Try-catch as an expression in Kotlin - Step-by-Step Execution

Concept Flow - Try-catch as an expression

Start

↓

Try block runs

↓

No Exception?

Yes→Try result used

↓

End

↓

Exception?

Yes→Catch block runs

↓

Catch result used

↓

End

The try block runs first. If no exception occurs, its result is used. If an exception happens, the catch block runs and its result is used instead.

Execution Sample

Kotlin

val result = try {
    "123".toInt()
} catch (e: NumberFormatException) {
    -1
}
println(result)

This code tries to convert a string to an integer. If it fails, it catches the exception and returns -1 instead.

Execution Table

Step	Action	Evaluation	Result
1	Execute try block: "123".toInt()	No exception	123
2	Assign try block result to 'result'	result = 123	123
3	Print 'result'	Output	123

💡 Try block succeeded, no exception thrown, result is 123

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
result	uninitialized	uninitialized	123	123

Key Moments - 2 Insights

Why does the catch block not run when the string is "123"?

Because the try block converts "123" to integer successfully without throwing an exception, as shown in execution_table step 1.

What happens if the string cannot be converted to an integer?

The try block throws a NumberFormatException, so the catch block runs and returns -1 instead, replacing the try block result.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 2?

A 123

B -1

C uninitialized

D Exception

💡 Hint

Check the 'Assign try block result to result' row in the execution_table.

At which step does the program print the output?

A Step 1

B Step 3

C Step 2

D No print occurs

💡 Hint

Look for the 'Print result' action in the execution_table.

If the string was "abc" instead of "123", what would happen?

A Program crashes without catch

B Try block returns "abc"

C Catch block runs and result is -1

D Result remains uninitialized

💡 Hint

Recall that "abc" cannot convert to Int, so exception triggers catch block.

Concept Snapshot

Try-catch in Kotlin can be used as an expression.
Syntax:
val result = try { /* code */ } catch(e: Exception) { /* fallback */ }
If try succeeds, its value is used.
If try throws, catch value is used instead.
This allows safe assignment with error handling in one expression.

Full Transcript

In Kotlin, try-catch can be used as an expression that returns a value. The try block runs first. If it completes without error, its result is assigned. If an exception occurs, the catch block runs and its result is assigned instead. For example, converting a string to an integer inside try returns the number if successful. If conversion fails, catch returns -1. This way, you can safely assign a value with error handling in one line. The execution table shows each step: running try, assigning result, and printing output. Variables track the value changes. Key moments clarify why catch runs only on exceptions. The quiz tests understanding of these steps and outcomes.