Java Stream API Practice Exercises for Java 17+

How to use? 🛠️

1. Fork this repository.
2. Remove the @Disabled Tags.
3. Solve the Problem.

NOTE: Don't forget to rate this to bookmark.

This repository is a Java Stream API practice project built with Java 17, Maven, and JUnit 5. It helps developers learn Java streams through step-by-step notes, intermediate and terminal operation examples, and hands-on coding exercises across easy, medium, and hard difficulty levels.

If you are searching for Java Stream API exercises, Java stream interview-style practice, functional programming examples in Java, or Stream API problems with solutions, this project is designed to be a practical learning resource.

Why this repository is useful

• Learn the Java Stream API from core concepts to advanced problem solving.
• Practice filter, map, flatMap, distinct, sorted, reduce, collect, and more.
• Solve JUnit-based exercises where your answer is compared against a reference solution.
• Explore related topics like functional interfaces and stream gatherers.
• Use a repository layout that is easy to study, fork, and extend.

Learning path

1. Start with learn/concepts
2. Review learn/intermediate_operations
3. Review learn/terminal_operations
4. Solve practice/a_easy_problems
5. Move to practice/b_medium_problems
6. Finish with practice/c_hard_problems
7. Explore practice/functional_interfaces
8. Explore gatherer notes in learn/gatherers and gatherer exercises in practice/gatherers

For a topic-by-topic index, open TOPICS.md.

Repository structure

src/test/java/com/github/streams/
├── learn/
│   ├── concepts/
│   ├── gatherers/
│   ├── intermediate_operations/
│   └── terminal_operations/
└── practice/
    ├── a_easy_problems/
    │   ├── numbers/
    │   └── strings/
    ├── b_medium_problems/
    │   ├── employee/
    │   ├── numbers/
    │   └── strings/
    ├── c_hard_problems/
    │   └── numbers/
    ├── functional_interfaces/
    └── gatherers/

Each exercise follows the same pattern:

1. Open a test class under src/test/java/com/github/streams/practice/...
2. Remove the @Disabled annotation
3. Write your Stream API solution in the yourSolution variable
4. Run the test and compare your implementation against mine.

What you will find inside

Java Stream API concept notes

• Beginner-friendly stream fundamentals
• Stream vs collection differences
• Lazy evaluation
• Optional results
• When not to use the Streams API

Intermediate operation guides

• filter
• map
• flatMap
• distinct
• sorted
• peek
• limit
• skip
• takeWhile
• dropWhile
• mapMulti

Terminal operation guides

• count
• forEach
• forEachOrdered
• toArray
• min / max
• anyMatch / allMatch / noneMatch
• findFirst / findAny
• reduce
• collect

Practice tracks

• Easy number and string problems
• Medium number, string, and employee aggregation problems
• Hard number challenges such as prime streams and consecutive sequence problems
• Functional interface exercises with Predicate, Function, Consumer, Supplier, and more
• Gatherer practice for modern stream processing patterns

Who this repository is for

• Java beginners learning the Stream API
• Intermediate Java developers looking to solidify their stream knowledge
• Experienced Java engineers preparing for coding interviews that involve stream-based problems
• Developers preparing for coding interviews involving streams
• Engineers who want Java functional programming examples
• Students looking for JUnit-based practice problems with solutions

Support the project

If this repository helps you learn Java streams, consider starring it so more developers can discover it.