Throw as an expression in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to throw an exception when the number is negative.

Kotlin

val number = -5
val result = if (number >= 0) number else throw [1]("Negative number")

Drag options to blanks, or click blank then click option'

AExceptionHandler

Bprintln

Creturn

DIllegalArgumentException

Attempts:

3 left

💡 Hint

Common Mistakes

Using println instead of throwing an exception

Trying to return a value instead of throwing

Using a non-exception class

In Kotlin, you throw an exception using the 'throw' keyword followed by an exception class instance like IllegalArgumentException.

2fill in blank

medium

Complete the code to throw an exception as an expression inside a function.

Kotlin

fun getPositiveNumber(number: Int): Int = if (number > 0) number else throw [1]("Number must be positive")

Drag options to blanks, or click blank then click option'

AError

BRuntimeException

CException

Dprintln

Attempts:

3 left

💡 Hint

Common Mistakes

Using println instead of throw

Using Error which is for serious system errors

Using Exception which is a checked exception in Java but less common in Kotlin

Throwing RuntimeException is a common way to signal an error condition in Kotlin expressions.

3fill in blank

hard

Fix the error in the code by completing the throw expression correctly.

Kotlin

val value = 10
val result = if (value < 0) throw [1]("Negative value") else value

Drag options to blanks, or click blank then click option'

AIllegalStateException

Bthrow

Creturn

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using print instead of throw

Using return inside an expression

Trying to throw the throw keyword itself

You must throw an exception object like IllegalStateException, not use print or return in this context.

4fill in blank

hard

Fill both blanks to throw an exception with a custom message when the input is zero.

Kotlin

fun checkInput(input: Int): Int {
    return if (input != 0) input else throw [1]("Input cannot be zero: " + [2])
}

Drag options to blanks, or click blank then click option'

AIllegalArgumentException

Binput

Cnumber

DRuntimeException

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong variable name for the message

Throwing a generic RuntimeException without message

Not including the input value in the message

Throw IllegalArgumentException with the input value to show the error message clearly.

5fill in blank

hard

Fill all three blanks to throw an exception as an expression inside a map operation.

Kotlin

val numbers = listOf(1, 2, 0, 4)
val results = numbers.mapIndexed { [2], n ->
    if (n != 0) n else throw [1]("Zero found at index " + [2])
}.mapIndexed { [3], value -> value * 2 }

Drag options to blanks, or click blank then click option'

AIllegalStateException

Bn

Cindex

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names for index

Throwing generic exceptions without message

Confusing variable names in mapIndexed

Throw IllegalStateException with the index variable in the message, and use 'i' as the index variable in mapIndexed.

Practice

1.

What does the throw keyword do when used as an expression in Kotlin?

easy

A. It declares a new variable.

B. It returns a value from a function.

C. It immediately stops execution and throws an exception.

D. It creates a new thread.

Solution

1. Step 1: Understand the role of throw

   The throw keyword is used to signal an error by throwing an exception.

2. Step 2: Recognize its effect in expressions

   When used as an expression, throw stops the current execution and throws the specified exception immediately.

3. Final Answer:

   It immediately stops execution and throws an exception. -> Option C

4. Quick Check:

   throw = stops and throws error [OK]

Hint: Throw always stops execution and signals an error [OK]

Common Mistakes:

• Thinking throw returns a normal value
• Confusing throw with return
• Assuming throw creates variables

2.

Which of the following is the correct way to use throw as an expression in Kotlin?

val number = if (input > 0) input else ???

easy

A. throw IllegalArgumentException("Input must be positive")

B. return IllegalArgumentException("Input must be positive")

C. println("Input must be positive")

D. throw new Exception("Input must be positive")

Solution

1. Step 1: Check Kotlin syntax for throw expression

   In Kotlin, throw is used without new keyword and can be used as an expression.

2. Step 2: Match the correct usage in the if-else expression

   The correct syntax is throw IllegalArgumentException("message") without new and it fits as an expression in else branch.

3. Final Answer:

   throw IllegalArgumentException("Input must be positive") -> Option A

4. Quick Check:

   Throw expression syntax = throw IllegalArgumentException("Input must be positive") [OK]

Hint: Use throw without 'new' keyword in Kotlin [OK]

Common Mistakes:

• Using 'new' keyword with throw
• Using return instead of throw
• Trying to print error instead of throwing

3.

What will be the output of this Kotlin code?

fun getName(name: String?): String {
    return name ?: throw IllegalArgumentException("Name required")
}

fun main() {
    println(getName(null))
}

medium

A. Compilation error

B. Name required

C. null

D. Exception in thread "main" java.lang.IllegalArgumentException: Name required

Solution

1. Step 1: Analyze the Elvis operator with throw

   The expression name ?: throw IllegalArgumentException("Name required") returns name if not null, else throws the exception.

2. Step 2: Check the input and output

   Since getName(null) is called, name is null, so the exception is thrown and program crashes with that message.

3. Final Answer:

   Exception in thread "main" java.lang.IllegalArgumentException: Name required -> Option D

4. Quick Check:

   Throw in Elvis operator throws exception [OK]

Hint: Elvis ?: with throw throws exception if left is null [OK]

Common Mistakes:

• Expecting null to print
• Thinking exception message prints normally
• Assuming code compiles but returns empty string

4.

Identify the error in this Kotlin code snippet:

val age = 20
val validAge = if (age >= 18) age else throw "Age must be 18 or older"

medium

A. Cannot throw a String, must throw an Exception object.

B. The if expression is missing braces.

C. The variable age should be nullable.

D. The else branch should return a value, not throw.

Solution

1. Step 1: Check the throw expression type

   In Kotlin, throw must throw an object of type Throwable, not a plain String.

2. Step 2: Identify the error in else branch

   The code tries to throw a String literal, which is invalid and causes a compilation error.

3. Final Answer:

   Cannot throw a String, must throw an Exception object. -> Option A

4. Quick Check:

   Throw must be Exception, not String [OK]

Hint: Throw only Exception objects, not strings [OK]

Common Mistakes:

• Throwing plain strings instead of exceptions
• Missing braces but code compiles anyway
• Confusing throw with return

5.

Given this Kotlin function:

fun parseNumber(input: String?): Int {
    val number = input?.toIntOrNull() ?: throw NumberFormatException("Invalid number")
    return number
}

What will parseNumber(null) do, and why is throw used as an expression here?

hard

A. Compiles but causes runtime error unrelated to throw.

B. Throws NumberFormatException because input is null; throw used to combine error and assignment.

C. Returns null because input is nullable; throw is a statement here.

D. Returns 0 because null converts to zero; throw is ignored.

Solution

1. Step 1: Understand the Elvis operator with throw

   The expression input?.toIntOrNull() ?: throw NumberFormatException("Invalid number") tries to convert input to Int or throws if null or invalid.

2. Step 2: Analyze behavior when input is null

   Since input is null, toIntOrNull() returns null, so the throw expression executes, throwing the exception immediately.

3. Step 3: Reason why throw is used as expression

   Using throw as expression allows combining error handling with assignment in one line, making code concise and readable.

4. Final Answer:

   Throws NumberFormatException because input is null; throw used to combine error and assignment. -> Option B

5. Quick Check:

   Throw expression combines error and value assignment [OK]

Hint: Throw as expression lets you handle errors inline [OK]

Common Mistakes:

• Thinking null converts to zero
• Confusing throw as statement only
• Ignoring exception thrown on null input