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Study Guide

This guide suggests one way to study the repository without treating every Java release as equally important on the first pass.

The repository is organized by release, but learning Java features is easier if you group them by theme: functional style, platform modernization, language simplification, pattern matching, concurrency, and recent preview work.

For release months and historical naming context, use java-release-timeline.md.

How To Use The Examples

For each feature, read in this order:

  1. The version README, to understand the problem the feature solved.
  2. The example or notes class, to see the smallest useful representation.
  3. The matching test, to see the expected behavior as executable documentation.

For example:

src/main/java/net/jrodolfo/java_evolution/java08/StreamExamples.java
src/test/java/net/jrodolfo/java_evolution/java08/StreamExamplesTest.java

Run the examples through tests:

make test

Use JavaDoc when you want a browsable API reference:

make docs

The generated site is written to:

target/site/apidocs/index.html

First Pass

Start here if the goal is interview preparation or practical Java fluency.

Phase Java Versions Focus
Early foundations 1-4 Objects, interfaces, exceptions, threads, I/O, collections, reflection, regex, NIO
Pre-Java-8 foundations 5-7 Generics, enums, annotations, concurrency utilities, Project Coin, NIO.2
Functional Java 8 Lambdas, streams, method references, Optional, default methods, date/time
Platform/API modernization 9-11 Collections factories, modules, HTTP Client, String/File APIs, var
Language simplification 12-16 Switch expressions, text blocks, records, pattern matching
Modern Java style 17-21 Sealed classes, virtual threads, record patterns, pattern switch, sequenced collections
Post-Java-21 expansion 22-24 FFM, Stream Gatherers, Class-File API, source simplification, runtime/security notes
LTS and current-release maturity 25-26 Final vs preview/incubator status, scoped values, compact source files, runtime/removal notes

Do not try to memorize every JEP number on the first pass. Focus on explaining why the feature exists and where you would use it.

Recommended Study Order

  1. Java 1-4 Study object-oriented basics, interfaces, checked exceptions, threads, java.io, collections, dynamic proxies, regex, NIO, logging, and chained exceptions. These are the foundation beneath later Java evolution.

  2. Java 5-7 Study generics, enums, annotations, java.util.concurrent, try-with-resources, multi-catch, NIO.2, and fork/join. These explain the baseline that Java 8 built on.

  3. Java 8 Study lambdas, streams, Optional, method references, CompletableFuture, default methods, and the date/time API. These are still daily-use features.

  4. Java 9-11 Study var, HttpClient, String APIs, files APIs, Predicate.not, and Optional.isEmpty. These show how Java became more concise without becoming dynamic.

  5. Java 12-16 Study switch expressions, text blocks, records, pattern matching for instanceof, and Stream.toList. These are common in modern code reviews and migration discussions.

  6. Java 17-21 Spend extra time here because Java 17 and Java 21 are LTS releases. Focus on sealed classes, strong encapsulation, virtual threads, record patterns, pattern matching for switch, and sequenced collections.

  7. Java 22-24 Study these as post-Java-21 expansion releases. Focus on final platform APIs, source simplification, and the difference between executable examples and explanatory modules.

  8. Java 25-26 Study these for LTS and current-version awareness. Pay attention to status labels: final, preview, incubator, notes-only, runtime, tooling, cryptography, or removal.

What To Skip Initially

It is reasonable to skim these on the first pass:

  • low-level runtime and GC notes
  • cryptography provider details
  • incubator APIs
  • native interop details
  • source launcher behavior
  • JFR/AOT operational features

These are useful, but they are less likely to matter before you understand the language and library features used in everyday code.

How To Review One Java Version

Use this checklist:

  1. Read the package README.
  2. Name each feature in one sentence.
  3. Explain what problem each feature solved.
  4. Open each example class and read the JavaDoc.
  5. Open the matching test and read the assertion messages.
  6. Run the tests for the whole project with make test.

For a quick version review, use feature-map.md.

For JEP lookup, use jep-index.md.

Interview Prep Path

If time is limited, prioritize these topics:

  • Java 8: lambdas, streams, Optional, date/time
  • Java 10: var
  • Java 11: HTTP Client and String APIs
  • Java 12-16: switch expressions, records, pattern matching
  • Java 17-21: sealed classes, virtual threads, record patterns, pattern matching for switch, sequenced collections
  • Java 22-24: post-Java-21 platform expansion
  • Java 25-26: final vs preview awareness and baseline discipline

The most convincing interview answer is not "I know Java 21." It is:

I can explain what changed, why it changed, and I have small tested examples here.