RunCatching for safe execution in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is RunCatching in Kotlin?

runCatching is a Kotlin function that helps you run code safely by catching exceptions automatically. It wraps the code block and returns a Result object that holds either the success value or the exception.

beginner

How do you get the successful result from a RunCatching block?

You can use getOrNull() to get the result if it succeeded, or null if there was an exception. Alternatively, use getOrElse { defaultValue } to provide a fallback value.

intermediate

What does onFailure do when used with RunCatching?

onFailure lets you run code only if the runCatching block caught an exception. It is useful for logging errors or handling failures without stopping the program.

beginner

Write a simple example of RunCatching that safely divides two numbers and returns 0 if division fails.

val result = runCatching { 10 / 0 }.getOrElse { 0 }
println(result) // Output: 0

intermediate

Why is RunCatching preferred over traditional try-catch blocks?

runCatching makes code cleaner and more readable by wrapping try-catch logic into a single expression. It also provides useful functions to handle success and failure in a functional style.

What does runCatching return after executing a block?

A A Result object containing success or failure

B The raw value or throws exception

C Always null

D Boolean true or false

runCatching returns a Result object that holds either the successful value or the caught exception.

Which function gets the value or returns a default if there was an exception?

A getOrNull()

B getOrElse { default }

C onFailure { }

D runCatching { }

getOrElse { default } returns the success value or the default if an exception occurred.

How can you run code only when runCatching catches an exception?

A Using onSuccess { }

B Using getOrElse { }

C Using onFailure { }

D Using getOrNull()

onFailure { } runs only if the block caught an exception.

What will runCatching { 5 / 0 }.getOrNull() return?

A null

B 0

C 5

D Throws exception

Division by zero throws an exception, so getOrNull() returns null.

Which is NOT a benefit of using runCatching?

A Cleaner code than try-catch

B Functional style error handling

C Easy chaining of success/failure handlers

D Automatically fixes bugs

runCatching does not fix bugs automatically; it only helps handle exceptions safely.

Explain how runCatching helps in safe execution of code in Kotlin.

Think about how it replaces try-catch and what it returns.

Describe how you can handle both success and failure cases using runCatching.

Consider the functions available on the Result object.

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