[Clean Architecture] 30. 업무 규칙: 엔터티와 유스케이스

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import java.math.BigDecimal;

record Money(BigDecimal amount) {
    Money multiply(double factor) {
        return new Money(amount.multiply(BigDecimal.valueOf(factor)));
    }
    Money divide(int n) {
        return new Money(amount.divide(BigDecimal.valueOf(n), 2, java.math.RoundingMode.HALF_UP));
    }
}

public class Loan {
    private final Money principal;
    private final double interestRate;
    private final int termInMonths;

    public Loan(Money principal, double interestRate, int termInMonths) {
        this.principal = principal;
        this.interestRate = interestRate;
        this.termInMonths = termInMonths;
    }

    // 핵심 업무 규칙
    public Money calculateInterest() {
        return principal.multiply(interestRate)
                       .multiply(termInMonths / 12.0);
    }

    public Money calculateMonthlyPayment() {
        // 핵심 업무 규칙: 원금과 이자를 합쳐 개월 수로 나눈 단순 상환액(실제 대출 상품의 복리 계산은 이보다 복잡할 수 있다)
        Money total = principal.multiply(1 + interestRate * termInMonths / 12.0);
        return total.divide(termInMonths);
    }
}

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import java.util.UUID;
import java.math.BigDecimal;

record Money(BigDecimal amount) {
    Money multiply(double factor) { return new Money(amount.multiply(BigDecimal.valueOf(factor))); }
}
class Loan {
    Loan(Money principal, double interestRate, int termInMonths) {}
}
record ApplyForLoanRequest(String customerId, Money principal, int term) {}
record ApplyForLoanResponse(String status, String loanId, String message) {
    static ApplyForLoanResponse approved(String loanId) { return new ApplyForLoanResponse("APPROVED", loanId, null); }
    static ApplyForLoanResponse rejected(String message) { return new ApplyForLoanResponse("REJECTED", null, message); }
}
record Customer(String id, String name) {}
record CreditScore(int value) {
    boolean isBelowMinimum() { return value < 600; }
}

interface CustomerRepository { Customer findById(String id); }
interface LoanRepository { void save(Loan loan); }
interface CreditService { CreditScore check(Customer customer); }

public class ApplyForLoanUseCase {
    private final CustomerRepository customers;
    private final LoanRepository loans;
    private final CreditService creditService;

    public ApplyForLoanUseCase(CustomerRepository customers, LoanRepository loans, CreditService creditService) {
        this.customers = customers;
        this.loans = loans;
        this.creditService = creditService;
    }

    public ApplyForLoanResponse execute(ApplyForLoanRequest request) {
        // 1. 고객 조회
        Customer customer = customers.findById(request.customerId());

        // 2. 신용 확인 (애플리케이션 규칙)
        CreditScore score = creditService.check(customer);
        if (score.isBelowMinimum()) {
            return ApplyForLoanResponse.rejected("신용 부족");
        }

        // 3. 대출 생성 (엔터티 사용)
        Loan loan = new Loan(
            request.principal(),
            determineRate(score),
            request.term()
        );
        String loanId = UUID.randomUUID().toString();

        // 4. 저장
        loans.save(loan);

        return ApplyForLoanResponse.approved(loanId);
    }

    private double determineRate(CreditScore score) {
        // 애플리케이션 규칙: 신용 점수가 높을수록 낮은 이율 적용
        return score.value() >= 750 ? 0.03 : 0.05;
    }
}

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import java.math.BigDecimal;

record Money(BigDecimal amount) {}

// 요청 모델
record ApplyForLoanRequest(String customerId, Money principal, int term) {}

// 응답 모델
record ApplyForLoanResponse(String status, String loanId, String message) {
    static ApplyForLoanResponse approved(String loanId) {
        return new ApplyForLoanResponse("APPROVED", loanId, null);
    }
    static ApplyForLoanResponse rejected(String message) {
        return new ApplyForLoanResponse("REJECTED", null, message);
    }
}