Native lesson simulator
Hash map lookup
Hash the key, jump to a bucket, then compare entries there.
Ada hashes to bucket 2; lookup only scans entries in that bucket.
Flash cards
Review the key moves
What is the main idea behind Java HashSet?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
import java.util.HashSet; // Import the HashSet ___Put the learning moves in the order that makes the concept easiest to apply.
A HashSet is a collection of elements where every element is unique .
It is part of the java.util package and implements the Set interface.
Create a HashSet
Example
Create a HashSet object called cars that will store strings:
import java.util.HashSet; // Import the HashSet class
HashSet<String> cars = new HashSet<String>();Now you can use methods like add() , contains() , and remove() to manage your collection of unique elements.
Add Elements
To add elements to a HashSet , use the add() method:
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 sets do not allow duplicate elements.
Check If an Element Exists
To check whether an element exists in a HashSet , use the contains() method:
cars.contains("Mazda");Remove an Element
To remove an element, use the remove() method:
cars.remove("Volvo");To remove all elements, use the clear() method:
cars.clear();HashSet Size
Use size() to count how many unique elements are in the set:
cars.size();Note
Duplicate values are not counted - only unique elements are included in the size.
Loop Through a HashSet
Loop through the elements of an HashSet with a for-each loop:
Example
for (String i : cars) {
System.out.println(i);
}Other Types
Elements in an HashSet are actually objects. In the examples above, we created elements (objects) of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int , you must specify an equivalent wrapper class : Integer . For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:
HashSetThe var Keyword
From Java 10, you can use the var keyword to declare a HashSet variable without writing the type twice. The compiler figures out the type from the value you assign.
This makes code shorter, but many developers still use the full type for clarity . Since var is valid Java, you may see it in other code, so it's good to know that it exists:
// Without var HashSet<String> cars = new HashSet<String>();
// With var var cars = new HashSet<String>();The Set Interface
Note
Sometimes you will see both Set and HashSet in Java code, like this:
import java.util.Set;
import java.util.HashSet;
Set<String> cars = new HashSet<>();This means the variable (cars) is declared as a Set (the interface), but it stores a HashSet object (the actual set). Since HashSet implements the Set interface, this is possible.
It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.
When Order Matters
In the next chapter, you will learn about TreeSet , which stores unique elements in sorted order .