Language Java

Java

Records and sealed types

Modern Java models data without boilerplate.

record Point(double x, double y) {}

sealed interface Shape permits Circle, Rectangle {}
record Circle(Point center, double radius) implements Shape {}
record Rectangle(Point topLeft, Point bottomRight) implements Shape {}

• Use record for immutable data carriers — equals, hashCode, toString, and accessors for free.
• sealed closes a hierarchy; exhaustive switch replaces instanceof chains.

double area = switch (shape) {
    case Circle c    -> Math.PI * c.radius() * c.radius();
    case Rectangle r -> Math.abs(r.bottomRight().x() - r.topLeft().x())
                        * Math.abs(r.bottomRight().y() - r.topLeft().y());
};

Optional and null discipline

• Return Optional from methods that may have no result; never use it as a field or parameter type.
• Optional is not a null check replacement — it signals "absence is a valid outcome."
• Annotate parameters and fields with @NonNull / @Nullable for static analysis.

Optional find(String id) { ... }
find(id).map(User::email).orElseThrow(() -> new NotFoundException(id));

Concurrency — virtual threads (JDK 21+)

Virtual threads (Project Loom) make blocking-style IO safe at scale.

try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future  user  = scope.fork(() -> fetchUser(id));
    Future order = scope.fork(() -> fetchOrder(orderId));
    scope.join().throwIfFailed();
    return new Response(user.get(), order.get());
}

• Executors.newVirtualThreadPerTaskExecutor() — drop-in for newCachedThreadPool() with virtual-thread semantics.
• Do not pool virtual threads; create-per-task is the idiom.
• Watch for carrier-thread pinning: synchronized blocks and some native calls pin a virtual thread to its carrier. Prefer ReentrantLock when high-throughput blocking is expected.
• StructuredTaskScope (JDK 21–24 preview — not yet standard; enable with --enable-preview) enforces structured concurrency: tasks are joined before the scope exits.

Error handling

• Prefer unchecked exceptions at boundaries; translate checked exceptions from libraries early.
• try-with-resources for anything AutoCloseable — never close in a finally block manually.
• Exception translation: catch a library-specific exception at the boundary, rethrow as your domain exception.

try (var conn = dataSource.getConnection()) {
    // ...
} catch (SQLException e) {
    throw new RepositoryException("fetch user " + id, e);
}

Streams and collections

• Stream for transformations; avoid imperative loops when a pipeline is clearer.
• .toList() (JDK 16+) over Collectors.toList() — returns an unmodifiable list.
• Use List.of, Map.of, Set.of for small immutable collections; Map.copyOf to defensively copy.

List emails = users.stream()
    .filter(User::isActive)
    .map(User::email)
    .toList();

Build and packaging

• Maven: pom.xml with ` for BOM imports; prefer the wrapper (./mvnw`).
• Gradle: build.gradle (Groovy) or build.gradle.kts (Kotlin DSL — preferred for IDE support).
• JPMS (module-info.java): adopt only when publishing a library that needs strong encapsulation.

Tooling

• Imports/Format: mvn spotless:apply / ./gradlew spotlessApply
• Lint: mvn checkstyle:check / ./gradlew checkstyleMain
• Test: mvn test / ./gradlew test (see Testing below)

Testing

• JUnit 5 (@Test, @ParameterizedTest, @MethodSource) — not JUnit 4.
• AssertJ for fluent assertions: assertThat(actual).isEqualTo(expected).
• Mockito for external boundaries; do not mock domain objects.

@ParameterizedTest
@MethodSource("provideInputs")
void computesTax(double income, double expectedTax) {
    assertThat(TaxCalculator.compute(income)).isCloseTo(expectedTax, within(0.01));
}

Avoid

• Raw types (List instead of List).
• Returning or passing null where Optional or a sentinel value communicates intent.
• Mutable static state outside of intentional singletons.
• equals without a matching hashCode override.
• Blocking inside a reactive pipeline or CompletableFuture chain.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: lugassawan
• Source: lugassawan/swe-workbench
• License: MIT

Install and usage instructions live in the source repository linked above.