Module 5: Handling Collections of Data

Beyond Single Variables

Welcome to Module 5! So far, we've worked with variables that hold a single value or object. But what if you need to store a list of 100 users, or a directory of phone numbers? Real-world applications require us to work with groups of data. This module introduces you to Java's powerful tools for managing data collections, starting with the basic **Array** and moving on to the more flexible and powerful classes within the **Java Collections Framework**.

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1Arrays: Fixed-Size Lists

An **Array** is the most fundamental data structure for storing a collection of elements of the same type. Think of it as a row of numbered boxes, where each box holds a value. The key characteristic of an array is its **fixed size**; once you create an array to hold 5 items, you cannot change it to hold 6.

Declaring and Initializing an Array

You declare an array by specifying the data type followed by square brackets `[]`.

// Declare an array of integers that can hold 5 elements
int[] numbers = new int[5];

// Assign values to elements using their index (starts at 0)
numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;
numbers[3] = 40;
numbers[4] = 50;

// Access and print an element
System.out.println("The first number is: " + numbers[0]); // Outputs 10

// A shorter way to initialize an array
String[] fruits = {"Apple", "Banana", "Orange"};

Looping Through an Array

The `for` loop is perfect for iterating through the elements of an array.

for (int i = 0; i < fruits.length; i++) {
    System.out.println(fruits[i]);
}

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2The `ArrayList`: A Dynamic Array

Arrays are great, but their fixed size can be a limitation. What if you don't know how many items you'll need to store? The `ArrayList` from the Java Collections Framework solves this problem. It's a resizable array that can grow and shrink as you add or remove elements.

import java.util.ArrayList; // Must be imported

public class Main {
    public static void main(String[] args) {
        // Create an ArrayList of Strings
        ArrayList<String> tasks = new ArrayList<>();

        // Add elements to the list
        tasks.add("Buy groceries");
        tasks.add("Walk the dog");
        tasks.add("Learn Java");

        // Get an element by index
        System.out.println("Second task: " + tasks.get(1));

        // Remove an element
        tasks.remove("Walk the dog");

        // Loop through the list with an enhanced for-loop
        for (String task : tasks) {
            System.out.println(task);
        }
    }
}

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3The `HashMap`: Key-Value Pairs

What if you want to store data not as a simple list, but as a set of key-value pairs? A `HashMap` is perfect for this. Think of it like a dictionary, where you look up a word (the **key**) to find its definition (the **value**). Keys in a `HashMap` must be unique.

import java.util.HashMap; // Must be imported

public class Main {
    public static void main(String[] args) {
        // Create a HashMap to store user scores (String key, Integer value)
        HashMap<String, Integer> userScores = new HashMap<>();
        
        // Add key-value pairs
        userScores.put("Alice", 95);
        userScores.put("Bob", 80);
        userScores.put("Charlie", 100);

        // Get a value by its key
        System.out.println("Bob's score is: " + userScores.get("Bob"));
        
        // Loop through the keys
        for (String userName : userScores.keySet()) {
            System.out.println(userName + " scored " + userScores.get(userName));
        }
    }
}