Java Language Patterns

Priority: P0 (CRITICAL)

Modern Java (21+) standards for concise, immutable, and expressive code.

Implementation Guidelines

• Records: Use record for DTOs/Value Objects. Avoid Lombok @Data where possible.
• Local Variables: Use var for clear, inferred types. Explicit types for interfaces.
• Switch: Use Switch Expressions -> and Pattern Matching over if/else chains.
• Text Blocks: Use """ for JSON/SQL strings. Avoid concatenation +.
• Pattern Matching: Use instanceof with binding: if (obj instanceof String s).
• Sealed Classes: Use sealed interface/class for restricted hierarchies (Domain Models).
• Collections: Use List.of(), Map.of(), Set.of() for unmodifiable collections.
• Streams: Use stream() for transformations. Prefer toList() (Java 16+) over collect(Collectors.toList()).
• Optional: Return Optional<T> over null. Use map, filter, ifPresent. Never use Optional fields/params.
• Virtual Threads: Prefer Executors.newVirtualThreadPerTaskExecutor() for high-throughput I/O.

Anti-Patterns

• No null: **No Nulls**: Return Optional or empty collections; avoid null parameters.
• No Raw Types: **Use Generics**: Never use raw types like List.
• No Old Switch: **No Fall-through**: Use Switch Expressions ->.
• Boilerplate: **No manual get/set**: Use Records or value objects.
• Blocking: **No synchronized**: Use java.util.concurrent utilities or Virtual Threads.

Code

// Record + Pattern Matching
public record User(String id, String name) {}

public String handle(Object obj) {
  return switch (obj) {
    case User(var id, var name) -> "User: " + name; // Record Patterns (Java 21)
    case String s when s.length() > 5 -> "Long String: " + s;
    case null -> "It's null";
    default -> "Unknown";
  };
}

// Virtual Threads (Loom)
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    var userDetails = scope.fork(() -> fetchUser());
    var orders = scope.fork(() -> fetchOrders());
    scope.join().throwIfFailed(); // Wait for both
}

Related Topics

best-practices | concurrency | testing