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Java HashSet

Learn about Java HashSet. Understand how to store unique elements, check for existence, and perform set operations.

A HashSet is a collection of items where every item is unique.

Think of a VIP Party List 🎟️.

  • You can't be on the list twice.
  • If you try to add "John" again, the bouncer says "He's already on the list!" and ignores you.
  • The order doesn't matter; you're either on the list or you're not.

Creating a HashSet

import java.util.HashSet;

HashSet<String> cars = new HashSet<String>();

Common Operations

1. Add Items

cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("BMW"); // Duplicate! Will be ignored.
cars.add("Mazda");

System.out.println(cars);
// Output: [Volvo, Mazda, Ford, BMW] (Order is not guaranteed!)

2. Check if Item Exists

System.out.println(cars.contains("Mazda")); // Output: true

3. Remove Items

cars.remove("Volvo");
cars.clear(); // Removes ALL items

4. Set Size

System.out.println(cars.size());

Loop Through a HashSet

for (String i : cars) {
  System.out.println(i);
}

Tip 💡: HashSet is super fast for checking if something exists (contains()). It's much faster than ArrayList for this!

Did you know? 🧐 Internally, a HashSet uses a HashMap to store its elements! The elements you add are stored as keys in the map, and a dummy object is used as the value.

What is the main feature of a HashSet?

Note : Java is a statically-typed language. It means that all variables must be declared before they can be used.

Challenge

Complete this chapter to unlock the next one.

Challenge

Task:

Create a HashSet of Integers. Add 1, 2, and 1 again. Print the size.

Java LinkedList

Learn about Java LinkedList. Understand how it differs from ArrayList, when to use it, and its special methods for manipulating the beginning and end of the list.

Java HashMap

Learn about Java HashMap. Understand how to store data in Key-Value pairs, access elements efficiently, and iterate through maps.