Variables, val and var

A student’s name can remain fixed while a corrected mark changes. Val prevents reassignment of a binding; var permits it. Kotlin can infer a type from an initial value, so you can often express both the value and its intended mutability without a separate type annotation.

What you’ll learn

Choose val or var from the required reassignment.
Distinguish a stable binding from mutable object contents.

Variables, val and var in the example

1
Score 72
A student’s name can remain fixed while a corrected mark changes.
```
Score 72
```
→2
Add bonus 8
An assignment stores a value, while equality uses a comparison operator.
```
Add bonus 8
```
→3
Display 80
Score starts at 72 and the compound assignment stores 80.
```
Display 80
```

Score starts at 72 and the compound assignment stores 80. The string template inserts the name and current score. The name is not reassigned, so val describes the intention accurately.

Understand variables, val and var

An assignment stores a value, while equality uses a comparison operator. Reassigning an Int variable to text is not an automatic conversion. A val reference to a mutable object can still allow operations on that object; preventing reassignment is different from deeply freezing data.

Follow variables, val and var

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

fun main() {
  val name = "Diya"
  var score = 72
  score += 8
  println("$name: $score")
}

Expected output

Diya: 80

Follow the execution

Score starts at 72 and the compound assignment stores 80. The string template inserts the name and current score. The name is not reassigned, so val describes the intention accurately.

Try this in the Kotlin compiler by pasting the complete program.

Practice: Variables, val and var

Your task: Which binding needs var if the score must be corrected?

Kotlin/JVM · complete program

fun main() {
  val name = "Diya"
  var score = 72
  score += 8
  println("$name: $score")
}

Worked answer

The score binding needs var. Keep name as val while its reference does not need reassignment; there is no reason to make every binding mutable.