\n\n소프트웨어 개체(클래스, 모듈, 함수 등등)는 확장에 대해 열려 있어야 하고, 수정에 대해서는 닫혀 있어야 한다.
\n
Movie를 새로운 컨텍스트에 사용되도록 확장할 수 있었던 것OCP를 수용하는 코드는 컴파일타임 의존성을 수정하지 않고도 런타임 의존성을 쉽게 변경할 수 있다. 의존성 관점에서 OCP를 따르는 설계란 컴파일타임 의존성은 유지하면서 런타임 의존성의 가능성을 확장하고 수정할 수 있는 구조라고 할 수 있다.
\nOCP의 핵심은 추상화에 의존하는 것
\npublic abstract class DiscountPolicy {\n private final List<DiscountCondition> conditions;\n \n protected DiscountPolicy(DiscountCondition... conditions) {\n this.conditions = Arrays.asList(conditions);\n }\n \n public Money calculateDiscountAmount(Screening screening) {\n for (DiscountCondition condition : conditions) {\n if (condition.isSatisfiedBy(screening)) {\n return getDiscountAmount(screening);\n }\n }\n return screening.getMovieFee();\n }\n protected abstract Money getDiscountAmount(Screening screening);\n}\n @AllArgsConstructor\npublic class Movie {\n\n private String title;\n\n private Duration runningTime;\n\n @Getter\n private Money fee;\n\n private DiscountPolicy discountPolicy;\n \n public Money calculateMovieFee(Screening screening) {\n return fee.minus(discountPolicy.calculateDiscountAmount(screening));\n }\n}\n Movie는 안정된 추상화인 DiscountPolicy에 의존하기 때문에 할인 정책을 추가하기 위해 DiscountPolicy의 자식 클래스를 추가하더라도 영향을 받지 않는다. 따라서 수정에 대해 닫혀 있다.추상화를 했다고 해서 모든 수정에 대해 설계가 폐쇄되는 것은 아니다. 추상화가 수정에 대해 닫혀 있을 수 있는 이유는 변경되지 않을 부분을 신중하게 결정하고 올바른 추상화를 주의 깊게 선택했기 때문이다.
\n결합도가 높아질수록 개방-폐쇄 원칙을 따르는 구조를 설계하기가 어려워진다.
\n유연하고 재사용 가능한 설계를 원한다면 객체에 대한 생성과 사용을 분리(seperating use from creation) 해야 한다.
\n사용으로부터 생성을 분리하는 데 사용되는 가장 보편적인 방법은 객체를 생성할 책임을 클라이언트로 옮기는 것
\n\n\n생성과 사용을 분리하기 위해 객체 생성에 특화된 객체를 FACTORY라고 부른다.
\n
public class Factory {\n public Movie createAvatarMovie() {\n return new Movie("아바타", Duration.ofMinutes(120), Money.wons(10000), new AmountDiscountPolicy(...));\n }\n}\n @AllArgsConstructor\npublic class Client {\n private Factory factory;\n\n public Money getAvatarFee() {\n Movie avatar = factory.createAvatarMovie();\n return avatar.getFee();\n }\n}\n 도메인 모델은 Information expert를 찾기 위해 참조할 수 있는 일차적인 재료다.
\n시스템을 객체로 분해하는 데는 크게 두 가지 방식이 존재
\n표현적 분해
\n행위적 분해
\n도메인 개념이 만족스럽지 못하다면 주저하지 말고 인공적인 객체를 창조하라. 객체지향이 실세계의 모방이라는 말은 옳지 않다.
