CodeBlanks Logo Icon CodeBlanks
Login Register

HashSet

Learn unique-element sets with HashSet.

Unique set: HashSet

When you want to drop duplicates automatically, you use a HashSet. If an element is already present when you add it again, the second add is a no-op; the set stays the same.

HashSet<String> tags = new HashSet<>();
tags.add("java");
tags.add("java");
tags.add("code");
// [java, code]

Checking existence with contains is very fast; much faster than indexOf on a list. A set does not guarantee order; if order matters, prefer LinkedHashSet.

Looping over a set

You can walk a HashSet with a for-each loop. The output order of the elements may not match the insertion order, because HashSet makes no ordering promise.

for (String tag : tags) {
  System.out.println(tag);
}

To compute the intersection or union of two sets, you use retainAll and addAll. They let you do set operations in a single line.

Practice Collect unique tags

Create a HashSet<String>, add "java", "code", "java" and "maven". Then print the size of the set.

Starter code

Main.java
import java.util.HashSet;

public class Main {
  public static void main(String[] args) {
    // Create a HashSet<String>
    // Add four tags with add (java twice)
    // Print the size with size()
  }
}

Sample solution

Main.java
HashSet<String> tags = new HashSet<>();
tags.add("java");
tags.add("code");
tags.add("java");
tags.add("maven");
System.out.println(tags.size());

Expected output

3

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match scenario to collection

Term Meaning
Ordered, indexed list ArrayList
Key-value table HashMap
Unique element set HashSet
Ordered unique set LinkedHashSet

2 Fill in the blank: is it in the set?

Question 
HashSet<String> tags = new HashSet<>();
tags.add("java");
boolean has = tags.____("java");
Answer 
HashSet<String> tags = new HashSet<>();
tags.add("java");
boolean has = tags.contains("java");

Explanation: contains("java") returns true.

3 What happens if you add the same element twice to a HashSet?

  • Two copies are stored
  • The second add is ignored; the set stays unchanged Correct answer
  • It throws an error
  • The old element is removed and the new one is added

Explanation: Set elements are unique; the second add is ignored.

All lessons in Beginner

1 Java basics

  1. What is Java?
  2. The main method
  3. Comments
  4. Class skeleton
  5. javac and java
  6. Summary - Java basics

2 Variables

  1. int and double
  2. boolean
  3. String
  4. Naming variables
  5. final variables
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Type casting
  3. String concatenation
  4. Math class
  5. BigInteger and BigDecimal
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Assignment and increment
  6. Summary - Operators

6 String basics

  1. length and charAt
  2. equals and equalsIgnoreCase
  3. indexOf and substring
  4. toUpperCase and trim
  5. String.format
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. else and else if
  3. Nested if
  4. switch
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. do-while
  4. break and continue
  5. for-each
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Filling arrays
  4. Multi-dimensional arrays
  5. Copying arrays
  6. Summary - Arrays

11 Array methods

  1. Arrays.sort
  2. Arrays.toString
  3. Arrays.binarySearch
  4. Arrays.copyOf
  5. Arrays.fill
  6. Summary - Array methods

12 Collections

  1. ArrayList
  2. add and get
  3. HashMap
  4. put and get
  5. HashSet
  6. Summary - Collections

13 Methods

  1. Defining methods
  2. return
  3. void methods
  4. Method overloading
  5. Scope
  6. Summary - Methods

14 Method parameters

  1. Primitive parameters
  2. Object parameters
  3. varargs
  4. Multiple values via array
  5. final parameters
  6. Summary - Method parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String builder

  1. StringBuilder
  2. append
  3. insert and delete
  4. reverse
  5. toString
  6. Summary - String builder

17 Input and output

  1. Scanner
  2. nextInt and nextLine
  3. System.out.printf
  4. BufferedReader
  5. try-with-resources
  6. Summary - Input and output

18 Enum

  1. Declaring enums
  2. values and valueOf
  3. Enum fields
  4. Enum methods
  5. switch with enum
  6. Summary - Enum

19 Exception basics

  1. try/catch
  2. finally
  3. throw
  4. Exception hierarchy
  5. Multiple catch
  6. Summary - Exception basics

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together