Nullable types with ? suffix in Kotlin - Step-by-Step Execution

Concept Flow - Nullable types with ? suffix

Declare variable with ?

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Variable can hold value or null

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Check if variable is null

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Handle null

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Continue

This flow shows how a variable declared with ? can hold a value or null, and how the program checks and handles null before using the variable.

Execution Sample

Kotlin

var name: String? = "Alice"
println(name?.length)
name = null
println(name?.length)

This code declares a nullable String, prints its length safely, then sets it to null and prints length again safely.

Execution Table

Step	Variable 'name'	Operation	Result	Output
1	undefined	Declare nullable String 'name' and assign "Alice"	name = "Alice"
2	"Alice"	Print length safely using name?.length	length = 5	5
3	"Alice"	Assign null to 'name'	name = null
4	null	Print length safely using name?.length	length = null (safe call)	null
5	null	End of program	-

💡 Program ends after safely printing length of nullable variable twice.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
name	undefined	"Alice"	"Alice"	null	null	null

Key Moments - 2 Insights

Why do we use the ?. operator when accessing length?

Because 'name' can be null, using ?. safely returns null instead of causing an error. See execution_table step 4 where name is null and length is safely null.

What happens if we try to access length without ? when name is null?

It causes a runtime error (NullPointerException). The ?. operator prevents this by checking for null first, as shown in steps 2 and 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the output when name is "Alice"?

A Error

B null

C 5

D 0

💡 Hint

Check the Output column at step 2 in execution_table.

At which step does the variable 'name' become null?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the Variable 'name' column in execution_table to see when it changes to null.

If we remove the ? from name?.length in step 4, what would happen?

A Output would be 0

B Program would crash with error

C Output would be null

D Output would be "null" string

💡 Hint

Recall key_moments about safe calls preventing errors when variable is null.

Concept Snapshot

Nullable types use ? after type name, e.g., String?.
Variables can hold a value or null.
Use ?. to safely access properties or methods.
Without ?, accessing null causes error.
Always check or use safe calls with nullable types.

Full Transcript

In Kotlin, adding a ? after a type means the variable can hold a value or null. For example, var name: String? means name can be a String or null. When using such variables, you must be careful to avoid errors. The ?. operator lets you safely access properties or methods; if the variable is null, it returns null instead of crashing. In the example, name starts as "Alice" and printing name?.length gives 5. After setting name to null, printing name?.length safely returns null without error. This helps prevent crashes from null values.