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vetoable

Learn vetoable properties with vetoable.

vetoable

vetoable lets you reject an assignment to a property based on a condition. Example: var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }. If the lambda returns true the assignment is accepted; if it returns false the assignment is rejected and the value stays unchanged.

This is useful for protecting invariants: age cannot be negative, email cannot be empty and so on. observable accepts every assignment; vetoable validates first. Both come from the Delegates object and use a similar lambda signature.

How vetoable works

The vetoable lambda takes three parameters: the property reference, the old value and the new value. The lambda returns a Boolean. If true the new value is assigned; if false the old value is kept.

Example: var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }. age = 25 is accepted (25 >= 0). age = -5 is rejected (-5 < 0); age stays 25. This is a clean way to protect a business rule in one line. Compare with observable: observable always accepts, vetoable is selective.

Practice Validate age with vetoable

Add import kotlin.properties.Delegates. Define var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }. In main assign age = 25 and call println(age). Then assign age = -5 and call println(age) again.

Starter code

Main.kt
import kotlin.properties.Delegates

// Define age with the vetoable delegate: reject negative values

fun main() {
    // Assign 25 and print, then assign -5 and print
}

Sample solution

Main.kt
import kotlin.properties.Delegates

var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }

fun main() {
    age = 25
    println(age)
    age = -5
    println(age)
}

Expected output

25
25

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does vetoable do?

  • Can reject by condition Correct answer
  • Never changes
  • Always accepts
  • Only reads

Explanation: vetoable rejects the assignment when the lambda returns false.

2 Complete the vetoable declaration

Question 
Delegates.____(0) { _, _, new -> new >= 0 }
Answer 
Delegates.vetoable(0) { _, _, new -> new >= 0 }

Explanation: vetoable rejects negative values.

3 Match the vetoable concepts

Term Meaning
vetoable Can reject
observable Tracks
true Accept
false Reject

All lessons in Intermediate

1 Classes in depth

  1. Primary constructor
  2. Secondary constructor
  3. Properties
  4. lateinit and lazy
  5. companion object
  6. Summary - Classes

2 Inheritance

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3 Data class

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4 Project: Helper library

  1. Module design
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  4. Date helpers
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5 Sealed class

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  2. when with sealed
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6 Object declaration

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7 Extension functions

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  3. Nullable extensions
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  5. Standard library
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8 Project: Quiz app

  1. Quiz model
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9 Generics

  1. Generic class
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  3. Variance
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  5. Reified
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10 Higher-order functions

  1. Function types
  2. Lambda
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11 Project: Library app

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12 Advanced collections

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  4. sortedBy and sortedWith
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13 Delegation

  1. by
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  3. by map
  4. vetoable
  5. Custom delegates
  6. Summary - Delegation

14 Value class

  1. value class
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15 Coroutine basics

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16 Flow basics

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17 File I/O

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18 Packages and modules

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19 Testing

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20 Intermediate final

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