Java — Collections

ArrayList

import java.util.*;

ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.add("Charlie");

System.out.println(names.get(0)); // Alice
System.out.println(names.size()); // 3
names.remove("Bob");
names.set(0, "Alicia");

LinkedList

LinkedList<Integer> nums = new LinkedList<>();
nums.addFirst(1);
nums.addLast(3);
nums.add(1, 2); // Add at index

System.out.println(nums.getFirst()); // 1
System.out.println(nums.getLast());  // 3

HashSet

HashSet<String> set = new HashSet<>();
set.add("Apple");
set.add("Banana");
set.add("Apple"); // Duplicate ignored

System.out.println(set.size()); // 2
System.out.println(set.contains("Banana")); // true

TreeSet

TreeSet<Integer> sorted = new TreeSet<>();
sorted.add(5);
sorted.add(2);
sorted.add(8);
sorted.add(1);

System.out.println(sorted); // [1, 2, 5, 8]
System.out.println(sorted.first()); // 1
System.out.println(sorted.last());  // 8

HashMap

HashMap<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);

System.out.println(ages.get("Alice")); // 30
System.out.println(ages.containsKey("Charlie")); // false

// Iterate
for (Map.Entry<String, Integer> entry : ages.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

Collections utilities

import java.util.*;

List<Integer> nums = Arrays.asList(5, 2, 8, 1, 9);

Collections.sort(nums);
Collections.reverse(nums);
Collections.shuffle(nums);
int max = Collections.max(nums);
int min = Collections.min(nums);

Iterating

List<String> names = List.of("Alice", "Bob", "Charlie");

// Enhanced for loop
for (String name : names) {
    System.out.println(name);
}

// forEach
names.forEach(System.out::println);

// Iterator
Iterator<String> it = names.iterator();
while (it.hasNext()) {
    System.out.println(it.next());
}

Mini Practice

Write Java code that:

1. Creates an ArrayList and manipulates it
2. Uses a HashMap to count word frequencies
3. Uses Collections.sort with a comparator
4. Iterates a map with entrySet

Up Next

In the next lesson, you'll learn about Generics — type-safe reusable code.