Part 4 · 1 chapters · ~8 min

Idioms

Effective Java in modern form: immutability by default, records over Lombok, composition over inheritance, small interfaces, dependency injection through constructors, streams used with restraint, exceptions as part of the API, try-with-resources, and package structure by feature.

Writing modern Java

code

// constructor injection: dependencies explicit, fields final, easy to test without a container
@Service
public class TransferService {
    private final LedgerRepository ledger; private final Clock clock;
    public TransferService(LedgerRepository ledger, Clock clock) { this.ledger = ledger; this.clock = clock; }
}

// try-with-resources: closed even on exceptions
try (var conn = dataSource.getConnection(); var ps = conn.prepareStatement(SQL)) { ... }

// streams with restraint: clear transformations, not puzzles
long pendingKobo = transfers.stream().filter(t -> t.state() == State.PENDING).mapToLong(Transfer::amountKobo).sum();

modern idiom	instead of
records for data	Lombok @Data classes, mutable POJOs
constructor injection	@Autowired fields
sealed types + switch	instanceof chains and visitor boilerplate
packages by feature (`transfers`, `ledger`)	packages by layer (`controllers`, `services`, `repositories`)
`Clock` injected	`LocalDateTime.now()` scattered (untestable)
`java.time` types	`Date` and `Calendar`