Collection in Java

What is Collection in Java?

A Collection in Java is a framework that provides an architecture to store and manage a group of objects. It includes interfaces (List, Set, Queue, Map) and classes (ArrayList, HashSet, LinkedList, etc.) for efficient data handling and manipulation.

The Java Collections Framework (JCF) provides reusable, high-performance data structures and algorithms that reduce programming effort and increase performance.

Collection Types Summary

Collection Implementation Examples

import java.util.ArrayList;
import java.util.List;

public class ListExample {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Mango");
        fruits.add("Apple"); // Duplicates allowed

        System.out.println("List elements: " + fruits);
        System.out.println("Element at index 1: " + fruits.get(1));
        System.out.println("Size: " + fruits.size());
    }
}

import java.util.HashSet;
import java.util.Set;

public class SetExample {
    public static void main(String[] args) {
        Set<Integer> numbers = new HashSet<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.add(10); // Duplicate ignored

        System.out.println("Set elements (unique): " + numbers);
        System.out.println("Contains 20? " + numbers.contains(20));
    }
}

import java.util.HashMap;
import java.util.Map;

public class MapExample {
    public static void main(String[] args) {
        Map<String, Integer> studentMarks = new HashMap<>();
        studentMarks.put("Alice", 90);
        studentMarks.put("Bob", 85);
        studentMarks.put("Charlie", 95);
        
        System.out.println("Student Marks: " + studentMarks);
        System.out.println("Alice's marks: " + studentMarks.get("Alice"));
        
        // Iterating through Map
        for (Map.Entry<String, Integer> entry : studentMarks.entrySet()) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
        }
    }
}

import java.util.PriorityQueue;
import java.util.Queue;

public class QueueExample {
    public static void main(String[] args) {
        Queue<String> queue = new PriorityQueue<>();
        queue.add("Task 1");
        queue.add("Task 2");
        queue.add("Task 3");
        
        System.out.println("Queue: " + queue);
        System.out.println("Removed: " + queue.poll());
        System.out.println("After removal: " + queue);
    }
}

Objectives of Collection Framework

• Represent and manipulate dynamic data groups efficiently
• Reduce programming effort using reusable data structures
• Provide high-performance, flexible data handling
• Allow interoperability between different collection types
• Support standard methods for common operations (add, remove, search, sort)

✅ Advantages

• Reduces development time with ready-to-use data structures
• Provides dynamic storage (no fixed size limitations)
• Improves performance with built-in algorithms
• Ensures consistency across different data types
• Includes thread-safe implementations (Vector, Hashtable)

⚠️ Limitations

• Can increase memory overhead compared to primitive arrays
• Type safety issues in non-generic collections (pre-Java 5)
• Performance trade-offs with certain operations (e.g., searching)
• Synchronization overhead in thread-safe collections

Collection Framework Hierarchy

The Collection Framework follows a well-defined hierarchy of interfaces and classes:

Best Practices for Java Collections