🧩 Kotlin – Sets (Mutable & Immutable): Unique Collection Explained

🧲 Introduction – Why Learn Kotlin Sets?

Sets are unordered collections of unique elements, ideal for ensuring that no duplicate data exists. Kotlin makes working with sets intuitive by offering both immutable and mutable variants. Sets are perfect for scenarios like tags, filters, unique IDs, and rule-based checks.

🎯 In this guide, you’ll learn:

• What Sets are in Kotlin and when to use them
• Difference between immutable and mutable sets
• How to add, remove, and check items in a set
• Best practices for handling uniqueness in collections

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🧮 What Is a Set in Kotlin?

A Set is a collection that:

• Holds unique elements (no duplicates)
• Does not guarantee order
• Supports both read-only (setOf) and modifiable (mutableSetOf) types

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🛡️ Immutable Sets – setOf()

An immutable set cannot be changed after creation.

val languages = setOf("Kotlin", "Java", "Python", "Kotlin")
println(languages)  // [Kotlin, Java, Python]

✔️ Only unique values are retained (duplicate "Kotlin" is removed).

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🛠️ Mutable Sets – mutableSetOf()

Mutable sets allow adding, removing, and modifying elements.

val tools = mutableSetOf("Git", "Docker", "Kubernetes")
tools.add("Gradle")
tools.remove("Docker")

println(tools)  // [Git, Kubernetes, Gradle]

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🔁 Iterating Over Sets

🔹 Using for loop:

for (tool in tools) {
    println("Tool: $tool")
}

🔹 Using forEach lambda:

languages.forEach { println("Lang: $it") }

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🔍 Set Functions & Operations

Function	Description
contains(x)	Checks if element exists
add(x)	Adds element (only in mutable sets)
remove(x)	Removes element
union(set)	Combines two sets
intersect(set)	Returns common elements
subtract(set)	Removes all elements from another set
isEmpty()	Checks if the set has no elements

🔹 Example:

val setA = setOf(1, 2, 3)
val setB = setOf(3, 4, 5)

println(setA union setB)       // [1, 2, 3, 4, 5]
println(setA intersect setB)   // [3]
println(setA subtract setB)    // [1, 2]

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🔃 Convert Between Mutable and Immutable Sets

val immutable = setOf("X", "Y")
val mutable = immutable.toMutableSet()
mutable.add("Z")

val backToImmutable = mutable.toSet()

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🧬 HashSet and LinkedHashSet in Kotlin

Type	Description
HashSet	Unordered, fast lookup
LinkedHashSet	Maintains insertion order

val orderedSet = linkedSetOf("A", "B", "C")
println(orderedSet)  // [A, B, C]

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✅ Best Practices for Kotlin Sets

Tip	Why It Matters
Use setOf() for fixed reference	Promotes immutability and thread safety
Use mutableSetOf() for flexibility	Enables add/remove operations
Use union(), intersect()	Cleaner than manually iterating for comparisons
Avoid duplicate checks manually	Let Set handle it automatically

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🚫 Common Mistakes with Sets

❌ Mistake	✅ Fix
Expecting duplicates in setOf()	Sets store only unique values
Modifying immutable set	Use mutableSetOf() or convert with toMutableSet()
Relying on order	Use List or LinkedHashSet if order is important
Confusing Set with Map	Set stores only values, not key-value pairs

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📌 Summary – Recap & Next Steps

Kotlin Sets are perfect for ensuring uniqueness and performing set-based operations like union and intersection. With both immutable and mutable options, they suit a wide range of real-world applications.

🔍 Key Takeaways:

• Sets store unique, unordered elements
• Use setOf() for immutable, mutableSetOf() for editable sets
• Built-in operations like union(), intersect() simplify logic
• Use LinkedHashSet to maintain insertion order

⚙️ Practical Use:
Sets are widely used in permission management, form validation, tag lists, and deduplication tasks in both Android and server-side Kotlin apps.

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❓ FAQs – Kotlin Sets

❓ Can Kotlin Sets contain duplicate elements?
✅ No. All elements in a set are unique. Duplicate entries are automatically removed.

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❓ What is the difference between setOf() and mutableSetOf()?
✅ setOf() is read-only and cannot be modified. mutableSetOf() allows add/remove operations.

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❓ How do I ensure my Set maintains insertion order?
✅ Use linkedSetOf() or LinkedHashSet.

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❓ Can I convert a list to a set?
✅ Yes. Use .toSet():

val names = listOf("A", "B", "A")
val uniqueNames = names.toSet()  // ["A", "B"]

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❓ Is a Set faster than a List in Kotlin?
✅ Sets offer faster lookup performance, especially for contains() operations, due to internal hashing.

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