Elvis operator (?:) for default values in Kotlin - Step-by-Step Execution

Concept Flow - Elvis operator (?:) for default values

Evaluate expression on left

↓

Is it null?

No→Use left value

Yes↓

Use right default value

The Elvis operator checks if the left expression is null; if not, it uses that value, otherwise it uses the right default value.

Execution Sample

Kotlin

val name: String? = null
val displayName = name ?: "Guest"
println(displayName)

This code assigns a default value "Guest" if name is null, then prints the result.

Execution Table

Step	Expression	Evaluation	Result	Action
1	name	null	null	Check if null
2	name ?: "Guest"	null ?: "Guest"	"Guest"	Use right default value
3	println(displayName)	"Guest"	Output: Guest	Print the result

💡 Execution ends after printing the default value because name was null

Variable Tracker

Variable	Start	After Elvis operator	Final
name	null	null	null
displayName	uninitialized	"Guest"	"Guest"

Key Moments - 2 Insights

Why does displayName become "Guest" when name is null?

Because the Elvis operator ?: checks if name is null (row 1), and since it is, it uses the right side "Guest" as the default value (row 2).

What happens if name is not null?

If name is not null, the Elvis operator returns the left value directly, skipping the default (not shown here but implied by the flow).

Visual Quiz - 3 Questions

Look at the execution table at step 2, what value does the Elvis operator produce?

A "Guest"

B null

C name

D Error

💡 Hint

Check the 'Result' column in step 2 of the execution_table

At which step does the program print the output?

A Step 1

B Step 2

C Step 3

D No print step

💡 Hint

Look for println in the 'Expression' column in execution_table

If name was "Alice", what would displayName be after the Elvis operator?

A "Guest"

B "Alice"

C null

D Error

💡 Hint

Recall Elvis operator returns left value if not null (concept_flow)

Concept Snapshot

Elvis operator ?: returns left value if not null,
otherwise returns right default value.
Syntax: val result = expr ?: default
Useful for providing fallback values
Avoids explicit null checks.

Full Transcript

The Elvis operator ?: in Kotlin helps assign default values when a variable might be null. It first checks the left expression; if it is not null, it uses that value. If it is null, it uses the right side as a default. In the example, name is null, so displayName becomes "Guest". The program then prints "Guest". If name had a value like "Alice", displayName would be "Alice". This operator simplifies code by avoiding manual null checks.