Collection Framework

Let’s dive deep into one of the most fundamental pillars of Java:

🧺 Java Collections Framework – Deep Dive

🧠 What Is the Collections Framework?#

The Java Collections Framework (JCF) is a unified architecture for representing and manipulating groups of objects.
It includes interfaces, classes, and algorithms to store, retrieve, and process data efficiently.

✅ Why Use Collections?#

🧱 Core Interfaces#

🔹 List Interface#

Ordered, indexed, allows duplicates

List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
list.add("A"); // duplicates allowed
System.out.println(list.get(1)); // Output: B

Key Implementations:#

🔹 Set Interface#

Unordered, no duplicates

Set<String> set = new HashSet<>();
set.add("A");
set.add("B");
set.add("A"); // ignored

Key Implementations:#

🔹 Queue Interface#

FIFO behavior, good for scheduling and processing

Queue<String> queue = new LinkedList<>();
queue.offer("A");
queue.offer("B");
System.out.println(queue.poll()); // A

🔹 Map Interface#

Key-value pairs, keys must be unique

Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(2, "B");
map.put(1, "C"); // overwrites value for key 1
System.out.println(map.get(1)); // Output: C

Key Implementations:#

📦 Comparison Summary#

🧠 Important Notes#

• List vs Set: Use List when order and duplicates matter. Use Set when uniqueness is required.
• Map is not a subtype of Collection. It’s a different design (key-value mapping).
• TreeSet/TreeMap use natural ordering or custom Comparator.

🧰 Common Algorithms in Collections class#

Collections.sort(list);
Collections.reverse(list);
Collections.shuffle(list);
Collections.max(list);
Collections.min(list);
Collections.frequency(list, "A");

🚨 Best Practices#

• Always program to the interface (List instead of ArrayList).
• Choose the right data structure based on performance needs.
• Use unmodifiable collections for immutability:

List<String> readOnly = Collections.unmodifiableList(list);

• Use EnumMap, EnumSet if your keys/elements are enums (super fast).

🏠 Homework for Students#

1. List Practice#

• Create an ArrayList<String>, add 5 names.
• Sort the list, reverse it, shuffle it.

2. Set Practice#

• Create a Set<Integer> with some duplicate values.
• Try printing it and verify that duplicates are removed.

3. Map Practice#

• Create a Map<String, Integer> for employee names and salaries.
• Print the name of the highest-paid employee.

4. TreeSet & TreeMap#

• Create a TreeSet<String> and add 5 names. Observe the sorting.
• Create a TreeMap<Integer, String> with roll numbers as keys.

5. Performance Comparison#

Write code to insert 1 million integers into:

• ArrayList
• LinkedList
• HashSet Compare the time taken using System.nanoTime().

Would you like to dive next into Concurrent Collections, or go into File I/O (java.nio or java.io), or Multithreading Basics next?