\n의존성 주입: 사용하는 객체가 아닌 외부의 독립적인 객체가 인스턴스를 생성한 후 이를 전달해서 의존성을 해결하는 방법
\nMovie avatar = new Movie("아바타", Duration.ofMinutes(120), Money.wons(10000), new AmountDiscountPolicy(...));\n avatar.setDiscountPolicy(new AmountDiscountPolicy(...));\n avatar.calculateDiscountAmount(screening, new AmountDiscountPolicy(...));\n Service Locator 패턴
\n@NoArgsConstructor(access = AccessLevel.PRIVATE)\npublic class ServiceLocator {\n \n private static final ServiceLocator soleInstance = new ServiceLocator();\n private DiscountPolicy discountPolicy;\n \n public static DiscountPolicy discountPolicy() {\n return soleInstance.discountPolicy;\n }\n \n public static void provide(DiscountPolicy discountPolicy) {\n soleInstance.discountPolicy = discountPolicy;\n }\n}\n ServiceLocator.provide(new AmountDiscountPolicy(...));\nMovie avatar = new Movie("아바타", Duration.ofMinutes(120), Money.wons(10000));\n 의존성을 구현 내부로 감출 경우
\n의존성 주입을 지원하는 프레임워크를 사용하지 못하는 경우나 깊은 호출 계층에 걸쳐 동일한 객체를 계속해서 전달해야 하는 고통을 견디기 어려운 경우에는 어쩔 수 없이 Service Locator 패턴을 사용하는 것을 고려하라.
\npublic class Movie {\n private AmountDiscountPolicy discountPolicy;\n}\n 의존성은 변경의 전파와 관련된 것이기 때문에 설계는 변경의 영향을 최소화하도록 의존성을 관리해야 한다.
\n가장 중요한 것은 추상화에 의존하는 것이다.
\n의존성 역전 원칙(Dependency Inversion Principle, DIP)
\n역전은 의존성의 방향뿐만 아니라 인터페이스의 소유권에도 적용된다.
\nMovie를 다양한 컨텍스트에서 재사용하기 위해서는 불필요한 클래스들이 Movie와 함께 배포해야한 한다.Separated interface 패턴
\n의존성 역전 원칙에 따라 상위 수준의 협력 흐름을 재사용하기 위해서는 추상화가 제공하는 인터페이스의 소유권 역시 역전시켜야 한다.
\n유연하고 재사용 가능한 설계가 항상 좋은 것은 아니다.
\n복잡성에 대한 걱정보다 유연하고 재사용 가능한 설계의 필요성이 더 크다면 코드의 구조와 실행 구조를 다르게 만들어라.
\n설계를 유연하게 만들기 위해서는 먼저 역할, 책임, 협력에 초점을 맞춰야 한다.
\n불필요한 Singleton 패턴은 객체 생성에 관해 너무 이른 시기에 고민하고 결정할 때 도입되는 경향이 있다.
\n이번 장에서 설명한 다양한 기법들을 적용하기 전에 역할, 책임, 협력의 모습이 선명하게 그려지지 않는다면 의존성을 관리하는 데 들이는 모든 노력이 물거품이 될 수도 있다는 사실을 명심하라.
","excerpt":"📖 9.1 개방-폐쇄 원칙 소프트웨어 개체(클래스, 모듈, 함수 등등)는 확장에 대해 열려 있어야 하고, 수정에 대해서는 닫혀 있어야 한다. 확장에 대해 열려 있다: 애플리케이션의 요구사항이 변경될 때 이 변경에 맞게 새로운 '동작'을 추가해서 애플리케이션의 기능을 확장할 수 있다. 수정에 대해 닫혀 있다: 기존의 '코드'를 수정하지 않고도 애플리케이션의 동작을 추가하거나 변경할 수 있다. 🔖 9.1.…","fields":{"slug":"/backend/object/chapter9/유연한_설계/"},"frontmatter":{"title":"Object - 9장 유연한 설계","thumbnail":{"childImageSharp":{"fluid":{"src":"/static/dd0e1990925e942697544e2bdcd9332e/9b73b/object.png"}}},"draft":false,"category":"Back-End","tags":["java"],"date":"October 22, 2023"}},"previous":{"id":"45d49f32-c00f-5da9-8b11-e6b443efa26c","html":"상속
\n합성
\n코드 재사용을 위해서는 객체 합성이 클래스 상속보다 더 좋은 방법이다.
