Java Patterns

Java Patterns

Intro

Modern Java leans on records, sealed types, pattern matching, and streams. Prefer immutable data carriers, exhaustive switches, and constructor injection over the Java-EE-era boilerplate.

Overview

Language features (17+)

Use record for immutable data carriers — you get equals, hashCode, toString for free. Use sealed interfaces to restrict hierarchies and unlock exhaustive switch. Use pattern matching with instanceof to skip casts. Use text blocks ("""...""") for multi-line strings. Prefer var for locals when the right-hand side makes the type obvious.

Records and sealed classes

Records replace boilerplate DTOs. Validate in the compact constructor:

record Age(int value) {
    Age {
        if (value < 0) throw new IllegalArgumentException();
    }
}

Sealed hierarchies plus records create algebraic data types:

sealed interface Result<T> permits Success, Failure {}
record Success<T>(T value) implements Result<T> {}
record Failure<T>(String error) implements Result<T> {}

Combined with pattern-matching switch, the compiler enforces exhaustiveness — adding a variant breaks the build at every consumer.

Stream API

Use streams for transformations, not side effects. Prefer toList() (Java 16+) over Collectors.toList(). The common pipeline is filter → map → collect. Use flatMap to flatten nested collections and groupingBy/partitioningBy for aggregation. Drop back to a for loop when it reads more clearly than a stream.

Optional

Return Optional<T> from methods that may not produce a result. Never use Optional as a field or parameter type. Chain with map, flatMap, and filter rather than isPresent() + get(). Use orElseThrow(() -> new NotFoundException(...)) for required values and Optional.empty() instead of returning null.

Spring Boot conventions

Use constructor injection, not field injection with @Autowired. Keep controllers thin — validate input, delegate to service, return a response. Put @Transactional on the service layer, not the controller. Configure via application.yml with typed @ConfigurationProperties classes and environment-specific profiles. Return ResponseEntity<T> for explicit status codes and centralize error handling in @RestControllerAdvice.

Testing

Use JUnit 5 with @DisplayName for readable output. Use @ParameterizedTest with @CsvSource or @MethodSource for data-driven cases. Mock with @Mock and @ExtendWith(MockitoExtension.class) rather than the legacy runner. Use AssertJ's fluent assertions (assertThat(result).isEqualTo(expected)). Use Spring test slices — @WebMvcTest for controllers, @DataJpaTest for repositories — to keep tests fast.

Gotchas

Agent-specific failure modes — provider-neutral pause-and-self-check items:

• Field injection with @Autowired on fields. Field injection hides dependencies, prevents immutability, and requires Spring's reflection to construct the object — making unit tests without Spring painful. Use constructor injection for all mandatory dependencies.
• Optional.get() without checking presence. Calling .get() on an empty Optional throws NoSuchElementException — the same crash you were trying to avoid with null. Use .orElseThrow(() -> new NotFoundException(...)) or .ifPresent(...) instead.
• Streams for trivial iteration. A stream over a single-step for loop adds indirection and hurts readability. Use a for loop when it's clearer; reach for streams only when the pipeline — filter → map → collect — adds genuine expressiveness.
• Wide @Transactional on controllers. Transactions should span a single unit of work inside the service layer. Annotating a controller method @Transactional creates a transaction that outlives the database interaction and crosses HTTP concern boundaries.
• Deep inheritance hierarchies over sealed types. Adding a new subclass to an open hierarchy requires updating every consumer that switches on type. Model closed sets with sealed interfaces and pattern-matching switch; the compiler enforces exhaustiveness.
• Collectors.toList() on Java 16+. The Collectors.toList() method returns a mutable list; Stream.toList() (Java 16+) returns an unmodifiable list and is shorter to write. Prefer toList() unless you need mutability.
• Optional as a field or method parameter. Optional is designed for return types only. Using it as a field adds boxing overhead and breaks serialization; using it as a parameter forces callers to wrap values unnecessarily. Use overloads or nullable with explicit documentation for parameters.

Full reference

Dependency injection

Prefer constructor injection — it makes dependencies explicit and enables testing without Spring. Inject interfaces, not concrete implementations. Use @Qualifier or custom annotations to disambiguate multiple beans. If a constructor has more than ~5 parameters, the class is probably doing too much — split it.

Code organization

Package by feature (com.app.order, com.app.user), not by layer (com.app.controllers, com.app.services). Keep one primary responsibility per class. Mark classes and fields final when you mean them to be immutable. Prefer composition and interfaces over deep inheritance. For larger systems, use module-info.java to enforce explicit module boundaries.

Pattern matching switch

String describe(Shape s) {
    return switch (s) {
        case Circle c    -> "circle r=" + c.radius();
        case Rectangle r -> "rect " + r.width() + "x" + r.height();
    };
}

With a sealed hierarchy, the compiler requires every permitted type to be handled — no default branch needed.

Anti-patterns

• Field injection with @Autowired — hides dependencies and requires reflection to test.
• Optional parameters or fields — use overloads or nullable with documentation instead.
• .get() on Optional without checking presence — use orElseThrow.
• Streams for trivial iteration where a for loop is clearer.
• Deep inheritance hierarchies of abstract classes — prefer composition and interfaces.
• Wide @Transactional annotations on controllers that cross business operations.
• Collectors.toList() on Java 16+ — use toList().

Common migrations

• POJO with getters/setters and equals/hashCode → record.
• Visitor pattern over an open hierarchy → sealed interface + pattern-matching switch.
• if/else if instanceof chains → pattern matching instanceof.
• Manual null-checks → Optional chains.
• Reflection-based DTO mappers between layers → records with explicit conversion methods.