Non-nullable types by default in Kotlin - Step-by-Step Execution

Concept Flow - Non-nullable types by default

Declare variable

↓

Type is non-nullable by default

↓

Assign non-null value

Yes→Works fine

No↓

Compile error: Null not allowed

↓

Use nullable type with ?

↓

Can assign null or non-null value

Kotlin variables are non-nullable by default, so you cannot assign null unless you explicitly allow it with a nullable type.

Execution Sample

Kotlin

var name: String = "Alice"
// name = null // Error: Null not allowed
var nullableName: String? = "Bob"
nullableName = null

This code shows a non-nullable String variable and a nullable String? variable, demonstrating assignment rules.

Execution Table

Step	Variable	Type	Assigned Value	Allowed?	Notes
1	name	String	"Alice"	Yes	Non-nullable variable assigned non-null value
2	name	String	null	No	Compile error: cannot assign null to non-nullable
3	nullableName	String?	"Bob"	Yes	Nullable variable assigned non-null value
4	nullableName	String?	null	Yes	Nullable variable assigned null value

💡 Execution stops because assigning null to non-nullable variable causes compile error

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
name	uninitialized	"Alice"	error (null assign)	error (null assign)	error (null assign)
nullableName	uninitialized	uninitialized	uninitialized	"Bob"	null

Key Moments - 2 Insights

Why does assigning null to 'name' cause an error?

Because 'name' is declared as a non-nullable String (see execution_table step 2), Kotlin does not allow null assignment to prevent null pointer errors.

How can a variable hold null in Kotlin?

By declaring the variable type with a question mark (String?), it becomes nullable and can hold null values (see execution_table steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what happens when we assign null to 'name' at step 2?

A It works and 'name' becomes null

B Compile error occurs

C 'name' becomes empty string

D Program crashes at runtime

💡 Hint

Check execution_table row for step 2 under 'Allowed?' and 'Notes'

At which step does 'nullableName' first hold a null value?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look at execution_table rows for 'nullableName' and see when 'Assigned Value' is null

If we remove the question mark from 'nullableName' type, what changes in the execution table?

A 'nullableName' can still hold null without error

B 'nullableName' becomes an integer

C Assigning null to 'nullableName' would cause a compile error

D No change in behavior

💡 Hint

Refer to key_moments about nullable types and null assignment

Concept Snapshot

Kotlin variables are non-nullable by default.
You cannot assign null to a non-nullable type.
Use '?' after type to allow null (nullable type).
Nullable variables can hold null or non-null values.
This helps avoid null pointer errors at runtime.

Full Transcript

In Kotlin, variables are non-nullable by default, meaning you cannot assign null to them. For example, a variable declared as 'var name: String' must always hold a non-null string. If you try to assign null to it, the compiler will show an error and stop the program from compiling. To allow a variable to hold null, you must declare it as nullable by adding a question mark after the type, like 'var nullableName: String?'. This variable can then hold either a string or null safely. This design helps prevent null pointer exceptions by catching null assignments at compile time.