빌더 패턴은 얼마나 유용한가?

 - 필드가 많은 객체를 생성할 경우 생성자를 이용하면 다루기도 어렵고 헷갈릿 수 있다,

private final Animal animal;
private final String petName;
private final String ownerName;
private final String address;
private final String telephone;
private final Date dateOfBirth;
private final String emailAddress;

 - 유효한 Pet 객체를 만들려면 필요한 모든 필드와 조합할 수 있는 옵션 필드를 포함하도록 많은 매개변수를 갖는 생성자가 최소한 4개는 필요하다.

 - 이러한 문제를 해결하는 한 가지 방법은 생성자를 사용하지 않는 것.

 - 필드에서 final을 제거하고 Pet 객체에 setter 객체를 사용

finla Pet p = new Pet();

p.setEmailAddress("owners@address.com");

- 유효하지 않는  Pet 객체를 생성할 수 있다는 점에 주의 해야 하고 Pet 객체를 정의 할 때는 animal, perName, ownerName, address, telephone 필드가 모두 채워져 있어야 한다,

 - 이런 상황일 때 빌더 패턴을 사용하면 도움이 된다.

public class PetTest {
   @Test
   public void legalBuild() {
      final Pet.Builder builder = new Pet.Builder();
      final  Pet pet = builder.withAnimal(Pet.Animal.CAT).withPetName("Squidge").withOwnerName("Simon Smith").withAddress("123 High Street")
            .withTelephone("0770777070").withEmailAddress("simon@email.com").build();
   }

   @Test(expected = IllegalStateException.class)
   public void illegalBuild() {
      final Pet.Builder builder = new Pet.Builder();
      final  Pet pet = builder.withAnimal(Pet.Animal.CAT).withPetName("Squidge").withOwnerName("Simon Smith").build();
   }
}

 - Builder 클래스는 Pet 클래스의 일부이며 Pet 객체를 생성하는 전적인 권한이 있다.

 - 생성자는 final 키워드를 뺀 private으로 선언할 수 있다.

public class Pet {
   public enum Animal {
      CAT, DOG, ETC
   }
   public static class Builder {
      private Animal animal;
      private String petName;
      private String ownerName;
      private String address;
      private String telephone;
      private Date dateOfBirth;
      private String emailAddress;

      public Builder withAnimal(final Animal animal) {
         this.animal = animal;
         return this;
      }
      public Builder withPetName(final String petName) {
         this.petName = petName;
         return this;
      }
      public Builder withOwnerName(final String ownerName) {
         this.ownerName = ownerName;
         return this;
      }
      public Builder withAddress(final String address) {
         this.address = address;
         return this;
      }
      public Builder withTelephone(final String telephone) {
         this.telephone = telephone;
         return this;
      }
      public Builder withDateOfBirth(final Date dateOfBirth) {
         this.dateOfBirth = dateOfBirth;
         return this;
      }
      public Builder withEmailAddress(final String emailAddress) {
         this.emailAddress = emailAddress;
         return this;
      }

      public Pet build() {
         if (animal == null || petName == null || ownerName == null || address == null || address == null || telephone == null) {
            throw new IllegalStateException("Connot create Pet");
         }

         return new Pet(animal, petName, ownerName, address, telephone, dateOfBirth, emailAddress);
      }
   }

   private final Animal animal;
   private final String petName;
   private final String ownerName;
   private final String address;
   private final String telephone;
   private final Date dateOfBirth;
   private final String emailAddress;

   private Pet(final Animal animal, final String petName, final String ownerName, final String address, final String telephone, final Date dateOfBirth, final String emailAddress) {
      this.animal = animal;
      this.petName = petName;
      this.ownerName = ownerName;
      this.address = address;
      this.telephone = telephone;
      this.dateOfBirth = dateOfBirth;
      this.emailAddress = emailAddress;
   }

public class LibraryBookTest {
   @Test
   public void fictionLibraryBook() {
      final  LibraryBook.Builder builder = new LibraryBook.Builder();
      final LibraryBook book = builder.withBookName("War and Peace").build();

      assertEquals(LibraryBook.BookType.FICTION, book.getBookType());
   }
}

public class LibraryBook {
   public enum BookType {
      FICTION, FILE, PET
   }
   public static class Builder {
      private BookType bookType =  BookType.FICTION;
      private String bookName;

      public Builder withBookType(final BookType bookType) {
         this.bookType = bookType;
         return this;
      }
      public Builder withBookName(final String bookName) {
         this.bookName = bookName;
         return this;
      }

      public LibraryBook build() {
         return new LibraryBook(bookType, bookName);
      }
   }

   private final BookType bookType;
   private final String bookName;

   public LibraryBook(final BookType bookType, final String bookName) {
      this.bookType = bookType;
      this.bookName = bookName;
   }

   public BookType getBookType() {
      return bookType;
   }
}