转换器（Converter)模式

在日常编码中，我们会遇到这样一个场景：把一个类型的对象转换成另一个对象，而这两者之前的转换强调的是"值（Value）"的等价转换，两者之间并没有继承与被继承的关系，也并不是像浮点数转整数这种语法意义上的转换关系。如下面举的这个例子："用户"这个对象定义了User和UserDto两种Bean class来表示，二者所代表的"值"都是一致的，只是一个是业务逻辑层面的，一个是数据访问层面的。二者之前常常会发生转换，这个时候可以使用转换器模式：

类图：

代码：

/*** User class*/
public class User {private String firstName;private String lastName;private boolean isActive;private String userId;/*** @param firstName user's first name* @param lastName  user's last name* @param isActive  flag indicating whether the user is active* @param userId user's identificator*/public User(String firstName, String lastName, boolean isActive, String userId) {this.firstName = firstName;this.lastName = lastName;this.isActive = isActive;this.userId = userId;}public String getFirstName() {return firstName;}public String getLastName() {return lastName;}public boolean isActive() {return isActive;}public String getUserId() {return userId;}@Override public boolean equals(Object o) {if (this == o) {return true;}if (o == null || getClass() != o.getClass()) {return false;}User user = (User) o;return isActive == user.isActive && Objects.equals(firstName, user.firstName) && Objects.equals(lastName, user.lastName) && Objects.equals(userId, user.userId);}@Override public int hashCode() {return Objects.hash(firstName, lastName, isActive, userId);}@Override public String toString() {return "User{" + "firstName='" + firstName + '\'' + ", lastName='" + lastName + '\''+ ", isActive=" + isActive + ", userId='" + userId + '\'' + '}';}
}

/*** User DTO class*/
public class UserDto {private String firstName;private String lastName;private boolean isActive;private String email;/*** @param firstName user's first name* @param lastName  user's last name* @param isActive  flag indicating whether the user is active* @param email     user's email address*/public UserDto(String firstName, String lastName, boolean isActive, String email) {this.firstName = firstName;this.lastName = lastName;this.isActive = isActive;this.email = email;}public String getFirstName() {return firstName;}public String getLastName() {return lastName;}public boolean isActive() {return isActive;}public String getEmail() {return email;}@Override public boolean equals(Object o) {if (this == o) {return true;}if (o == null || getClass() != o.getClass()) {return false;}UserDto userDto = (UserDto) o;return isActive == userDto.isActive && Objects.equals(firstName, userDto.firstName) && Objects.equals(lastName, userDto.lastName) && Objects.equals(email, userDto.email);}@Override public int hashCode() {return Objects.hash(firstName, lastName, isActive, email);}@Override public String toString() {return "UserDto{" + "firstName='" + firstName + '\'' + ", lastName='" + lastName + '\''+ ", isActive=" + isActive + ", email='" + email + '\'' + '}';}
}

/*** Generic converter, thanks to Java8 features not only provides a way of generic bidirectional* conversion between coresponding types, but also a common way of converting a collection of objects* of the same type, reducing boilerplate code to the absolute minimum.* @param <T> DTO representation's type* @param <U> Domain representation's type*/
public class Converter<T, U> {private final Function<T, U> fromDto;private final Function<U, T> fromEntity;/*** @param fromDto Function that converts given dto entity into the domain entity.* @param fromEntity Function that converts given domain entity into the dto entity.*/public Converter(final Function<T, U> fromDto, final Function<U, T> fromEntity) {this.fromDto = fromDto;this.fromEntity = fromEntity;}/*** @param userDto DTO entity* @return The domain representation - the result of the converting function application on dto entity.*/public final U convertFromDto(final T userDto) {return fromDto.apply(userDto);}/*** @param user domain entity* @return The DTO representation - the result of the converting function application on domain entity.*/public final T convertFromEntity(final U user) {return fromEntity.apply(user);}/*** @param dtoUsers collection of DTO entities* @return List of domain representation of provided entities retrieved by*        mapping each of them with the convertion function*/public final List<U> createFromDtos(final Collection<T> dtoUsers) {return dtoUsers.stream().map(this::convertFromDto).collect(Collectors.toList());}/*** @param users collection of domain entities* @return List of domain representation of provided entities retrieved by*        mapping each of them with the convertion function*/public final List<T> createFromEntities(final Collection<U> users) {return users.stream().map(this::convertFromEntity).collect(Collectors.toList());}}

/*** Example implementation of the simple User converter.*/
public class UserConverter extends Converter<UserDto, User> {/*** Constructor.*/public UserConverter() {super(userDto -> new User(userDto.getFirstName(), userDto.getLastName(), userDto.isActive(),userDto.getEmail()),user -> new UserDto(user.getFirstName(), user.getLastName(), user.isActive(),user.getUserId()));}
}

/*** The Converter pattern is a behavioral design pattern which allows a common way of bidirectional* conversion between corresponding types (e.g. DTO and domain representations of the logically* isomorphic types). Moreover, the pattern introduces a common way of converting a collection of* objects between types.*/
public class App {/*** Program entry point** @param args command line args*/public static void main(String[] args) {Converter<UserDto, User> userConverter = new Converter<>(userDto -> new User(userDto.getFirstName(), userDto.getLastName(), userDto.isActive(),userDto.getEmail()),user -> new UserDto(user.getFirstName(), user.getLastName(), user.isActive(), user.getUserId()));UserDto dtoUser = new UserDto("John", "Doe", true, "whatever[at]wherever.com");User user = userConverter.convertFromDto(dtoUser);System.out.println("Entity converted from DTO:" + user);ArrayList<User> users = Lists.newArrayList(new User("Camile", "Tough", false, "124sad"),new User("Marti", "Luther", true, "42309fd"), new User("Kate", "Smith", true, "if0243"));System.out.println("Domain entities:");users.forEach(System.out::println);System.out.println("DTO entities converted from domain:");List<UserDto> dtoEntities = userConverter.createFromEntities(users);dtoEntities.forEach(System.out::println);}
}

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