Collection vs Map: What is Difference?

In Java, Collection and Map are two key parts of the Java Collection Framework (JCF), but they have distinct roles:

Collection is a root interface that represents a group of objects (like List, Set, Queue).
Map is a separate interface used for key-value pair storage.

1. What is Collection?

Collection is the root interface for all collections that store a group of objects in Java.

Features of Collection Interface:

✔ Represents a group of elements (single values).
✔ Extended by List, Set, and Queue.
✔ Provides methods like add(), remove(), size(), contains().
✔ Does not store key-value pairs (only elements).
✔ Cannot be instantiated directly (must use classes like ArrayList or HashSet).

Collection Hierarchy

plaintextCopy codeCollection (Interface)
├── List (Interface) → ArrayList, LinkedList, Vector
├── Set (Interface) → HashSet, TreeSet, LinkedHashSet
└── Queue (Interface) → PriorityQueue, Deque

Example Using Collection Interface

import java.util.*;

public class CollectionExample {
    public static void main(String[] args) {
        Collection<String> names = new ArrayList<>(); // Collection reference
        names.add("Sujit");
        names.add("Patel");
        
        System.out.println(names); // Output: [Sujit, Patel]
    }
}

🔹 Here, Collection is used to store individual elements.

2. What is Map?

Map is an interface that stores key-value pairs (not a subtype of Collection).

Features of Map Interface:

✔ Stores key-value pairs (key → value).
✔ Keys are unique (no duplicates), but values can be duplicated.
✔ Provides methods like put(), get(), remove(), containsKey(), containsValue().
✔ Implemented by HashMap, TreeMap, LinkedHashMap, Hashtable.
✔ Does not extend Collection because it works differently (keys + values).

Map Hierarchy

plaintextCopy codeMap (Interface)
├── HashMap
├── TreeMap
├── LinkedHashMap
└── Hashtable

Example Using Map Interface

import java.util.*;

public class MapExample {
    public static void main(String[] args) {
        Map<Integer, String> studentMap = new HashMap<>(); // Map reference
        studentMap.put(1, "Sujit");
        studentMap.put(2, "Patel");
        
        System.out.println(studentMap.get(1)); // Output: Sujit
    }
}

🔹 Here, Map stores key-value pairs, allowing quick lookups using keys.

3. Key Differences Between Collection and Map

Feature	Collection	Map
Type	Interface	Interface
Function	Stores a group of objects	Stores key-value pairs
Duplicates	Depends on `List` (allows) or `Set` (disallows)	Keys must be unique, but values can be duplicated
Order of Elements	`List` maintains order, `Set` does not	`HashMap` is unordered, `TreeMap` is sorted
Key-Value Storage	No (only individual elements)	Yes (key-value pairs)
Implementations	`ArrayList`, `HashSet`, `LinkedList`	`HashMap`, `TreeMap`, `LinkedHashMap`

4. When to Use Collection vs Map?

Use Collection when storing a group of elements (List, Set, Queue).
Use Map when storing key-value pairs and needing quick lookups.

5. Conclusion

Collection is a root interface for data structures like List, Set, and Queue.
Map is a separate interface used for key-value storage.
Map does not extend Collection because it follows a different structure.

Would you like a comparison between HashMap and TreeMap? 🚀

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