Properties with val and var in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the difference between val and var in Kotlin properties?

val declares a read-only property (like a constant). You cannot change its value after initialization. var declares a mutable property, which means you can change its value later.

beginner

Can a val property be assigned a value after the object is created?

No. A val property must be assigned once during initialization and cannot be reassigned later.

beginner

How do you declare a mutable property named age of type Int in Kotlin?

You declare it using var age: Int. This means you can change the value of age anytime.

beginner

What happens if you try to reassign a val property in Kotlin?

The compiler will give an error because val properties are read-only and cannot be reassigned after initialization.

beginner

Example: What will this code print?

val name = "Anna"
// name = "Bob" // What if uncommented?

The code will print nothing because it only declares name. If you uncomment the reassignment, the compiler will show an error because name is a val and cannot be changed.

Which keyword declares a property that can be changed after initialization?

A var

B val

C const

D let

var declares a mutable property that can be reassigned. val is read-only.

What happens if you try to assign a new value to a val property?

A It works fine

B Value silently changes

C Runtime error

D Compiler error

The Kotlin compiler prevents reassignment of val properties, causing a compile-time error.

How do you declare a read-only property in Kotlin?

A val

B var

C let

D const

val declares a read-only property that cannot be reassigned.

Which of these is true about var properties?

A They cannot be changed after initialization

B They must be initialized with a value

C They can be reassigned any number of times

D They are constants

var properties are mutable and can be reassigned multiple times.

If you want a property that never changes after creation, which keyword do you use?

A var

B val

C mutable

D final

val creates a read-only property that cannot be changed after initialization.

Explain the difference between val and var properties in Kotlin with examples.

Think about constants versus variables in everyday life.

What error do you get if you try to change a val property after it is set? Why does Kotlin enforce this?

Consider why some things in life should not change once set.

Practice

1. What is the main difference between val and var properties in Kotlin?

easy

A. val can be changed anytime, var is read-only after initialization.

B. val is read-only after initialization, var can be changed anytime.

C. val and var both can be changed anytime.

D. val and var are exactly the same.

Solution

1. Step 1: Understand val behavior

   val properties are assigned once and cannot be reassigned later.

2. Step 2: Understand var behavior

   var properties can be reassigned multiple times after initialization.

3. Final Answer:

   val is read-only after initialization, var can be changed anytime. -> Option B

4. Quick Check:

   val = read-only, var = mutable [OK]

Hint: Remember: val = fixed, var = variable [OK]

Common Mistakes:

• Confusing val as mutable
• Thinking var is read-only
• Believing val and var behave the same

2. Which of the following is the correct way to declare a mutable property named age of type Int in Kotlin?

easy

A. var age: Int = 25

B. val age: Int = 25

C. val age = 25

D. var age: Int

Solution

1. Step 1: Identify mutable property keyword

   Mutable properties use var keyword.

2. Step 2: Check initialization

   Property must be initialized or declared nullable; here age is initialized with 25.

3. Final Answer:

   var age: Int = 25 -> Option A

4. Quick Check:

   Mutable property uses var [OK]

Hint: Use var for changeable properties [OK]

Common Mistakes:

• Using val for mutable property
• Not initializing non-nullable property
• Omitting type without initialization

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

class Person {
    val name = "Anna"
    var age = 30
}

fun main() {
    val p = Person()
    println(p.name)
    p.age = 31
    println(p.age)
}

medium

A. Anna
30

B. Anna
null

C. Error: Cannot assign to val property

D. Anna
31

Solution

1. Step 1: Check val name usage

   name is a val, so it prints "Anna" and cannot be changed.

2. Step 2: Check var age reassignment

   age is a var, so it can be changed from 30 to 31 before printing.

3. Final Answer:

   Anna 31 -> Option D

4. Quick Check:

   val prints original, var prints updated [OK]

Hint: val prints original, var can be updated [OK]

Common Mistakes:

• Expecting error on changing var
• Thinking val can be reassigned
• Confusing printed values

4. Identify the error in the following Kotlin code:

class Car {
    val model: String
    var year: Int

    init {
        model = "Tesla"
        year = 2020
    }
}

fun main() {
    val car = Car()
    car.model = "BMW"
    println(car.model)
}

medium

A. Cannot assign a new value to val property 'model'.

B. Property 'year' is not initialized.

C. Missing constructor parameters.

D. No error, code runs fine.

Solution

1. Step 1: Check val model reassignment

   model is declared as val, so it cannot be reassigned after initialization.

2. Step 2: Verify property initialization

   model and year are both initialized in init, so no initialization error.

3. Final Answer:

   Cannot assign a new value to val property 'model'. -> Option A

4. Quick Check:

   val properties cannot be reassigned [OK]

Hint: val properties cannot be changed after init [OK]

Common Mistakes:

• Trying to reassign val property
• Ignoring uninitialized var warnings
• Assuming val can be reassigned

5. You want to create a Kotlin class Book where the title should never change after creation, but the price can be updated. Which declaration is correct?

hard

A. class Book(val title: String, val price: Double)

B. class Book(var title: String, var price: Double)

C. class Book(val title: String, var price: Double)

D. class Book(var title: String, val price: Double)

Solution

1. Step 1: Determine immutability for title

   title should not change, so declare it with val.

2. Step 2: Determine mutability for price

   price can change, so declare it with var.

3. Final Answer:

   class Book(val title: String, var price: Double) -> Option C

4. Quick Check:

   Immutable title = val, mutable price = var [OK]

Hint: Use val for fixed, var for changeable properties [OK]

Common Mistakes:

• Using var for title which should be fixed
• Using val for price which should be changeable
• Mixing up val and var keywords