Collection vs List Java: What is Difference?

Collection vs List in Java

In Java, Collection and List are both part of the Java Collection Framework (JCF), but they have different roles:

Collection is a root interface that represents a group of objects.
List is a subinterface of Collection that maintains an ordered collection of elements.

1. What is Collection?

Collection is the top-level interface in the Java Collection Framework that defines basic methods for managing a group of objects.

Features of Collection Interface:

✔ Root interface for all collection classes (List, Set, Queue).
✔ Defines fundamental methods like add(), remove(), size(), and contains().
✔ Does not allow direct object instantiation (needs implementation classes like ArrayList, 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<Integer> numbers = new ArrayList<>(); // Collection reference
        numbers.add(10);
        numbers.add(20);
        
        System.out.println(numbers); // Output: [10, 20]
    }
}

🔹 Here, Collection is used as a reference type, but ArrayList is the actual implementation.

2. What is List?

List is a subinterface of Collection that represents an ordered collection (sequence) of elements.

Features of List Interface:

✔ Maintains insertion order (unlike Set).
✔ Allows duplicate elements.
✔ Supports index-based access (get(index)).
✔ Implemented by ArrayList, LinkedList, Vector.

Example Using List Interface

import java.util.*;

public class ListExample {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(); // List reference
        names.add("Sujit");
        names.add("Patel");
        names.add("Sujit"); // Allows duplicates
        
        System.out.println(names.get(1)); // Output: Patel
    }
}

🔹 List allows duplicate values and supports index-based access.

3. Key Differences Between Collection and List

Feature	Collection	List
Type	Interface	Subinterface of `Collection`
Function	General framework for collections	Ordered collection of elements
Order of Elements	No specific order	Maintains insertion order
Duplicates	Varies (`Set` disallows, `List` allows)	Allows duplicates
Index Access	No direct index access	Supports `get(index)` and `set(index, value)`
Implementation Classes	`ArrayList`, `HashSet`, `TreeSet`, `LinkedList`, etc.	`ArrayList`, `LinkedList`, `Vector`

4. When to Use Collection vs List?

Use Collection when you need a flexible reference type for different data structures (List, Set, Queue).
Use List when you need ordered storage and index-based access.

5. Conclusion

Collection is a generic interface that covers List, Set, and Queue.
List is a specific subinterface of Collection that maintains order and allows duplicates.

Would you like a comparison between ArrayList and LinkedList? 🚀

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