Modern Java Roadmap

The flow

1. 1.Choose baseline
2. 2.Model data
3. 3.Handle variants
4. 4.Compile
5. 5.Verify

A practical upgrade sequence

Keep the existing Java 8 topics: streams, lambdas and Optional remain useful. Add records (stable in Java 16), sealed types (Java 17), and pattern switch and virtual threads (Java 21) as explicit milestones. Do not label every feature in a new JDK as final.

This curriculum targets Java 21 without preview flags. Running a newer JDK does not automatically make the application compatible with an older runtime: use --release to compile against the chosen Java API and bytecode level.

Make states explicit

The example combines a sealed result family with record values and an exhaustive switch. Adding another permitted result forces the switch to be reconsidered at compilation, rather than hiding an unhandled case behind a default branch.

Use closed hierarchies when the variants are controlled together. Plugin ecosystems with unknown third-party extensions may need open interfaces instead.

Upgrade the system, not only syntax

Check framework, build plugin, agent and library support; compare startup, GC behavior and representative workloads. Keep a rollback artifact. Features such as structured concurrency must be checked against the exact JDK release and preview policy before becoming a production dependency.

Try it

import java.util.List;
public class ModernJava {
    sealed interface Result permits Success, Failure {}
    record Success(List<String> topics) implements Result {
        Success { topics = List.copyOf(topics); }
    }
    record Failure(String reason) implements Result {}
    static String describe(Result result) {
        return switch (result) {
            case Success(var topics) -> "Topics: " + topics.size();
            case Failure(var reason) -> "Failed: " + reason;
        };
    }
    public static void main(String[] args) {
        if (!describe(new Success(List.of("records"))).equals("Topics: 1")) {
            throw new AssertionError("Unexpected success branch");
        }
        if (!describe(new Failure("missing")).equals("Failed: missing")) {
            throw new AssertionError("Unexpected failure branch");
        }
        System.out.println("Java 21: records, sealed types and exhaustive pattern switch.");
    }
}

Run npm run examples:java with JDK 21+. Compiled with --release 21, without preview flags.

Interview questions

Official references

• Java language changes

Continue learning

• Java Records →
• Virtual Threads →