\n상속을 남용할 때 직면하는 세 가지 문제
\npublic class Properties {\n private Hashtable<String, String> properties = new Hashtable<>();\n\n public String setProperty(String key, String value) {\n return properties.put(key, value);\n }\n\n public String getProperty(String key) {\n return properties.get(key);\n }\n}\n Hashtable의 오퍼레이션들이 퍼블릭 인터페이스를 오염시키지 않는다.Stack 또한 Vector의 인스턴스 변수를 Stack 클래스의 인스턴스 변수로 선언함으로써 합성 관계로 변경할 수 있다.@RequiredArgsConstructor\npublic class InstrumentedHashSet<E> implements Set<E> {\n \n private static final int addCount = 0;\n \n private final Set<E> set;\n\n ... // Overriding\n}\n HashSet에 대한 구현 결합도는 제거하면서 퍼블릭 인터페이스는 그대로 상속포워딩
\nSet의 오버레이션을 오버라이딩한 인스턴스 메서드에서 내부의 HashSet 인스턴스에게 동일한 메서드 호출을 그대로 전달public class PersonalPlaylist {\n\n private Playlist playlist = new Playlist();\n\n public void append(Song song) {\n playlist.append(song);\n }\n\n public void remove(Song song) {\n playlist.getTracks().remove(song);\n playlist.getSingers().remove(song.getSinger());\n }\n}\n Playlist의 내부 구현을 변경하더라도 파급효과를 최대한 PersonalPlaylist 내부로 캡슐화할 수 있다.몽키 패치
\n기본 정책
\n부가 정책
\n@RequiredArgsConstructor\npublic abstract class Phone {\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return result;\n }\n\n protected abstract Money calculateCallFee(Call call);\n}\n @RequiredArgsConstructor\npublic class RegularPhone extends Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n return amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPhone extends Phone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n\n private final Money regularAmount;\n\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n return nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n return regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n public class TaxableRegularPhone extends RegularPhone {\n\n private final double taxRate;\n\n public TaxableRegularPhone(Money amount, Duration seconds, double taxRate) {\n super(amount, seconds);\n this.taxRate = taxRate;\n }\n\n @Override\n public Money calculateFee() {\n Money fee = super.calculateFee();\n return fee.plus(fee.times(taxRate));\n }\n}\n @RequiredArgsConstructor\npublic abstract class Phone {\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return afterCalculated(result);\n }\n\n protected abstract Money calculateCallFee(Call call);\n \n protected abstract Money afterCalculated(Money fee);\n}\n\n @RequiredArgsConstructor\npublic class RegularPhone extends Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n return amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee;\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPhone extends Phone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n\n private final Money regularAmount;\n\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n return nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n return regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee;\n }\n}\n @RequiredArgsConstructor\npublic abstract class Phone {\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return afterCalculated(result);\n }\n\n protected Money afterCalculated(Money fee) {\n return fee;\n }\n\n protected abstract Money calculateCallFee(Call call);\n}\n public class TaxableRegularPhone extends RegularPhone {\n\n private final double taxRate;\n\n public TaxableRegularPhone(Money amount, Duration seconds, double taxRate) {\n super(amount, seconds);\n this.taxRate = taxRate;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee.plus(fee.times(taxRate));\n }\n}\n public class TaxableNightlyDiscountPhone extends NightlyDiscountPhone {\n\n private final double taxRate;\n\n public TaxableNightlyDiscountPhone(Money nightlyAmount, Money regularAmount, Duration seconds, double taxRate) {\n super(nightlyAmount, regularAmount, seconds);\n this.taxRate = taxRate;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee.plus(fee.times(taxRate));\n }\n}\n public class RateDiscountableRegularPhone extends RegularPhone {\n\n private final Money discountAmount;\n\n public RateDiscountableRegularPhone(Money amount, Duration seconds, Money discountAmount) {\n super(amount, seconds);\n this.discountAmount = discountAmount;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee.minus(discountAmount);\n }\n}\n public class RateDiscountableNightlyDiscountPhone extends NightlyDiscountPhone {\n\n private final Money discountAmount;\n\n public RateDiscountableNightlyDiscountPhone(Money nightlyAmount, Money regularAmount, Duration seconds, Money discountAmount) {\n super(nightlyAmount, regularAmount, seconds);\n this.discountAmount = discountAmount;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee.minus(discountAmount);\n }\n}\n 상속을 이용한 해결 방법은 모든 가능한 조합별로 자식 클래스를 하나씩 추가하는 것이다.
