RunCatching for safe execution in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this RunCatching example?

Consider this Kotlin code using runCatching. What will it print?

Kotlin

val result = runCatching {
    val number = "123a".toInt()
    number * 2
}.getOrElse {
    -1
}
println(result)

A Exception in thread "main" java.lang.NumberFormatException

B 0

C -1

D 246

💡 Hint

Think about what happens when toInt() fails inside runCatching.

The string "123a" cannot be converted to an integer, so toInt() throws a NumberFormatException. The runCatching catches this exception, and getOrElse returns the fallback value -1.

❓ Predict Output

intermediate

What does this runCatching block return?

Analyze this Kotlin snippet using runCatching. What is the value of output?

Kotlin

val output = runCatching {
    val list = listOf(1, 2, 3)
    list[5]
}.getOrNull()
println(output)

A IndexOutOfBoundsException

B null

C 3

D 5

💡 Hint

What happens when you access an invalid index inside runCatching?

Accessing list[5] throws an IndexOutOfBoundsException. runCatching catches it, so getOrNull() returns null.

🔧 Debug

advanced

Why does this runCatching block not catch the exception?

Look at this Kotlin code. Why does the exception still crash the program?

Kotlin

val result = runCatching {
    throw IllegalStateException("Error happened")
}.getOrElse {
    throw it
}
println("Done")

A Because getOrElse rethrows the exception, so it is not handled.

B Because println("Done") is unreachable and causes a compile error.

C Because the exception is thrown outside runCatching block.

D Because runCatching only catches IOException, not IllegalStateException.

💡 Hint

Check what getOrElse does with the caught exception.

The runCatching catches the exception, but getOrElse rethrows it with throw it. So the exception is not handled and crashes the program.

📝 Syntax

advanced

Which option correctly uses runCatching to safely parse an integer?

Choose the Kotlin code snippet that safely parses a string to an integer using runCatching and returns 0 if parsing fails.

A val number = runCatching { "42".toInt() }.getOrElse { 0 }

B val number = runCatching { "42".toInt() } ?: 0

C val number = runCatching { "42".toInt() }.getOrDefault(0)

D val number = runCatching { "42".toInt() }.getOrNull() ?: 0

💡 Hint

Remember how getOrElse works compared to getOrDefault and getOrNull.

getOrElse returns the result if successful, or the lambda value if an exception occurred. ?: operator does not work directly on Result. getOrNull returns null on failure, so the Elvis operator works but returns null if success is null.

🚀 Application

expert

How many items are in the list after this runCatching chain?

Given this Kotlin code, how many items does results contain after execution?

Kotlin

val inputs = listOf("10", "abc", "20", "-5")
val results = inputs.mapNotNull { input ->
    runCatching { input.toInt() }
        .getOrNull()
}.filter { it > 0 }

A 3

B 1

C 4

D 2

💡 Hint

Count how many strings convert to positive integers successfully.

"10" and "20" convert successfully to positive integers. "abc" fails and returns null, "-5" converts but is filtered out because it is not > 0. So the final list has 2 items.

Practice

1. What is the main purpose of runCatching in Kotlin?

easy

A. To execute code and catch exceptions safely without crashing the program

B. To run code only if a condition is true

C. To create a new thread for running code

D. To convert a value to a string safely

Solution

1. Step 1: Understand what runCatching does

   runCatching runs a block of code and catches any exceptions that happen inside it.

2. Step 2: Identify the main purpose

   This helps keep the program running smoothly by handling errors safely without crashing.

3. Final Answer:

   To execute code and catch exceptions safely without crashing the program -> Option A

4. Quick Check:

   runCatching = safe execution [OK]

Hint: runCatching catches errors to keep code safe [OK]

Common Mistakes:

• Thinking runCatching runs code conditionally
• Confusing runCatching with threading
• Assuming it converts types automatically

2. Which of the following is the correct syntax to use runCatching in Kotlin?

easy

A. val result = runCatching -> someFunction()

B. val result = runCatching(someFunction())

C. val result = runCatching { someFunction() }

D. val result = runCatching.someFunction()

Solution

1. Step 1: Recall the syntax of runCatching

   runCatching takes a lambda block with curly braces to run code safely.

2. Step 2: Match the correct syntax

   val result = runCatching { someFunction() } uses curly braces and calls someFunction() inside, which is correct.

3. Final Answer:

   val result = runCatching { someFunction() } -> Option C

4. Quick Check:

   runCatching uses braces { } [OK]

Hint: Use curly braces {} to pass code block to runCatching [OK]

Common Mistakes:

• Using parentheses instead of braces
• Trying to call runCatching like a function with arrow
• Accessing runCatching as an object

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

val result = runCatching { 10 / 0 }
println(result.isSuccess)

medium

A. true

B. null

C. Exception thrown

D. false

Solution

1. Step 1: Analyze the code inside runCatching

   The code tries to divide 10 by 0, which causes an exception (ArithmeticException).

2. Step 2: Understand runCatching result

   runCatching catches the exception, so no crash happens. The isSuccess property will be false because the block failed.

3. Final Answer:

   false -> Option D

4. Quick Check:

   Exception caught means isSuccess = false [OK]

Hint: Divide by zero causes failure, so isSuccess is false [OK]

Common Mistakes:

• Expecting true because code runs
• Thinking exception is thrown outside runCatching
• Confusing isSuccess with isFailure

4. Identify the error in this Kotlin code using runCatching:

val result = runCatching {
    val number = "abc".toInt()
}
println(result.getOrNull())

medium

A. No error, prints null because conversion fails

B. Throws NumberFormatException outside runCatching

C. Syntax error in runCatching block

D. getOrNull() returns the string "abc"

Solution

1. Step 1: Understand the code inside runCatching

   Trying to convert "abc" to Int causes NumberFormatException, but runCatching catches it.

2. Step 2: Check what getOrNull() returns

   Since exception happened, getOrNull() returns null safely without crashing.

3. Final Answer:

   No error, prints null because conversion fails -> Option A

4. Quick Check:

   Exception caught means getOrNull() = null [OK]

Hint: runCatching catches exceptions, getOrNull returns null on failure [OK]

Common Mistakes:

• Expecting exception to crash program
• Thinking getOrNull returns original string
• Assuming syntax error in code block

5. You want to safely parse a list of strings to integers using runCatching and collect only the successful results. Which Kotlin code snippet correctly does this?

hard

A. val numbers = list.map { runCatching { it.toInt() }.exceptionOrNull() }

B. val numbers = list.map { runCatching { it.toInt() }.getOrNull() }.filterNotNull()

C. val numbers = list.map { it.toInt() }.filter { it != null }

D. val numbers = list.map { runCatching { it.toInt() }.getOrThrow() }

Solution

1. Step 1: Understand the goal

   We want to convert strings to integers safely, ignoring failures, and collect only successful integers.

2. Step 2: Analyze each option

   val numbers = list.map { runCatching { it.toInt() }.getOrNull() }.filterNotNull() uses runCatching to catch exceptions, then getOrNull() returns integer or null, and filterNotNull() removes nulls, collecting only successful parses.

3. Final Answer:

   val numbers = list.map { runCatching { it.toInt() }.getOrNull() }.filterNotNull() -> Option B

4. Quick Check:

   Safe parse + filter null = val numbers = list.map { runCatching { it.toInt() }.getOrNull() }.filterNotNull() [OK]

Hint: Use getOrNull() then filterNotNull() to keep successful parses [OK]

Common Mistakes:

• Using getOrThrow() which throws on failure
• Filtering nulls before catching exceptions
• Using exceptionOrNull() which returns exceptions, not values