Constructors and initialization checks

A mark should never begin outside zero through one hundred. A primary constructor declares the inputs for creating an instance, while init blocks establish additional rules and state. Secondary constructors must delegate under Kotlin’s construction rules.

What you’ll learn

Validate constructor inputs.
Avoid partially valid domain records.

Constructors and initialization checks in the example

1
Constructor input
A mark should never begin outside zero through one hundred.
```
Constructor input
```
→2
Range requirement
Use require for an argument contract when failure should be an exception.
```
Range requirement
```
→3
Valid instance or failure
The valid constructor completes with value 80.
```
Valid instance or failure
```

The valid constructor completes with value 80. The second construction fails its requirement, and the catch reports Invalid score. No usable Mark with value 120 is returned.

Understand constructors and initialization checks

Use require for an argument contract when failure should be an exception. Init executes during construction, and property initializers and init blocks follow their declared order. A successful type conversion does not itself establish a valid domain value.

Follow constructors and initialization checks

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

Kotlin/JVM · complete program

class Mark(val value: Int) {
  init { require(value in 0..100) { "Score must be 0..100" } }
}
fun main() {
  println(Mark(80).value)
  try { Mark(120) }
  catch (e: IllegalArgumentException) { println("Invalid score") }
}

Expected output

80
Invalid score

Follow the execution

The valid constructor completes with value 80. The second construction fails its requirement, and the catch reports Invalid score. No usable Mark with value 120 is returned.

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

Practice: Constructors and initialization checks

Your task: What happens when constructing Mark(-5)?

Kotlin/JVM · complete program

class Mark(val value: Int) {
  init { require(value in 0..100) { "Score must be 0..100" } }
}
fun main() {
  println(Mark(80).value)
  try { Mark(120) }
  catch (e: IllegalArgumentException) { println("Invalid score") }
}

Worked answer

The requirement fails and throws IllegalArgumentException. The caller must handle that failure if invalid input is an ordinary possibility.