Chapter 17: Systematic Application Guide

What You’ll Learn

• 8 checkpoints for consistent JBCT application
• Quick reference tables for immediate lookup
• Violation -> Fix patterns for common mistakes
• Application order for new code and reviews

Prerequisites: Chapter 16: Project Structure

The Checkpoint Approach

JBCT application works through checkpoints - specific moments during coding where you pause and verify compliance. Each checkpoint has:

• Trigger: When to apply this checkpoint
• Rules: What to check
• Fixes: How to correct violations

The goal is 100% compliance through systematic verification, not heroic effort.

CHECKPOINT 1: Before Writing Any Lambda

Trigger: You’re about to write ->

Rules to Check

Allowed Lambda Forms (Exhaustive)

// Method references (ALWAYS PREFERRED)
.map(Email::new)
.flatMap(this::validate)

// Single-value expression (no braces)
.map(value -> expression)
.filter(s -> !s.isBlank())

// Multi-value expression (no braces)
.map((a, b) -> new Pair(a, b))

Extraction Pattern

// BEFORE (violation)
.map(data -> {
    cache.put(key, data);
    log.info("Cached: {}", data);
    return data.size();
})

// AFTER (compliant)
.map(this::cacheAndCount)

private int cacheAndCount(Data data) {
    cache.put(key, data);
    log.info("Cached: {}", data);
    return data.size();
}

CHECKPOINT 2: Choosing Return Type

Trigger: Writing a method signature

Decision Tree

Can this operation fail?
|-- NO: Can the value be absent?
|   |-- NO -> return T
|   |-- YES -> return Option<T>
|-- YES: Is it async/IO?
    |-- NO -> return Result<T>
    |-- YES -> return Promise<T>

Rules to Check

CHECKPOINT 3: Writing Factory Methods

Trigger: Creating construction logic for a type

Rules to Check

Pattern

public record Email(String value) {
    // Factory with validation -> Result<T>
    public static Result<Email> email(String raw) {
        return Verify.ensure(raw, Verify.Is::present)
            .map(String::trim)
            .filter(INVALID_EMAIL, s -> PATTERN.matcher(s).matches())
            .map(Email::new);
    }
}

public record Config(DbUrl url, DbPassword pass) {
    // Factory without validation -> T
    public static Config config(DbUrl url, DbPassword pass) {
        return new Config(url, pass);
    }
}

CHECKPOINT 4: Designing a Class/Interface

Trigger: Creating a new type

Rules to Check

Zone Placement

Zone A (Entry): Controllers, handlers, main
  -> Business action verbs: handleRegistration(), processOrder()

Zone B (Domain): Use cases, value objects, domain services
  -> Domain vocabulary: Email.email(), ValidRequest.validRequest()

Zone C (Infrastructure): DB, external APIs, config loading
  -> Technical names: loadAllGenerations(), saveUser(), readFile()

CHECKPOINT 5: Writing Monadic Chains

Trigger: Chaining .map()/.flatMap()/.filter() etc.

Rules to Check

Pattern Separation

// VIOLATION: Mixing Sequencer + Fork-Join
return validate(request)
    .flatMap(req -> Result.all(
        checkInventory(req),
        validatePayment(req)
    ).map((inv, pay) -> proceed(req)));

// FIX: Extract Fork-Join
return validate(request)
    .flatMap(this::validateOrder)
    .flatMap(this::processOrder);

private Result<ValidRequest> validateOrder(ValidRequest req) {
    return Result.all(checkInventory(req), validatePayment(req))
        .map((inv, pay) -> req);
}

CHECKPOINT 6: Adding Logging

Trigger: Adding log statements

Rules to Check

Ownership Pattern

// VIOLATION: Caller logs for callee
cache.refresh()
    .onSuccess(count -> log.debug("Refreshed {}", count));

// FIX: Cache owns its logging
// In GenerationCache:
public Result<Integer> refresh() {
    return doRefresh()
        .onSuccess(count -> log.debug("Refreshed {}", count));
}

CHECKPOINT 7: Review Completeness

Trigger: Before submitting code for review

Verification Checklist

• [ ] Every lambda checked against L1-L5
• [ ] Every return type checked against R1-R5
• [ ] Every factory method checked against F1-F4
• [ ] Every new type checked against D1-D4
• [ ] Every monadic chain checked against M1-M4
• [ ] Every log statement checked against G1-G4
• [ ] No FQCNs in code (use imports)
• [ ] Test names follow methodName_outcome_condition

CHECKPOINT 8: Implementation Patterns

Trigger: Choosing implementation style

Nested Record vs Lambda Pattern

Does implementation need state beyond parameters?
|-- YES -> Use nested record pattern
|-- NO: Is it a functional interface?
    |-- YES -> Use lambda pattern
    |-- NO -> Use nested record pattern

Nested record pattern (state needed):

static GenerationCache generationCache(WorkConfig config) {
    record generationCache(
        AtomicReference<Instant> lastRefreshTime,
        ConcurrentMap<String, Generation> cache,
        WorkConfig config
    ) implements GenerationCache {
        @Override
        public Result<Unit> refresh() { ... }
    }

    return new generationCache(
        new AtomicReference<>(Instant.EPOCH),
        new ConcurrentHashMap<>(),
        config
    );
}

Lambda pattern (functional interface, no state):

static Step validate(Validator validator) {
    return ctx -> validator.validate(ctx.input());
}

Quick Reference: Violation -> Fix Patterns

Application Order

When Implementing New Code

1. Design phase: Apply Checkpoint 4 (class design)
2. Signature phase: Apply Checkpoint 2 (return types), Checkpoint 3 (factories)
3. Implementation phase: Apply Checkpoint 1 (lambdas), Checkpoint 5 (chains), Checkpoint 8 (patterns)
4. Polish phase: Apply Checkpoint 6 (logging)
5. Completion phase: Apply Checkpoint 7 (review)

When Reviewing Existing Code

1. Scan for lambdas (->) - apply Checkpoint 1
2. Scan for return types - apply Checkpoint 2
3. Scan for factories - apply Checkpoint 3
4. Scan for mixed responsibilities - apply Checkpoint 4
5. Scan for logging - apply Checkpoint 6
6. Final verification - apply Checkpoint 7

Key Takeaways

1. 8 checkpoints - Cover all aspects of JBCT
2. Apply systematically - Violations caught early, not in review
3. Quick reference tables - For immediate lookup
4. Order matters - Design -> Signature -> Implementation -> Polish
5. 100% compliance - Achievable through systematic verification

Exercises

See Appendix B for exercises on:

• Exercise 6.1: Checkpoint application practice
• Exercise 6.3: Code review using checkpoints

What’s Next

Chapter 18 covers migration strategies for adopting JBCT in existing codebases.