🗺️ Kotlin – Maps (Key-Value Pairs): Organize Data with Keys

🧲 Introduction – Why Learn Kotlin Maps?

Maps are one of the most essential collection types in Kotlin, allowing you to store key-value pairs efficiently. They’re perfect for scenarios like lookup tables, configuration settings, or JSON-like structures. Kotlin provides immutable and mutable Maps with powerful utilities and clean syntax.

🎯 In this guide, you’ll learn:

• How to declare and use Kotlin Maps
• The difference between immutable (mapOf) and mutable (mutableMapOf) maps
• Access, update, and iterate over key-value data
• Best practices for map operations in real apps

---

🧾 What Is a Map in Kotlin?

A Map is a collection of key-value pairs. Each key is unique and maps to a value.

• Keys → Unique
• Values → Can be duplicated
• Keys are used to retrieve or update values

---

🛡️ Immutable Maps – mapOf()

val countries = mapOf("IN" to "India", "US" to "United States")

println(countries["IN"])  // India
println(countries["FR"])  // null (not found)

• Cannot be modified after creation
• Great for constants and static data

---

🛠️ Mutable Maps – mutableMapOf()

val codes = mutableMapOf("A" to 1, "B" to 2)
codes["C"] = 3
codes.remove("A")

println(codes)  // {B=2, C=3}

• You can add, remove, or update entries freely

---

🔁 Iterating Over a Map

🔹 Loop with entries:

for ((key, value) in countries) {
    println("$key => $value")
}

🔹 Loop over keys or values only:

for (key in countries.keys) {
    println("Code: $key")
}

for (value in countries.values) {
    println("Country: $value")
}

---

🧪 Common Map Functions

Function	Description
get(key)	Returns value for the given key
getOrDefault(key, v)	Returns value or default if key not found
containsKey(key)	Checks if a key exists in the map
containsValue(value)	Checks if a value exists
put(key, value)	Adds or updates entry (mutable only)
remove(key)	Removes entry by key (mutable only)
isEmpty()	Checks if the map has no entries

---

🧬 Map Type Variants

Type	Description
HashMap	Unordered, fast lookup
LinkedHashMap	Maintains insertion order
TreeMap (Java)	Sorted by key (requires Java interop)

val orderedMap = linkedMapOf("one" to 1, "two" to 2)
println(orderedMap)  // {one=1, two=2}

---

🔄 Convert Lists to Maps

val users = listOf("Alice", "Bob")
val userMap = users.associateWith { it.length }

println(userMap)  // {Alice=5, Bob=3}

✔️ Turns a list into a map based on a transformation.

---

🚫 Common Mistakes

❌ Mistake	✅ Fix
Trying to modify mapOf()	Use mutableMapOf() or convert using .toMutableMap()
Using missing keys without null check	Use getOrDefault() or getOrElse() for safety
Confusing map with set/list	Map uses keys and values; not just values
Assuming key order in mapOf()	Use linkedMapOf() to maintain order

---

✅ Best Practices for Kotlin Maps

Tip	Why It Helps
Use mapOf() for static configurations	Prevents accidental mutations
Use getOrDefault() for safe access	Avoids null crashes
Use entries, keys, values for iteration	Enhances readability
Choose correct map type (LinkedHashMap, HashMap)	Based on ordering needs

---

📌 Summary – Recap & Next Steps

Kotlin Maps are ideal for keyed data access, such as ID-to-name lookups, field-value mappings, or configuration tables. With both immutable and mutable forms, plus built-in functions, Maps offer a clean and safe way to organize and access data.

🔍 Key Takeaways:

• Use mapOf() for immutable maps, mutableMapOf() for editable ones
• Each key is unique; values can repeat
• Retrieve data using keys, safely handle missing entries
• Use map functions to filter, transform, and combine data

⚙️ Practical Use:
Maps are essential for user ID-to-data maps, API headers, language/localization dictionaries, and dynamic form field storage in Android and backend Kotlin applications.

---

❓ FAQs – Kotlin Maps

❓ What is a Kotlin Map used for?
✅ A Map stores key-value pairs for fast lookup, transformation, or matching logic.

---

❓ How do I create a mutable Map in Kotlin?
✅ Use mutableMapOf():

val m = mutableMapOf("A" to 1)
m["B"] = 2

---

❓ Can I access a key that doesn’t exist in a map?
✅ Yes, but it returns null or use getOrDefault():

map.getOrDefault("X", "Unknown")

---

❓ Are Maps ordered in Kotlin?
✅ By default, no. Use linkedMapOf() or LinkedHashMap for insertion order.

---

❓ Can I filter a map by values or keys?
✅ Yes:

val filtered = map.filter { it.value > 10 }

---

Share Now :