Try-catch as an expression in Kotlin - Mini Project: Build & Apply

Try-catch as an expression

📖 Scenario: Imagine you are writing a small Kotlin program that tries to convert user input into a number. Sometimes the input might not be a valid number, so you want to handle that error smoothly.

🎯 Goal: You will build a Kotlin program that uses try-catch as an expression to safely convert a string to an integer. If the conversion fails, it will return a default value.

📋 What You'll Learn

Create a variable with a string that might not be a number

Create a default integer value for fallback

Use try-catch as an expression to convert the string to an integer or use the default

Print the final integer value

💡 Why This Matters

🌍 Real World

Handling user input or data from files where the format might be incorrect is common. Using try-catch as an expression helps keep code clean and safe.

💼 Career

Many Kotlin jobs require safe error handling and working with user input or external data. This skill is essential for writing robust applications.

1

Create the input string

Create a variable called input and set it to the string "abc123".

Kotlin

# Create a variable called input and set it to "abc123"
# Your code here

Hint

Use val input = "abc123" to create the string variable.

2

Create the default fallback value

Create a variable called defaultValue and set it to the integer 0.

Kotlin

val input = "abc123"
# Create a variable called defaultValue and set it to 0
# Your code here

Hint

Use val defaultValue = 0 to create the fallback integer.

3

Use try-catch as an expression to convert input

Create a variable called number and set it to the result of a try-catch expression that tries to convert input to an integer using input.toInt(). If an exception occurs, return defaultValue.

Kotlin

val input = "abc123"
val defaultValue = 0
# Use try-catch as an expression to convert input to an integer or use defaultValue
# Your code here

Hint

Use val number = try { input.toInt() } catch (e: Exception) { defaultValue } to handle conversion safely.

4

Print the result

Write a println statement to display the value of number.

Kotlin

val input = "abc123"
val defaultValue = 0
val number = try {
    input.toInt()
} catch (e: Exception) {
    defaultValue
}
# Print the value of number
# Your code here

Hint

Use println(number) to show the final integer.

Practice

1. What does it mean that try-catch is an expression in Kotlin?

easy

A. It can only be used inside functions with no return type.

B. It only catches exceptions but cannot return values.

C. It must always be followed by a finally block.

D. It can return a value that can be assigned to a variable.

Solution

1. Step 1: Understand try-catch as an expression

   In Kotlin, try-catch can produce a value, unlike in some languages where it is only a statement.

2. Step 2: Recognize value assignment

   This means you can assign the result of a try-catch block directly to a variable.

3. Final Answer:

   It can return a value that can be assigned to a variable. -> Option D

4. Quick Check:

   Try-catch returns value = D [OK]

Hint: Try-catch can be assigned like any expression [OK]

Common Mistakes:

• Thinking try-catch cannot return values
• Confusing try-catch with only statements
• Believing finally is always required

2. Which of the following is the correct syntax to assign a value using try-catch as an expression in Kotlin?

easy

A. val result = try { 10 / 0 } catch (e: Exception) { 0 }

B. val result try { 10 / 0 } catch (e: Exception) { 0 }

C. val result = try { 10 / 0 } finally { 0 }

D. val result = catch (e: Exception) { 0 } try { 10 / 0 }

Solution

1. Step 1: Check assignment syntax

   The correct way to assign a value from try-catch is using val result = try { ... } catch { ... }.

2. Step 2: Validate each option

   val result = try { 10 / 0 } catch (e: Exception) { 0 } uses correct syntax with try-catch returning a value. Options A, B, and D have syntax errors or misuse of finally/catch.

3. Final Answer:

   val result = try { 10 / 0 } catch (e: Exception) { 0 } -> Option A

4. Quick Check:

   Correct try-catch assignment syntax = C [OK]

Hint: Remember: val x = try { } catch { } [OK]

Common Mistakes:

• Omitting '=' in assignment
• Using finally block to return value
• Placing catch before try

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

val result = try {
    val num = 10 / 2
    "Success: $num"
} catch (e: Exception) {
    "Error"
}
println(result)

medium

A. Error

B. Success: 5

C. 10 / 2

D. Exception thrown

Solution

1. Step 1: Evaluate try block

   The division 10 / 2 equals 5, so the try block returns the string "Success: 5".

2. Step 2: Check for exceptions

   No exception occurs, so the catch block is skipped.

3. Final Answer:

   Success: 5 -> Option B

4. Quick Check:

   Try succeeds, output = Success: 5 [OK]

Hint: If no error, try block value is returned [OK]

Common Mistakes:

• Assuming catch runs even without error
• Printing the expression instead of result
• Confusing integer division with string output

4. Identify the error in this Kotlin code using try-catch as an expression:

val result = try {
    val x = 10 / 0
    x
} catch {
    0
}
println(result)

medium

A. Cannot assign try-catch to a val

B. Division by zero is allowed without catch

C. Missing exception parameter in catch block

D. println cannot print variables from try-catch

Solution

1. Step 1: Check catch block syntax

   The catch block must declare an exception parameter like catch (e: Exception).

2. Step 2: Identify the error

   Here, catch is missing the exception parameter, causing a syntax error.

3. Final Answer:

   Missing exception parameter in catch block -> Option C

4. Quick Check:

   Catch needs exception parameter = B [OK]

Hint: Always include exception variable in catch (e: Exception) [OK]

Common Mistakes:

• Omitting exception parameter in catch
• Ignoring division by zero error
• Thinking try-catch can't be assigned

5. Given this Kotlin function using try-catch as an expression, what will parseIntOrZero("abc") return?

fun parseIntOrZero(input: String): Int = try {
    input.toInt()
} catch (e: NumberFormatException) {
    0
}

hard

A. Returns 0

B. Throws NumberFormatException

C. Returns null

D. Returns input string

Solution

1. Step 1: Analyze try block behavior

   Calling input.toInt() on "abc" throws NumberFormatException.

2. Step 2: Catch block returns fallback

   The catch block catches this exception and returns 0 as fallback.

3. Final Answer:

   Returns 0 -> Option A

4. Quick Check:

   Exception caught, fallback 0 returned = A [OK]

Hint: Catch returns fallback value on error [OK]

Common Mistakes:

• Expecting exception to propagate
• Thinking null is returned on error
• Confusing return type with input type