Mutable vs immutable interfaces in Kotlin

Introduction

We use mutable and immutable interfaces to control if data can be changed or not. This helps keep our programs safe and clear.

When you want to share data but prevent others from changing it.

When you need to update data safely in one place.

When you want to avoid accidental changes to important information.

When designing APIs to clearly show which parts can be changed.

When working with collections where some should stay fixed and others can change.

Syntax

Kotlin

interface ReadOnlyList<T> {
    fun get(index: Int): T
    val size: Int
}

interface MutableList<T> : ReadOnlyList<T> {
    fun add(item: T)
    fun removeAt(index: Int)
}

Immutable interfaces only allow reading data, no changes.

Mutable interfaces extend immutable ones and add methods to change data.

Examples

This interface allows only reading items and size, no changes.

Kotlin

interface ReadOnlyList<T> {
    fun get(index: Int): T
    val size: Int
}

This interface allows adding and removing items, so it is mutable.

Kotlin

interface MutableList<T> : ReadOnlyList<T> {
    fun add(item: T)
    fun removeAt(index: Int)
}

You can use a read-only interface to prevent changes.

Kotlin

val readOnly: ReadOnlyList<String> = ...
// readOnly.get(0) works
// readOnly.add("new") does NOT work

Mutable interface lets you read and change the list.

Kotlin

val mutable: MutableList<String> = ...
mutable.add("new") // works
mutable.get(0) // works

Sample Program

This program shows a mutable list that you can add to and remove from. It prints the size and first item before and after removal.

Kotlin

interface ReadOnlyList<T> {
    fun get(index: Int): T
    val size: Int
}

interface MutableList<T> : ReadOnlyList<T> {
    fun add(item: T)
    fun removeAt(index: Int)
}

class SimpleList<T> : MutableList<T> {
    private val items = mutableListOf<T>()
    override fun get(index: Int) = items[index]
    override val size get() = items.size
    override fun add(item: T) { items.add(item) }
    override fun removeAt(index: Int) { items.removeAt(index) }
}

fun main() {
    val list: MutableList<String> = SimpleList()
    list.add("apple")
    list.add("banana")
    println("Size: ${list.size}")
    println("First item: ${list.get(0)}")
    list.removeAt(0)
    println("Size after removal: ${list.size}")
    println("First item now: ${list.get(0)}")
}

Important Notes

Immutable interfaces help avoid bugs by preventing unwanted changes.

Mutable interfaces should be used carefully to keep data consistent.

In Kotlin, many standard collections have both mutable and read-only versions.

Summary

Immutable interfaces allow only reading data.

Mutable interfaces allow changing data and usually extend immutable ones.

Use immutable interfaces to protect data and mutable ones when changes are needed.

Practice

1. Which statement best describes the difference between mutable and immutable interfaces in Kotlin?

easy

A. Immutable interfaces are used only for primitive types.

B. Mutable interfaces only allow reading data, immutable interfaces allow modification.

C. Both mutable and immutable interfaces allow data modification.

D. Immutable interfaces allow only reading data, while mutable interfaces allow data modification.

Solution

1. Step 1: Understand interface roles

   Immutable interfaces provide read-only access to data, preventing changes.

2. Step 2: Compare mutable interfaces

   Mutable interfaces extend immutable ones and allow data to be changed.

3. Final Answer:

   Immutable interfaces allow only reading data, while mutable interfaces allow data modification. -> Option D

4. Quick Check:

   Immutable = read-only, Mutable = read-write [OK]

Hint: Immutable = read-only, mutable = can change data [OK]

Common Mistakes:

• Confusing which interface allows modification
• Thinking both interfaces allow changes
• Assuming immutable interfaces are only for primitives

2. Which of the following is the correct way to declare a mutable list interface in Kotlin?

easy

A. val list: MutableList<Int> = listOf(1, 2, 3)

B. val list: List<Int> = mutableListOf(1, 2, 3)

C. val list: MutableList<Int> = mutableListOf(1, 2, 3)

D. val list: List<Int> = listOf(1, 2, 3)

Solution

1. Step 1: Identify mutable list declaration

   MutableList is the interface for mutable lists, and mutableListOf() creates a mutable list instance.

2. Step 2: Check each option

   val list: MutableList<Int> = mutableListOf(1, 2, 3) correctly declares a MutableList and initializes it with mutableListOf().

3. Final Answer:

   val list: MutableList<Int> = mutableListOf(1, 2, 3) -> Option C

4. Quick Check:

   MutableList + mutableListOf() = mutable list [OK]

Hint: Use MutableList with mutableListOf() for mutable lists [OK]

Common Mistakes:

• Using listOf() with MutableList type
• Declaring mutable list as List type
• Mixing mutable and immutable functions

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

val immutableList: List<Int> = listOf(1, 2, 3)
val mutableList: MutableList<Int> = mutableListOf(4, 5, 6)

println(immutableList.size)
mutableList.add(7)
println(mutableList)

medium

A. 3
[4, 5, 6, 7]

B. 3
[4, 5, 6]

C. 4
[4, 5, 6, 7]

D. Compilation error

Solution

1. Step 1: Check immutable list size

   immutableList has 3 elements, so size is 3.

2. Step 2: Add element to mutable list and print

   mutableList adds 7, so it becomes [4, 5, 6, 7].

3. Final Answer:

   3 [4, 5, 6, 7] -> Option A

4. Quick Check:

   Immutable size 3, mutable list adds 7 [OK]

Hint: Immutable size fixed; mutable can add elements [OK]

Common Mistakes:

• Trying to add to immutable list causing error
• Expecting immutable list size to change
• Confusing output format

4. Identify the error in this Kotlin code snippet:

val numbers: List<Int> = listOf(1, 2, 3)
numbers.add(4)

medium

A. Cannot add to a List because it is immutable

B. Syntax error in listOf() usage

C. No error, code runs fine

D. Variable numbers should be declared as var

Solution

1. Step 1: Understand List immutability

   List interface in Kotlin is immutable; it does not support add() method.

2. Step 2: Analyze the error

   Calling add(4) on immutable List causes a compilation error.

3. Final Answer:

   Cannot add to a List because it is immutable -> Option A

4. Quick Check:

   Immutable List blocks add() method [OK]

Hint: List is immutable; use MutableList to add elements [OK]

Common Mistakes:

• Trying to add to immutable List
• Assuming var allows modification of immutable List
• Confusing syntax error with immutability error

5. You have a function that accepts a List<String> parameter but you want to modify the list inside the function. Which is the best approach to handle this in Kotlin?

hard

A. Change the parameter type to MutableList<String> and pass a mutable list.

B. Create a new mutable copy of the list inside the function and modify that.

C. Use val keyword inside the function to modify the list.

D. Keep the parameter as List<String> and cast it to MutableList inside the function.

Solution

1. Step 1: Understand parameter immutability

   List<String> is immutable, so it cannot be modified directly.

2. Step 2: Evaluate safe modification methods

   Casting to MutableList is unsafe and may cause runtime errors; changing parameter type affects API design.

3. Step 3: Use safe copy for modification

   Creating a mutable copy inside the function allows safe modifications without affecting original list.

4. Final Answer:

   Create a new mutable copy of the list inside the function and modify that. -> Option B

5. Quick Check:

   Copy immutable list to mutable for safe changes [OK]

Hint: Copy immutable list to mutable before modifying [OK]

Common Mistakes:

• Casting immutable list to mutable unsafely
• Changing parameter type breaking API contracts
• Assuming val allows modification