Module 11: Collections Framework in Practice

Sets for Uniqueness

Use sets to remove duplicates and check membership efficiently.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-17

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, manually edited for clarity, and checked against current Java documentation and runnable examples.

Learning goals

Sets are about uniqueness, not position: If duplicates should collapse into one value, a set may be the right abstraction.

HashSet is the usual default: It is fast for membership tests and does not promise iteration order.

LinkedHashSet keeps insertion order, TreeSet keeps sorted order: Those ordering guarantees come with different tradeoffs.

Practical question: Do you need uniqueness only, uniqueness plus input order, or uniqueness plus sorted order?

Runnable examples

Duplicates are ignored in a set

java.util.Set<String> tags = new java.util.HashSet<>();
tags.add("java");
tags.add("java");
tags.add("streams");
System.out.println(tags.size());

Expected output

Mini exercise

Use a set to remove duplicate course tags from a list of repeated strings.

Summary

Next step

After sets, move to maps when values must be found by key.

Sources used