Collections Framework Overview

Data Structures, Ready-Made

The Collections Framework in java.util provides ready-made implementations of the most useful data structures, so you do not have to write linked lists or hash maps yourself.

The Three Main Interfaces

List in Practice

import java.util.*;

List<String> cities = new ArrayList<>();
cities.add("Delhi");
cities.add("Pune");
cities.add("Delhi");   // duplicates allowed
System.out.println(cities.get(1)); // Pune
System.out.println(cities.size()); // 3

Set in Practice

Set<String> unique = new HashSet<>();
unique.add("apple");
unique.add("banana");
unique.add("apple");   // ignored - no duplicates
System.out.println(unique); // [apple, banana]

Map in Practice

Map<String, Integer> scores = new HashMap<>();
scores.put("Asha", 92);
scores.put("Bimal", 85);
System.out.println(scores.get("Asha"));   // 92
System.out.println(scores.containsKey("Bimal")); // true

Choosing the Right Collection

• ArrayList: fast random access, fast append.
• LinkedList: fast insert/delete at ends for big lists.
• HashSet: fastest uniqueness checks; no order.
• TreeSet: unique elements kept sorted.
• HashMap: fastest key lookup; unordered.
• TreeMap: keys kept in sorted order.

Iterating

for (String c : cities) {
    System.out.println(c);
}
for (Map.Entry<String, Integer> e : scores.entrySet()) {
    System.out.println(e.getKey() + " = " + e.getValue());
}

Key Points

• Lists allow duplicates, Sets do not, Maps pair keys with values.
• Choose the simplest structure that meets your needs.
• Use generic types List<String> to keep collections type safe.