\npublic class TaxableAndRateDiscountableRegularPhone extends TaxableRegularPhone {\n\n private final Money discountAmount;\n\n public TaxableAndRateDiscountableRegularPhone(Money amount, Duration seconds, double taxRate, Money discountAmount) {\n super(amount, seconds, taxRate);\n this.discountAmount = discountAmount;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return super.afterCalculated(fee).minus(discountAmount);\n }\n}\n public class RateDiscountableAndTaxableRegularPhone extends RateDiscountableRegularPhone {\n\n private final double taxRate;\n\n public RateDiscountableAndTaxableRegularPhone(Money amount, Duration seconds, Money discountAmount, double taxRate) {\n super(amount, seconds, discountAmount);\n this.taxRate = taxRate;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return super.afterCalculated(fee).plus(fee.times(taxRate));\n }\n}\n public class TaxableAndDiscountableNightlyDiscountPhone extends TaxableNightlyDiscountPhone {\n\n private final Money discountAmount;\n\n public TaxableAndDiscountableNightlyDiscountPhone(Money nightlyAmount, Money regularAmount, Duration seconds, double taxRate, Money discountAmount) {\n super(nightlyAmount, regularAmount, seconds, taxRate);\n this.discountAmount = discountAmount;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return super.afterCalculated(fee).minus(discountAmount);\n }\n}\n public class RateDiscountableAndTaxableNightlyDiscountPhone extends RateDiscountableNightlyDiscountPhone {\n\n private final double taxRate;\n\n public RateDiscountableAndTaxableNightlyDiscountPhone(Money nightlyAmount, Money regularAmount, Duration seconds, Money discountAmount, double taxRate) {\n super(nightlyAmount, regularAmount, seconds, discountAmount);\n this.taxRate = taxRate;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return super.afterCalculated(fee).plus(fee.times(taxRate));\n }\n}\n\n 합성을 사용하면 구현이 아닌 퍼블릭 인터페이스에 대해서만 의존할 수 있기 때문에 런타임에 객체의 관계를 변경할 수 있다.
\npublic interface RatePolicy {\n Money calculateFee(Phone phone);\n}\n public abstract class BasicRatePolicy implements RatePolicy{\n\n @Override\n public Money calculateFee(Phone phone) {\n Money result = Money.ZERO;\n\n for (Call call : phone.getCalls()) {\n result.plus(calculateCallFee(call));\n }\n\n return result;\n }\n\n protected abstract Money calculateCallFee(Call call);\n}\n @RequiredArgsConstructor\npublic class RegularPolicy extends BasicRatePolicy {\n\n private final Money amount;\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n return amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPolicy extends BasicRatePolicy {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n private final Money regularAmount;\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n return nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n return regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n @RequiredArgsConstructor\npublic class Phone {\n\n private final RatePolicy ratePolicy;\n\n @Getter\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n return ratePolicy.calculateFee(this);\n }\n}\n @RequiredArgsConstructor\npublic abstract class AdditionalRatePolicy implements RatePolicy {\n\n private final RatePolicy next;\n\n @Override\n public Money calculateFee(Phone phone) {\n Money fee = next.calculateFee(phone);\n return afterCalculated(fee);\n }\n\n protected abstract Money afterCalculated(Money fee);\n}\n public class TaxablePolicy extends AdditionalRatePolicy {\n \n private final double taxRatio;\n\n public TaxablePolicy(RatePolicy next, double taxRatio) {\n super(next);\n this.taxRatio = taxRatio;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee.plus(fee.times(taxRatio));\n }\n}\n public class RateDiscountablePolicy extends AdditionalRatePolicy {\n\n private final Money discountAmount;\n\n public RateDiscountablePolicy(RatePolicy next, Money discountAmount) {\n super(next);\n this.discountAmount = discountAmount;\n }\n\n @Override\n protected Money afterCalculated(Money fee) {\n return fee.minus(discountAmount);\n }\n}\n Phone phone = new Phone(new taxablePolicy(0.05, new RegularPolicy(...)))
코드를 재사용하려는 강력한 동기 이면에는 중복된 코드를 제거하려는 욕망이 숨어 있다.
\n중복 코드는 사람들의 마음속에 의심과 불신의 씨앗을 뿌린다.
\n중복 코드는 변경을 방해한다. 이것이 중복 코드를 제거해야 하는 가장 큰 이유다.
