Skip to content

WHAT_IS_IT.md

Latest commit

 

History

History
123 lines (86 loc) · 5.11 KB

WHAT_IS_IT.md

File metadata and controls

123 lines (86 loc) · 5.11 KB
Raw

What Is This Project?

Overview

Java8Practice is a structured, example-driven learning project for developers who want to deeply understand modern Java features (Java 8 → 21) by reading and running real code — not just reading theory.

The project now targets Java 21 (LTS) and uses its features throughout: var, switch expressions, Stream.toList(), String.formatted(), Optional.isEmpty(), and more.

Each package in the project represents one concept, with production-style code and tests that demonstrate the concept in action.

🎯 Who Is This For?

  • Java developers transitioning from Java 7 to Java 8+
  • Developers who have heard about Optional, lambdas, and functional interfaces but want to see them used properly
  • Anyone who wants to understand why these features exist and when to use them in real projects

🧠 What Will You Learn?

1. Optional<T> — Eliminating NullPointerException

Location: src/main/java/optionaldemoone/ and src/test/java/optionaldemoWithPerson/

Optional is a container object that may or may not hold a non-null value. It forces the caller to explicitly handle the "no value" case, making your code safer and more self-documenting.

Real-world scenario covered:

  • A Customer may or may not have a MemberCard
  • A MemberCard may or may not qualify for a discount
  • The DiscountService chains these optionals without a single null check

Key methods practiced:

Method What it does
Optional.of(value) Creates an Optional with a non-null value
Optional.ofNullable(value) Creates an Optional that handles null
Optional.empty() Creates an empty Optional
.map(fn) Transforms the value if present
.flatMap(fn) Like map, but for functions that return Optional
.orElse(default) Returns value or a default
.orElseGet(supplier) Like orElse, but lazy (computed only when needed)
.orElseThrow(supplier) Returns value or throws an exception
.ifPresent(action) Runs an action if value is present
.ifPresentOrElse(action, emptyAction) Handles both cases explicitly
.filter(predicate) Keeps the value only if it matches a condition
.or(supplier) Returns this Optional or another one if empty (Java 9+)
Optional::stream Converts Optional to a Stream (Java 9+)

2. Function<T, R> — Functional Interface Basics

Location: src/main/java/functionalInterface/function/myimpl/

Function<T, R> is one of the core functional interfaces in Java 8. It takes one input and produces one output.

Key concepts practiced:

  • Implementing Function<T, R> as a class (PerformSquare)
  • Using lambdas as Function
  • andThen(Function) — compose functions left-to-right
  • compose(Function) — compose functions right-to-left

3. JUnit 5 Tagging & Test Organisation

Location: src/test/java/junitdemo/

In real projects, you run different subsets of tests in different phases of your CI/CD pipeline (unit tests, integration tests, sanity checks). JUnit 5 @Tag enables this.

Key concepts practiced:

  • @Tag("integration") — mark a class or method as an integration test
  • @Tag("ecomm-unit-tests") — domain-specific tags
  • @DisplayName("...") — human-readable test names
  • Running tagged groups with Maven: mvn test -Dgroups="integration"

📦 Package Breakdown

Package Type Topic
functionalInterface.function.myimpl Main Function<T,R>, andThen, compose
optionaldemoone Main Optional with Customer/MemberCard domain
jenkins Main CI/CD placeholder (empty)
junitdemo Test JUnit 5 @Tag, @DisplayName
optionaldemoone Test DiscountService Optional pipeline tests
optionaldemoWithPerson Test Optional with streams, flatMap, filter

🔑 Key Design Philosophy

"Make the impossible states unrepresentable."

This project demonstrates that instead of sprinkling if (x != null) checks everywhere, you can model optional values explicitly using Optional<T>. This approach:

  1. Self-documents your API — callers know a field might be absent
  2. Forces handling of the absent case — no more silent NullPointerException
  3. Enables chaining — clean, readable pipelines without nested null checks
  4. Works beautifully with streams — Optional::stream and flatMap on Stream<Optional<T>>

▶️ Quick Start

# Clone and run all tests
mvn test

# Run only the Optional discount tests
mvn test -Dgroups="Rel1,Rel2,Rel3,Rel4"

📖 Further Reading