java

Java

Design

• Composition over inheritance. Extend only for true is-a relationships
• Immutability by default. final fields, no setters unless mutation required
• Records for data carriers (16+). No boilerplate POJOs
• Sealed classes for closed type hierarchies (17+). Use exhaustive switch expressions over them
• Pattern matching instanceof (16+): if (obj instanceof String s) — no explicit cast
• Small interfaces. One concern per interface
• Factory methods or builders over telescoping constructors
• No null in public APIs. Optional<T> for absent values. Never Optional as field type
• Objects.requireNonNull(param, "param") at method entry for non-null enforcement

Errors

• Checked exceptions for recoverable conditions callers must handle. Unchecked for programming errors
• Never catch Exception or Throwable except at boundaries (HTTP handler, queue consumer)
• Log full stack trace. Never swallow exceptions silently
• try-with-resources for all Closeable. No manual finally close blocks
• Catch specific exceptions. Never use exceptions for flow control

Modern Java

• var where type is obvious from right-hand side. Not to obscure types
• Streams for transformation pipelines. Stream.toList() (16+) over Collectors.toList()
• List.of(), Map.of(), Set.of() for immutable collections. List.copyOf() for defensive copy
• Sequenced collections (21+): SequencedCollection, getFirst(), getLast() over index hacks
• switch expressions over statements. Pattern matching in switch (21+) with exhaustive coverage
• Unnamed patterns and variables (22+, stable 25): catch (IOException _), case Point(int x, _)
• Record patterns (21+): if (obj instanceof Point(int x, int y))
• Text blocks for multiline strings (SQL, JSON, HTML). No string concat across lines
• Stream gatherers (22+, stable 25): stream.gather(...) for custom intermediate operations
• instanceof checks before casts eliminated — use pattern matching everywhere