JavaData Structures
Java Data Structures
Data structures are ways to store and organize data so you can use it efficiently.
Anarray is an example of a data structure, which allows multiple elements to be stored in a single variable.
Java includes many other data structures as well, in thejava.util package. Each is used to handle data in different ways.Some of the most common are:
ArrayListHashSetHashMap
Tip: Data structures are like supercharged arrays - more flexible and feature-rich!
We'll explore all of these - and many more - in detail later, but for now, here's a quick introduction to each one.
ArrayList
AnArrayList is a resizable array that can grow as needed.
It allows you to store elements and access them by index.
Example
// Import the ArrayList classimport java.util.ArrayList;public class Main { public static void main(String[] args) { ArrayList<String> cars = new ArrayList<String>(); cars.add("Volvo"); cars.add("BMW"); cars.add("Ford"); cars.add("Mazda"); System.out.println(cars);}}
HashSet
AHashSet is a collection where every element is unique - no duplicates are allowed.
Example
// Import the HashSet classimport java.util.HashSet;public class Main { public static void main(String[] args) { HashSet<String> cars = new HashSet<String>(); cars.add("Volvo"); cars.add("BMW"); cars.add("Ford"); cars.add("BMW"); // Duplicate cars.add("Mazda"); System.out.println(cars); }}Note: In the example above, even though BMW is added twice it only appears once in the set because every element in a set has to be unique.
HashMap
AHashMap storeskey-value pairs, which are great when you want to store values and find them by a key (like a name or ID):
Example
// Import the HashMap classimport java.util.HashMap;public class Main { public static void main(String[] args) { // Create a HashMap object called capitalCities HashMap<String, String> capitalCities = new HashMap<String, String>(); // Add keys and values (Country, City) capitalCities.put("England", "London"); capitalCities.put("Germany", "Berlin"); capitalCities.put("Norway", "Oslo"); capitalCities.put("USA", "Washington DC"); System.out.println(capitalCities); }}Data Structures Overview
| Data Structure | Stores | Keeps Order? | Allows Duplicates? | Best For |
|---|---|---|---|---|
| ArrayList | Ordered elements | Yes | Yes | Accessing elements by index |
| HashSet | Unique elements | No | No | Avoiding duplicates, fast checks |
| HashMap | Key-value pairs | No | Yes (keys are unique) | Fast lookup by key |
Iterators
When learning about data structures, you will often hear about iterators too.
An iterator is a way to loop through elements in a data structure.
It is called an "iterator" because "iterating" is the technical term for looping.
Example
Using an Iterator with ArrayList:
import java.util.ArrayList;import java.util.Iterator;public class Main { public static void main(String[] args) { // Create an ArrayList of Strings ArrayList<String> cars = new ArrayList<String>(); cars.add("Volvo"); cars.add("BMW"); cars.add("Ford"); cars.add("Mazda"); // Get an iterator for the ArrayList Iterator<String> it = cars.iterator(); // Iterate through the list using the iterator while(it.hasNext()) { System.out.println(it.next()); } }}Next, let's take a closer look at each data structure in more detail.
