Java Learning Path: Start Here

In short: you learn core Java in 7 milestones, building one small, tested project in each. Then you move on to the SDET track, where Java is used most on this site.

How to use this page

Read this page once to see the plan. Then, for each milestone, follow the same four steps: learn → build → test → commit. Come back here whenever you're unsure what to do next.

The pages in this learning path

In short: each page has one job, so nothing is explained twice.

Page	What it's for	When to open it
This roadmap	The plan: what each milestone covers and how to check yourself	At the start of each milestone
Java cheat sheet	Learn each idea: short explanation, example, exercise	The "learn" step of every milestone
Milestones & Mini-Projects	A full working project with tests for each milestone	The "build" and "test" steps
Quick Reference	Look up syntax and library calls	Any time you're coding
Coding Best Practices	Habits that keep code clean and safe	After Milestone 5, then before every commit
Java: The Complete Guide	JVM internals and interview questions	When you want the deeper "why"

Part 1 — Core Java: Milestones 1–7

In short: everyone does these seven, in order. Each one ends with a working project in your portfolio.

Milestone	You learn	You build
1	Setup, types, text, switch, methods	Temperature converter
2	Lists, sets, maps, sorting	Word counter
3	Loops, conditions, exceptions	Password validator
4	Generics, lambdas, functional interfaces	Retry helper
5	Classes, inheritance, interfaces, records, enums	Bank accounts
6	Streams, Optional, files	Log analyzer
7	Build tools, JUnit, concurrency	Concurrent health checker

For each milestone:

1. Learn — read the cheat-sheet sections listed for that milestone below. Type the examples into jshell.
2. Build — follow its project in Milestones & Mini-Projects. Type the code; don't paste it.
3. Test — run mvn test until everything passes. Then do the project's Try it.
4. Commit — save your progress with git:

git add .
git commit -m "Milestone 1: temperature converter, 8 tests passing"
git push

Milestone 1: Getting started

Learn: Your first program · Variables & types · Text & numbers · Making decisions & loops · Methods

Build: Temperature converter

Check yourself:

• What's the difference between int and Integer?
• Why does 7 / 2 give 3, and how do you get 3.5?
• What does static mean on a method?

Milestone 2: Collections

Learn: Arrays & lists · Sets, maps & queues · Quick Reference: Sorting

Build: Word counter

Check yourself:

• When would you pick a HashSet over an ArrayList?
• What does map.merge(key, 1, Integer::sum) do when the key is missing?
• How do you sort by one field, then another?

Milestone 3: Control flow & errors

Learn: Handling errors · Reading & writing files · Common mistakes

Build: Password validator

Check yourself:

• What's the difference between a checked and an unchecked exception?
• Why should you compare strings with equals, not ==?
• What does try-with-resources do for you?

Milestone 4: Generics & lambdas

Learn: Generics · Lambdas

Build: Retry helper

Check yourself:

• What does the <T> in static <T> T first(List<T> items) mean?
• Name three functional interfaces and what each takes and returns.
• Why can't a lambda change a local variable from outside it?

Milestone 5: Object-oriented design

Learn: Classes & objects · Records · Inheritance & interfaces · equals, hashCode & toString · Enums & switch patterns

Build: Bank accounts

Then read: Coding Best Practices, sections 1–5.

Check yourself:

• When would you use an interface, an abstract class, and a record?
• What goes wrong if you override equals but not hashCode?
• Why store money as long cents instead of double?

Milestone 6: Streams & files

Learn: Streams · Optional · Immutability · Quick Reference: Collectors, Files

Build: Log analyzer

Check yourself:

• Why does nothing happen until a stream's last step?
• What does groupingBy return?
• When should a method return Optional, and when an empty list?

Milestone 7: Build, test & concurrency

Learn: Build tools · Testing with JUnit · Threads & thread pools · Sharing data safely · CompletableFuture · The JVM & memory

Build: Concurrent health checker

Check yourself:

• What's a race condition, and name two ways to prevent one?
• When are virtual threads a good fit, and when aren't they?
• How did putting the network call behind an interface make the tests possible?

Part 2 — Your role track: Milestone 8 onwards

In short: on this site Java is taught for SDET work (Software Development Engineer in Test) — building automated tests and test frameworks.

Step	Read	You build
8	Java for SDET, sections 2–6: unit tests, fixtures, data-driven tests, mocks, API tests	Tests for your Milestone 5 and 7 projects
9	Java for SDET, sections 7–12: page objects, waits, test data builders, tags, reports, CI	A small UI test framework that runs in CI
10	Tool guides: JUnit, TestNG, Rest Assured, Selenium	Pick the tools your team uses
Final	Java for SDET: practice projects	A full test automation suite running in CI (Continuous Integration)

Working as an SDE or SRE? The same Milestones 1–7 apply; then use Java: The Complete Guide for the deeper language topics.

Your portfolio

In short: by Milestone 7 you'll have seven small, tested projects in one GitHub repository.

java-mastery/
├── pom.xml
├── README.md            # one line per milestone: what it does, how to run it
└── src/
    ├── main/java/com/example/m1 … m7/
    └── test/java/com/example/m1 … m7/

Your role track then adds one larger final project.

How you know you're ready to move on

After Milestone 7:

• All seven projects built; mvn test passes.
• You can explain when to use a List, Set, or Map.
• You can read a file, handle errors, and test both the good and bad paths.
• Your git history shows steady progress.

After your role track:

• You finished the final project, and it runs in CI.
• You can explain the design choices in it to someone else.

If you get stuck

Problem	What to do
The code won't compile	Read the first error only — later ones are often caused by it. The line number and ^ marker point at the problem.
mvn test finds no tests	Test classes must end in Test, sit under src/test/java, and use org.junit.jupiter.api.Test.
A milestone feels too hard	Re-read its cheat-sheet sections, and run each example on its own in jshell.
"I already know Java"	Still do Milestones 1–7, but do every Try it and add more tests.
Not enough time	Do less, but regularly. One milestone done well beats four rushed.

Start now

Install an LTS (Long-Term Support) JDK — Java 21 or 25 — and Maven, then create the project from Project setup.

java -version            # should say 21 or higher
mvn -version
mkdir -p ~/projects/java-mastery && cd ~/projects/java-mastery
git init
jshell                   # try a few lines, then /exit

Helpful tools:

Tool	What it does
IntelliJ IDEA Community	Free editor with Java-aware hints and a test runner
jshell	Try any snippet instantly
SDKMAN! (sdk install java)	Install and switch between Java versions

Good books to read alongside: Effective Java (Joshua Bloch) for idiomatic Java, and The Pragmatic Programmer (Hunt & Thomas) for professional habits.