When as expression returning value in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a when expression in Kotlin?

A when expression in Kotlin is a control structure that checks a value against multiple conditions and returns a result based on the first matching condition.

beginner

How do you use when as an expression to return a value?

You assign the when expression to a variable. Each branch returns a value, and the whole when expression evaluates to that value.

intermediate

What happens if no branch matches in a when expression used as a value?

If no branch matches, and there is no else branch, the code will not compile. You must provide an else branch to cover all cases.

intermediate

Can when expression branches return different types?

No. All branches must return values of the same type or compatible types so the when expression has a consistent return type.

beginner

Example: What is the value of result after this code?<br>

val x = 2
val result = when (x) {
  1 -> "One"
  2 -> "Two"
  else -> "Other"
}

The value of result is "Two" because the when expression matches the case 2 and returns "Two".

What must a when expression used as a value always include?

A A <code>break</code> statement

B A loop inside each branch

C A <code>return</code> keyword in each branch

D An <code>else</code> branch to cover unmatched cases

When used as an expression, when must cover all cases, usually by including an else branch.

What type of value does a when expression return?

A The type common to all branch results

B The type of the first branch only

C Always a String

D No value, it returns Unit

The when expression returns a value whose type matches all branch results.

What happens if you omit the else branch in a when expression used as a value?

A The program compiles but throws an error at runtime

B The program does not compile

C The first branch is used as default

D The program compiles and returns null

Without an else branch, the compiler cannot guarantee all cases are covered, so it does not compile.

Which of these is a valid way to assign a value using when expression?

A val result = when (x) { 1: 10, 2: 20 }

B val result = when (x) { 1 -> 10; 2 -> 20 }

C val result = when (x) { 1 -> 10 2 -> 20 else -> 0 }

D val result = when (x) { 1 => 10, 2 => 20 }

The correct syntax uses arrows (->) and includes an else branch to cover all cases.

Can a when expression be used without an argument?

A Yes, it can check arbitrary conditions in branches

B No, it always requires an argument

C Only if used inside a function

D Only with numeric arguments

A when expression can be used without an argument to check conditions directly in branches.

Explain how a when expression returns a value in Kotlin and why the else branch is important.

Think about how the program decides what value to give back.

Describe the rules for the types of values returned by each branch in a when expression used as a value.

Consider what happens if branches return different kinds of values.