How can this code throw a NoWhenBranchMatchedException?

How Can This Code Throw a NoWhenBranchMatchedException?

Understanding NoWhenBranchMatchedException

The NoWhenBranchMatchedException is an exception thrown by Kotlin’s `when` expression when none of the provided branches match the subject of the expression. This typically occurs when you have exhaustive `when` branches, meaning you’ve explicitly handled all possible values, but the runtime value doesn’t match any of them. Let’s explore scenarios where this exception might arise and how to avoid it.

Scenarios Leading to NoWhenBranchMatchedException

• Missing Case: If your `when` expression doesn’t cover all possible values, you might encounter this exception.
• Type Mismatch: When the type of the subject of the `when` expression doesn’t match the types specified in the branches.
• Unexpected Input: If the code receives unexpected input or data that isn’t handled in your `when` expression, the exception can occur.

Example Code

fun processStatus(status: Int): String {
    when (status) {
        1 -> return "Active"
        2 -> return "Inactive"
        3 -> return "Pending"
    }
    throw NoWhenBranchMatchedException()
}

fun main() {
    println(processStatus(4))
}

Exception in thread "main" kotlin.NoWhenBranchMatchedException
	at MainKt.processStatus(Main.kt:6)
	at MainKt.main(Main.kt:9)

In this example, the processStatus function uses a `when` expression to handle status codes. The code throws a NoWhenBranchMatchedException because the function is called with the input `4`, which doesn’t have a corresponding branch in the `when` expression.

How to Avoid NoWhenBranchMatchedException

• Handle All Cases: Ensure that your `when` expression covers all possible values. This often involves adding a default or “else” branch.
• Use `else` Branch: Incorporate an `else` branch to handle situations where no other branch matches. This is a common and robust approach to prevent the exception.
• Type Checking: Validate the type of the input before using it in your `when` expression. This prevents potential type mismatches.
• Input Validation: Implement input validation to ensure that the received data adheres to the expected format and range.

Example with an Else Branch

fun processStatus(status: Int): String {
    when (status) {
        1 -> return "Active"
        2 -> return "Inactive"
        3 -> return "Pending"
        else -> return "Unknown"
    }
}

Unknown

By adding an `else` branch, we now handle the input `4`, returning “Unknown.” The exception is avoided because the `else` branch acts as a fallback.

Table Comparing Approaches

when (status) {
    1 -> "Active"
    2 -> "Inactive"
}

when (status) {
    1 -> "Active"
    2 -> "Inactive"
    else -> "Unknown"
}

Conclusion

The NoWhenBranchMatchedException can indicate an oversight in your code where you’re not explicitly handling all potential values. By incorporating the best practices described above, you can mitigate the risk of encountering this exception and ensure your code handles all input scenarios gracefully.