Custom getters and setters in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom getter in Kotlin?

A custom getter is a special function that runs when you access a property. It lets you control or change the value returned without storing it directly.

beginner

How do you define a custom setter in Kotlin?

You define a custom setter by writing a set function inside the property. It runs when you assign a value, letting you check or modify the value before saving it.

intermediate

Why use custom getters and setters instead of simple properties?

They let you add extra logic like validation, formatting, or lazy calculation when getting or setting a value, making your code safer and clearer.

beginner

Show a simple Kotlin property with a custom getter that returns the length of a string property.

val nameLength: Int get() = name.length This means whenever you ask for nameLength, it calculates and returns the length of name.

intermediate

What happens if you define a custom setter but no backing field is used?

Without a backing field, the setter cannot store the value. You must handle storage yourself or use a backing field with the 'field' keyword to save the value.

In Kotlin, how do you access the backing field inside a custom setter?

A Using 'this'

B Using 'super'

C Using the property name

D Using the keyword 'field'

Inside a custom setter or getter, 'field' refers to the actual storage of the property.

What is the default behavior if you do not define a custom getter or setter?

A The property is read-only

B The property has a default getter and setter that directly access the backing field

C The property cannot be accessed

D The property throws an error when accessed

Kotlin automatically provides default getter and setter that read and write the backing field.

Which keyword is used to define a custom getter in Kotlin?

A get()

B set()

C getter()

D fetch()

The 'get()' keyword defines a custom getter function for a property.

What happens if you try to use 'field' outside a getter or setter?

A It causes a compile error

B It works normally

C It returns null

D It returns the property name

'field' is only valid inside custom getters and setters; outside, it causes a compile error.

Why might you want to add validation in a custom setter?

A To make the property read-only

B To speed up the program

C To prevent invalid values from being stored

D To automatically print the value

Validation in setters helps ensure only correct values are saved, avoiding bugs.

Explain how custom getters and setters work in Kotlin and why they are useful.

Think about how you control reading and writing a property.

Write a Kotlin property with a custom setter that only accepts positive numbers and throws an error otherwise.

Use 'set(value)' and check if value is positive.

Practice

1. What is the main purpose of custom getters and setters in Kotlin?

easy

A. To create new classes automatically

B. To define functions outside a class

C. To control how properties are read and written

D. To import external libraries

Solution

1. Step 1: Understand property access

   Custom getters and setters allow you to define how a property value is retrieved or changed.

2. Step 2: Identify their purpose

   They help control and protect the data by customizing read/write behavior.

3. Final Answer:

   To control how properties are read and written -> Option C

4. Quick Check:

   Custom getters/setters = control property access [OK]

Hint: Remember: getters/setters control property reading/writing [OK]

Common Mistakes:

• Confusing getters/setters with class creation
• Thinking they define functions outside classes
• Assuming they import libraries

2. Which of the following is the correct syntax to define a custom setter for a Kotlin property age?

easy

A. var age: Int
set(value) { field = value }

B. var age: Int
get(value) { field = value }

C. val age: Int
set(value) { field = value }

D. var age: Int
set { value = field }

Solution

1. Step 1: Check property mutability

   Setter can only be defined for var, not val.

2. Step 2: Verify setter syntax

   The setter uses set(value) { field = value } to assign the new value.

3. Final Answer:

   var age: Int set(value) { field = value } -> Option A

4. Quick Check:

   Setter syntax = set(value) { field = value } [OK]

Hint: Setter needs var and set(value) syntax [OK]

Common Mistakes:

• Using get instead of set for setter
• Defining setter on val property
• Reversing field and value in setter

3. What will be the output of this Kotlin code?

class Person {
    var name: String = ""
        get() = field.uppercase()
        set(value) { field = value.trim() }
}

fun main() {
    val p = Person()
    p.name = "  alice  "
    println(p.name)
}

medium

A. ALICE

B. alice

C. ecila

D. Alice

Solution

1. Step 1: Analyze setter behavior

   Setter trims the input, so " alice " becomes "alice" stored in field.

2. Step 2: Analyze getter behavior

   Getter returns the stored value in uppercase, so "alice" becomes "ALICE".

3. Final Answer:

   ALICE -> Option A

4. Quick Check:

   Setter trims, getter uppercases = ALICE [OK]

Hint: Setter trims, getter uppercases output [OK]

Common Mistakes:

• Ignoring trimming in setter
• Not converting to uppercase in getter
• Assuming original input prints

4. Identify the error in this Kotlin class with custom getter and setter:

class Box {
    var size: Int = 0
        get() = field
        set(value) {
            if (value > 0) field = value
            else field = 0
        }

    fun printSize() {
        println(size)
    }
}

medium

A. Setter does not assign value correctly

B. No error, code works fine

C. Getter should not use field

D. Property size should be val, not var

Solution

1. Step 1: Check setter logic

   Setter assigns field = value if positive, else 0. This is valid.

2. Step 2: Check getter and property type

   Getter returns field, which is correct. Property is var, so setter allowed.

3. Final Answer:

   No error, code works fine -> Option B

4. Quick Check:

   Setter and getter correct = no error [OK]

Hint: Setter with condition is valid; getter uses field correctly [OK]

Common Mistakes:

• Thinking getter can't use field
• Assuming setter must always assign value
• Confusing var and val for setters

5. You want a Kotlin property score that never stores a negative value. Which custom setter implementation ensures this correctly?

hard

A. var score: Int = 0
set(value) { field = value * -1 }

B. var score: Int = 0
set(value) { if (value < 0) field = 0 }

C. var score: Int = 0
set(value) { field = value }

D. var score: Int = 0
set(value) { field = if (value < 0) 0 else value }

Solution

1. Step 1: Understand requirement

   Property must never store negative values; negative inputs become zero.

2. Step 2: Evaluate each setter

   var score: Int = 0 set(value) { field = if (value < 0) 0 else value } sets field to 0 if value is negative, else stores value correctly.

3. Step 3: Check other options

   A stores negative of value, which can be positive or negative incorrectly; B only sets field if negative, ignoring positive values; C stores negatives unchanged.

4. Final Answer:

   var score: Int = 0 set(value) { field = if (value < 0) 0 else value } -> Option D

5. Quick Check:

   Setter with if-else ensures no negatives [OK]

Hint: Use if-else in setter to block negatives [OK]

Common Mistakes:

• Not assigning field for positive values
• Allowing negative values to be stored
• Incorrectly negating values