\n\n\nDon't Repeat Yourself
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@RequiredArgsConstructor\npublic class Call {\n \n @Getter\n private final LocalDateTime from;\n \n private final LocalDateTime to;\n\n public Duration getDuration() {\n return Duration.between(from, to);\n }\n}\n @Getter\n@RequiredArgsConstructor\npublic class Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n\n private List<Call> calls = new ArrayList<>();\n\n public void call(Call call) {\n calls.add(call);\n }\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n }\n\n return result;\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPhone {\n \n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n\n private final Money regularAmount;\n\n private final Duration seconds;\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateCallFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n result = result.plus(nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n } else {\n result = result.plus(regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n }\n }\n\n return result;\n }\n}\n 새로운 요구사항: 통화 요금에 부과할 세금 계산
\n@Getter\n@RequiredArgsConstructor\npublic class Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n \n private final double taxRate;\n\n private List<Call> calls = new ArrayList<>();\n\n public void call(Call call) {\n calls.add(call);\n }\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n }\n\n return result.plus(result.times(taxRate));\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPhone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n \n private final Money regularAmount;\n \n private final Duration seconds;\n \n private final double taxRate;\n \n private List<Call> calls = new ArrayList<>();\n\n public Money calculateCallFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n result = result.plus(nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n } else {\n result = result.plus(regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n }\n }\n\n return result.minus(result.times(taxRate));\n }\n}\n minus를 호출하고 있다.@Getter\n@RequiredArgsConstructor\npublic class Phone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n enum PhoneType {REGULAR, NIGHTLY}\n\n private final PhoneType type;\n\n private final Money amount;\n\n private final Money regularAmount;\n\n private final Money nightlyAmount;\n\n private final Duration seconds;\n\n private List<Call> calls = new ArrayList<>();\n\n public Phone(Money amount, Duration seconds) {\n this(PhoneType.REGULAR, amount, Money.ZERO, Money.ZERO, seconds);\n }\n\n public Phone(Money regularAmount, Money nightlyAmount, Duration seconds) {\n this(PhoneType.NIGHTLY, Money.ZERO, regularAmount, nightlyAmount, seconds);\n }\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n if (type == PhoneType.REGULAR) {\n result = result.plus(amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n } else {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n result = result.plus(nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n } else {\n result = result.plus(regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n }\n }\n }\n\n return result;\n }\n}\n public class NightlyDiscountPhone extends Phone {\n private static final int LATE_NIGHT_HOUR = 22;\n\n private Money nightlyAmount;\n\n public NightlyDiscountPhone(Money nightlyAmount, Money regularAmount, Duration seconds) {\n super(regularAmount, seconds);\n this.nightlyAmount = nightlyAmount;\n }\n\n @Override\n public Money calculateFee() {\n Money result = super.calculateFee();\n \n Money nightlyFee = Money.ZERO;\n for (Call call : getCalls()) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n nightlyFee = nightlyFee.plus(getAmount().minus(nightlyAmount.times((double) call.getDuration().getSeconds() / getSeconds().getSeconds())));\n }\n }\n \n return result.minus(nightlyFee);\n }\n}\n 새로운 요구사항: 세금 부과
\n@Getter\n@RequiredArgsConstructor\npublic class Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n\n private final double taxRate;\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds()));\n }\n\n return result.plus(result.times(taxRate));\n }\n}\n public class NightlyDiscountPhone extends Phone {\n private static final int LATE_NIGHT_HOUR = 22;\n\n private Money nightlyAmount;\n\n public NightlyDiscountPhone(Money nightlyAmount, Money regularAmount, Duration seconds, double taxRate) {\n super(regularAmount, seconds, taxRate);\n this.nightlyAmount = nightlyAmount;\n }\n\n @Override\n public Money calculateFee() {\n Money result = super.calculateFee();\n\n Money nightlyFee = Money.ZERO;\n for (Call call : getCalls()) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n nightlyFee = nightlyFee.plus(getAmount().minus(nightlyAmount.times((double) call.getDuration().getSeconds() / getSeconds().getSeconds())));\n }\n }\n\n return result.minus(nightlyFee.plus(nightlyFee.times(getTaxRate())));\n }\n}\n \n\n자식 클래스의 메서드 안에서 super 참조를 이용해 부모 클래스의 메서드를 직접 호출할 경우 두 클래스는 강하게 결합된다. super 호출을 제거할 수 있는 방법을 찾아 결합도를 제거하라.
