Java Collections Framework: List, Set, Map 

🗂️ 1. List Interface

A List is an ordered collection (also known as a sequence) that allows duplicate elements. Elements can be accessed by their index.

🔹 Common Implementations:

ArrayList

LinkedList

Vector (rarely used now)

✅ Key Features:

Maintains insertion order

Allows duplicates

Supports random access (in ArrayList)

🔧 Example:

java

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import java.util.*;

List<String> names = new ArrayList<>();

names.add("Alice");

names.add("Bob");

names.add("Alice"); // Duplicate allowed

System.out.println(names); // [Alice, Bob, Alice]

📦 2. Set Interface

A Set is a collection that does not allow duplicate elements. It models the mathematical set abstraction.

🔹 Common Implementations:

HashSet

LinkedHashSet

TreeSet

✅ Key Features:

No duplicates allowed

HashSet: No order

LinkedHashSet: Maintains insertion order

TreeSet: Sorted order (uses Comparable or Comparator)

🔧 Example:

java

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import java.util.*;

Set<String> fruits = new HashSet<>();

fruits.add("Apple");

fruits.add("Banana");

fruits.add("Apple"); // Duplicate ignored

System.out.println(fruits); // [Apple, Banana] (order not guaranteed)

🗺️ 3. Map Interface

A Map is an object that maps keys to values. It cannot contain duplicate keys; each key maps to at most one value.

🔹 Common Implementations:

HashMap

LinkedHashMap

TreeMap

✅ Key Features:

Key-value pairs

No duplicate keys

HashMap: No order

LinkedHashMap: Insertion order

TreeMap: Sorted by key

🔧 Example:

java

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import java.util.*;

Map<Integer, String> idMap = new HashMap<>();

idMap.put(1, "Alice");

idMap.put(2, "Bob");

idMap.put(1, "Charlie"); // Overwrites previous value for key 1

System.out.println(idMap); // {1=Charlie, 2=Bob}

🧠 Summary Table

Feature List Set Map

Allows Duplicates ✅ Yes ❌ No ✅ Values, ❌ Keys

Maintains Order ✅ (most) ✅ (LinkedHashSet) ✅ (LinkedHashMap)

Access by Index ✅ Yes ❌ No ❌ No (access by key)

Key-Value Storage ❌ No ❌ No ✅ Yes

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