Java OOP & SOLID Design

Overview

Java is chosen precisely when the domain is rich (see backend-language-decision). That value is only realized if you model it with real objects that protect their invariants — not anemic data bags with a pile of setters and logic scattered in services.

Core principle: Objects own their invariants. If a rule about an entity can be violated from outside the entity, the model is broken.

SOLID, concretely

• S — Single Responsibility: one reason to change per class. A class that parses HTTP, applies business rules, and writes SQL is three classes.
• O — Open/Closed: extend behavior via new types/strategies, not by editing a growing switch. New payment method → new PaymentMethod implementation, not another case.
• L — Liskov: a subtype must honor the supertype's contract. If Square extends Rectangle breaks setWidth, the hierarchy is wrong — prefer composition.
• I — Interface Segregation: many small role interfaces over one fat one. A caller that needs read shouldn't depend on write.
• D — Dependency Inversion: services depend on interfaces (ports), not concrete adapters. Inject the interface via the constructor.

Encapsulation over anemic models

// ❌ anemic: invariant lives nowhere, anyone can break it
class Task {
    public String title;      // no bound enforced
    public Status status;
}
task.title = "x".repeat(500); // invalid state, no guard

// ✅ rich: the entity enforces its own rules
public final class Task {
    private final TaskId id;
    private String title;
    private Status status;

    private Task(TaskId id, String title) { this.id = id; this.title = title; }

    public static Task create(String title) {
        if (title == null || title.isBlank() || title.length() > 200)
            throw new InvalidTaskTitle(title);
        return new Task(TaskId.newId(), title);
    }

    public void rename(String title) { /* same guard, reused */ }
    public void complete() {
        if (status == Status.DONE) throw new IllegalTransition("already done");
        this.status = Status.DONE;
    }
    // getters, no public setters
}

Value objects over primitives

Wrap meaningful primitives: TaskId, Email, Money — not raw String/long/BigDecimal. A TaskId can't be accidentally passed where a UserId is expected, and validation lives in one place. Java record makes these cheap.

Prefer

• record for immutable value objects and DTOs.
• sealed interfaces + pattern matching for closed hierarchies (states, commands).
• Immutability by default; expose behavior methods, not setters.
• Throw domain exceptions (InvalidTaskTitle) mapped to HTTP status at the boundary — not raw IllegalArgumentException leaking out.

Prefer a sealed interface + exhaustive switch pattern matching (final since Java 21) for a closed set of states or commands over a type enum plus if/instanceof chains — the compiler then refuses to let a new case go unhandled.

Common Mistakes

• Public setters that let callers build invalid state.
• Business rules in the service that should be on the entity.
• String/long everywhere instead of value objects.
• A god-service with dozens of methods and no domain objects.
• Lombok @Data/@EqualsAndHashCode on a JPA entity — it pulls in lazy associations (forcing a load, or crashing outside the session) and generates a mutable, settable id that breaks equals/hashCode once the entity is persisted. Write equals/hashCode on the id only, or skip Lombok on entities.

Red Flags

• An entity with only getters/setters and no behavior.
• The same validation copy-pasted in two services → it belongs on the entity/value object.
• A switch over a type enum that grows every feature → missing polymorphism.