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Java 초기 버전의 대표적인 사례
\njava.util.Stack\njava.util.Properties\n\n\n상속받은 부모 클래스의 메서드가 자식 클래스의 내부 구조에 대한 규칙을 깨트릴 수 있다.
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\n\n자식 클래스가 부모 클래스의 메서드를 오버라이딩할 경우 부모 클래스가 자식의 메서드를 사용하는 방법에 자식 클래스가 결합될 수 있다.
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@Getter\n@RequiredArgsConstructor\npublic class Song {\n\n private final String singer;\n \n private final String title;\n}\n public class Playlist {\n\n @Getter\n private final List<Song> tracks = new ArrayList<>();\n\n public void append(Song song) {\n getTracks().add(song);\n }\n}\n public class PersonalPlaylist extends Playlist {\n \n public void remove(Song song) {\n getTracks().remove(song);\n }\n}\n @Getter\npublic class Playlist {\n\n private final List<Song> tracks = new ArrayList<>();\n\n private final Map<String, String> singers = new HashMap<>();\n\n public void append(Song song) {\n getTracks().add(song);\n singers.put(song.getSinger(), song.getTitle());\n }\n}\n public class PersonalPlaylist extends Playlist {\n\n public void remove(Song song) {\n getTracks().remove(song);\n getSingers().remove(song.getSinger());\n }\n}\n \n\n클래스를 상속하면 결합도로 인해 자식 클래스와 부모 클래스의 구현을 영원히 변경하지 않거나, 자식 클래스와 부모 클래스를 동시에 변경하거나 둘 중 하나를 선택할 수 밖에 없다.
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코드 중복을 제거하기 위해 상속을 도입할 때 따르는 원칙
\n@Getter\n@RequiredArgsConstructor\npublic class Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return result;\n }\n\n private Money calculateCallFee(Call call) {\n return amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPhone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n\n private final Money regularAmount;\n\n private final Duration seconds;\n\n private List<Call> calls = new ArrayList<>();\n\n public Money calculateCallFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return result;\n }\n\n private Money calculateCallFee(Call call) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n return nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n return regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n public abstract class AbstractPhone {\n\n private final List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return result;\n }\n\n protected abstract Money calculateCallFee(Call call);\n}\n @Getter\n@RequiredArgsConstructor\npublic class Phone extends AbstractPhone{\n\n private final Money amount;\n\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n return amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n @RequiredArgsConstructor\npublic class NightlyDiscountPhone extends AbstractPhone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n\n private final Money regularAmount;\n\n private final Duration seconds;\n\n @Override\n protected Money calculateCallFee(Call call) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n return nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n return regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n public abstract class Phone {\n}\n\npublic class RegularPhone extends Phone {\n}\n\npublic class NightlyDiscountPhone extends Phone {\n}\n @RequiredArgsConstructor\npublic abstract class Phone {\n \n private final double taxRate;\n \n private List<Call> calls = new ArrayList<>();\n\n public Money calculateFee() {\n Money result = Money.ZERO;\n\n for (Call call : calls) {\n result = result.plus(calculateCallFee(call));\n }\n\n return result.plus(result.times(taxRate));\n }\n\n protected abstract Money calculateCallFee(Call call);\n}\n @Getter\npublic class RegularPhone extends Phone {\n\n private final Money amount;\n\n private final Duration seconds;\n\n public RegularPhone(double taxRate, Money amount, Duration seconds) {\n super(taxRate);\n this.amount = amount;\n this.seconds = seconds;\n }\n\n @Override\n protected Money calculateCallFee(Call call) {\n return amount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n public class NightlyDiscountPhone extends Phone {\n\n private static final int LATE_NIGHT_HOUR = 22;\n\n private final Money nightlyAmount;\n\n private final Money regularAmount;\n\n private final Duration seconds;\n\n public NightlyDiscountPhone(double taxRate, Money nightlyAmount, Money regularAmount, Duration seconds) {\n super(taxRate);\n this.nightlyAmount = nightlyAmount;\n this.regularAmount = regularAmount;\n this.seconds = seconds;\n }\n\n @Override\n protected Money calculateCallFee(Call call) {\n if (call.getFrom().getHour() >= LATE_NIGHT_HOUR) {\n return nightlyAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n return regularAmount.times((double) call.getDuration().getSeconds() / seconds.getSeconds());\n }\n}\n