Java Data Structures

Java Data Structures

Data structures are ways to store and organize data so you can use it efficiently.

An array is an example of a data structure, which allows multiple elements to be stored in a single variable.

Some of the most common are:

• ArrayList
• HashSet
• HashMap

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

An ArrayList is a resizable array that can grow as needed.

It allows you to store elements and access them by index.

Example

// Import the ArrayList class
import 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

A HashSet is a collection where every element is unique - no duplicates are allowed.

Example

// Import the HashSet class
import 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

A HashMap stores key-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 class
import 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

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

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.