Explicit type declaration in Kotlin - Step-by-Step Execution

Concept Flow - Explicit type declaration

Start

↓

Declare variable with type

↓

Assign value matching type

↓

Use variable

↓

End

This flow shows declaring a variable with a specific type, assigning a matching value, then using it.

Execution Sample

Kotlin

var name: String
name = "Alice"
var age: Int
age = 30
println("Name: $name, Age: $age")

Declares variables with explicit types and prints their values.

Execution Table

Step	Action	Variable	Type	Value	Output
1	Declare 'name' with type String	name	String	uninitialized
2	Assign "Alice" to 'name'	name	String	"Alice"
3	Declare 'age' with type Int	age	Int	uninitialized
4	Assign 30 to 'age'	age	Int	30
5	Print values				Name: Alice, Age: 30

💡 Program ends after printing the output.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
name	uninitialized	"Alice"	"Alice"	"Alice"
age	uninitialized	uninitialized	30	30

Key Moments - 2 Insights

Why do we write ': String' or ': Int' after variable names?

This tells Kotlin the exact type the variable must hold, so it only accepts matching values. See steps 1 and 3 in the execution table.

What happens if we assign a value of the wrong type?

Kotlin will give an error because the value does not match the declared type. This is why in steps 2 and 4, the assigned values match the declared types.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'name' after step 2?

A uninitialized

B "Alice"

C 30

D null

💡 Hint

Check the 'Value' column for 'name' at step 2 in the execution table.

At which step is the variable 'age' assigned a value?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look for the assignment action for 'age' in the execution table.

If we remove ': Int' from 'age' declaration, what changes in the execution?

A Kotlin infers the type automatically

B The program will not compile

C The variable 'age' becomes a String

D The variable 'age' can hold any type

💡 Hint

Kotlin can infer types if explicit declaration is missing, so no error occurs.

Concept Snapshot

Explicit type declaration in Kotlin:
val variableName: Type = value
var variableName: Type = value
- Declares variable with fixed type
- Helps catch type errors early
- Value must match declared type

Full Transcript

This example shows how to declare variables in Kotlin with explicit types. First, we declare 'name' as a String and assign "Alice" to it. Then, we declare 'age' as an Int and assign 30. Finally, we print both values. Explicit type declaration means telling Kotlin exactly what type a variable holds, so it only accepts matching values. This helps avoid mistakes. The execution table shows each step: declaration, assignment, and printing. The variable tracker shows how values change. Remember, if you assign a wrong type, Kotlin will give an error. You can also let Kotlin guess the type by skipping explicit declaration, but here we show how to write it clearly.