Custom exceptions = clear error types [OK]

Hint: Custom exceptions give clear names to specific errors [OK]

Common Mistakes:

• Thinking custom exceptions fix errors automatically
• Believing they speed up code execution
• Confusing them with replacing built-in exceptions

2. Which of the following is the correct way to declare a custom exception class named MyError in Kotlin?

easy

A. exception MyError extends Exception

B. fun MyError() : Exception

C. class MyError inherits Exception

D. class MyError : Exception()

Solution

1. Step 1: Recall Kotlin class declaration syntax

   In Kotlin, classes are declared with class ClassName.

2. Step 2: Inherit from Exception class

   Use colon : to inherit, so class MyError : Exception() is correct.

3. Final Answer:

   class MyError : Exception() -> Option D

4. Quick Check:

   Custom exception syntax = class Name : Exception() [OK]

Hint: Use colon : to inherit Exception in class declaration [OK]

Common Mistakes:

• Using 'fun' instead of 'class' to declare exception
• Using 'extends' or 'inherits' which are not Kotlin keywords
• Missing parentheses after Exception

3. What will be the output of this Kotlin code?

class MyException : Exception("Custom error")

fun test() {
  throw MyException()
}

fun main() {
  try {
    test()
  } catch (e: MyException) {
    println(e.message)
  }
}

medium

A. Custom error

B. MyException

C. null

D. No output, program crashes

Solution

1. Step 1: Understand the custom exception message

   The class passes "Custom error" to Exception constructor, so e.message is "Custom error".

2. Step 2: Catch block prints the exception message

   The catch block catches MyException and prints e.message, which is "Custom error".

3. Final Answer:

   Custom error -> Option A

4. Quick Check:

   Exception message printed = Custom error [OK]

Hint: Exception message prints from e.message in catch block [OK]

Common Mistakes:

• Expecting class name instead of message
• Thinking program crashes without output
• Assuming message is null if not set explicitly

4. Identify the error in this Kotlin custom exception class:

class MyException extends Exception() {}

medium

A. Class body cannot be empty

B. Missing parentheses after class name

C. Using 'extends' instead of ':' for inheritance

D. Exception class cannot be inherited

Solution

1. Step 1: Check Kotlin inheritance syntax

   Kotlin uses colon ':' for inheritance, not 'extends'.

2. Step 2: Identify the incorrect keyword

   The code uses 'extends' which is from Java, causing syntax error.

3. Final Answer:

   Using 'extends' instead of ':' for inheritance -> Option C

4. Quick Check:

   Kotlin inheritance uses ':' not 'extends' [OK]

Hint: Use ':' not 'extends' for inheritance in Kotlin [OK]

Common Mistakes:

• Confusing Java syntax with Kotlin
• Thinking empty class body is invalid
• Believing Exception cannot be subclassed

5. You want to create a custom exception InvalidAgeException that takes an age value as a property and shows a message like "Invalid age: 15". Which Kotlin code correctly implements this?

hard

A. class InvalidAgeException(val age: Int) : Exception("Invalid age: $age")

B. class InvalidAgeException(age: Int) : Exception("Invalid age: ${age}")

C. class InvalidAgeException(age: Int) : Exception("Invalid age: $age")

D. class InvalidAgeException(age: Int) : Exception("Invalid age: " + age)

Solution

1. Step 1: Understand primary constructor properties and string interpolation

   In Kotlin, val param: Type creates a read-only property accessible as e.age.

2. Step 2: Constructor param available in super call

   Params (with or without val) are in scope for superclass constructor, use $age for interpolation (braces optional for simple identifiers).

3. Step 3: Evaluate options

   A correctly uses val age: Int for property + $age interpolation. B/C/D lack val, so no age property (only private param). D uses concatenation (valid but less idiomatic).

4. Final Answer:

   class InvalidAgeException(val age: Int) : Exception("Invalid age: $age") -> Option A

5. Quick Check:

   val age: Int + $age interpolation for property + message [OK]

Hint: Use val age: Int and $age for property + message interpolation [OK]

Common Mistakes:

• Omitting 'val' (no public age property)
• Using unnecessary braces ${age} (optional for simple vars)
• Using string concatenation instead of interpolation