java - 使用复制构造函数而不是 Object.clone 进行深度复制的正确方法

Question

我有一些代码使用 Object.clone 执行深层复制，但我正在尝试使用更“可接受”的复制构造函数技术重写它。下面是我正在尝试做的两个简单示例，第一个使用克隆，第二个使用复制构造函数。

使用克隆的深拷贝

 import java.util.*;

 abstract class Person implements Cloneable {
     String name;
     public Object clone() throws CloneNotSupportedException {
         return super.clone();
     }
 }

 class Teacher extends Person implements Cloneable {
     int courses;
     public String toString() { return name + ": courses=" + courses; }
 }

 class Student extends Person implements Cloneable {
     double gpa;
     public String toString() { return name + ": gpa=" + gpa; }
 }

 public class DeepCopy_Clone {
     private static List<Person> deepCopy(List<Person> people) throws CloneNotSupportedException {
         List<Person> copy = new ArrayList<Person>();
         for (Person person : people) {
             copy.add((Person)person.clone());
         }
         return copy;
     }

     public static void main(String[] args) throws CloneNotSupportedException {
         ArrayList<Person> people = new ArrayList<Person>();

         Teacher teacher = new Teacher();
         teacher.name = "Teacher";
         teacher.courses = 5;
         people.add(teacher);

         Student student = new Student();
         student.name = "Student";
         student.gpa = 4.0;
         people.add(student);

         List<Person> peopleCopy = deepCopy(people);

         // Invalidate the original data to prove a deep copy occurred
         teacher.name = null;
         teacher.courses = -1;
         student.name = null;
         student.gpa = -1;

         for (Person person : peopleCopy) {
             System.out.println(person.toString());
         }
     }
 }

使用拷贝构造函数进行深拷贝

 import java.util.*;

 abstract class Person {
     String name;
     public Person() {}
     public Person(Person other) {
         this.name = other.name;
     }
     public Person deepCopy() {
         if (this instanceof Teacher) {
             return new Teacher((Teacher)this);
         } else if (this instanceof Student) {
             return new Student((Student)this);
         }

         throw new Error("Unknown type of person");
     }
 }

 class Teacher extends Person {
     int courses;
     public Teacher() {}
     public Teacher(Teacher other) {
         super(other);
         this.courses = other.courses;
     }
     public String toString() { return name + ": courses=" + courses; }
 }

 class Student extends Person {
     double gpa;
     public Student() {}
     public Student(Student other) {
         super(other);
         this.gpa = other.gpa;
     }
     public String toString() { return name + ": gpa=" + gpa; }
 }

 public class DeepCopy_ConstructorAlternative {
     private static List<Person> deepCopy(List<Person> people) {
         List<Person> copy = new ArrayList<Person>();
         for (Person person : people) {
             copy.add(person.deepCopy());
         }
         return copy;
     }

     public static void main(String[] args) {
         ArrayList<Person> people = new ArrayList<Person>();

         Teacher teacher = new Teacher();
         teacher.name = "Teacher";
         teacher.courses = 5;
         people.add(teacher);

         Student student = new Student();
         student.name = "Student";
         student.gpa = 4.0;
         people.add(student);

         List<Person> peopleCopy = deepCopy(people);

         // Invalidate the original data to prove a deep copy occurred
         teacher.name = null;
         teacher.courses = -1;
         student.name = null;
         student.gpa = -1;

         for (Person person : peopleCopy) {
             System.out.println(person.toString());
         }
     }
 }

我发现有趣的是，尽管在 Java 中一直在谈论克隆的弊端，但克隆替代方案需要更少的代码和更少的转换(至少在这种特殊情况下是这样)。

对于复制构造函数替代方案，我将不胜感激。你会做不同的事吗？谢谢。

Answer 1

代替:

 public Object clone() throws CloneNotSupportedException {
     return super.clone();
 }

我更愿意:

public Person clone() {
    try {
        return (Person) clone();
    } catch (CloneNotSupportedException e) {
        throw new RuntimeException("This should be impossible ...");
    }
}

因此调用者不必处理永远不会发生的异常，也不必强制转换。

在复制构造函数方法中，类型切换更好地以多态方式处理:

abstract class Person {
    ...
    public abstract Person deepCopy();
}

class Student {
    ...
    public Student deepCopy() {
        return new Student(this);
    }
}

class Teacher {
    ...
    public Teacher deepCopy() {
        return new Teacher(this);
    }
}

现在编译器可以检查您是否为所有子类型提供了深拷贝，并且您不需要任何转换。

最后，请注意克隆和复制构造函数方法都具有相同的公共(public) api(无论该方法称为 clone() 还是 deepCopy() 都没有很重要)，所以你使用哪种方法是一个实现细节。复制构造函数方法更加冗长，因为您同时提供构造函数和调用该构造函数的方法，但它可以更容易地泛化为通用类型转换工具，允许诸如:

public Teacher(Person p) {
    ...
    say("Yay, I got a job");
}

建议:如果您只想要一个相同的副本，请使用克隆；如果您的调用者可能希望请求特定类型的实例，请使用复制构造函数。