介紹

一直以來只知道Java有clone方法，該方法屬於Object的，對於什麼是淺克隆與深克隆就比較模糊了，現在就來補充學習一下。

概念

淺拷貝（淺克隆）複製出來的物件的所有變數都含有與原來的物件相同的值，而所有的對其他物件的引用仍然指向原來的物件。

深拷貝（深克隆）複製出來的所有變數都含有與原來的物件相同的值，那些引用其他物件的變數將指向複製出來的新物件，而不再是原有的那些被引用的物件。換言之，深複製把要複製的物件所引用的物件都複製了一遍。

實現

Student.java

public class Student implements Cloneable {
 private
 
 String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
}

Teacher.java

public class Teacher implements Cloneable {
 private String name;
 private String course;
 public Teacher
 
(String name, String course) {
 this.name = name;
 this.course = course;
 }
}

Student類中包含有name，age和Teacher物件。

淺克隆

克隆物件實現Cloneable介面（該介面是一個標記介面），在克隆的方法裡面呼叫super.clone(),就會返回克隆後的物件。

public class Student implements Cloneable {
 private String name;
 private int age;
 private Teacher teacher;
 public
 
 Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
 public Student clone() {
 Student student = null;
 try {
 student = (Student) super.clone();
 } catch (CloneNotSupportedException e) {
 e.printStackTrace();
 }
 return student;
 }
}

測試

 public static void main(String args[]) throws IOException, ClassNotFoundException {
 Teacher teacher = new Teacher("王老師", "英語");
 Student student = new Student("小明", 11, teacher);
 Student clone = student.clone();
 clone.setName("小強");
 clone.setAge(20);
 clone.getTeacher().setName("李老師");
 System.out.println(student.getName() + " " + student.getAge());
 System.out.println(clone.getName() + " " + clone.getAge());
 System.out.println(clone.getTeacher() == student.getTeacher());
 }

輸出：

小明 11
小強 20

true複製程式碼從上面結果可知，克隆出來的Student物件裡的name和age是新的，但是teacher是和原來的共享的，這就是淺克隆。

深克隆

Student.java

public class Student implements Cloneable {
 private String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
 public Student clone() {
 Student student = null;
 try {
 student = (Student) super.clone();
 Teacher teacher = this.teacher.clone();
 student.setTeacher(teacher);
 } catch (CloneNotSupportedException e) {
 e.printStackTrace();
 }
 return student;
 }
}

Teacher.java

public class Teacher implements Cloneable {
 private String name;
 private String course;
 public Teacher(String name, String course) {
 this.name = name;
 this.course = course;
 }
 public Teacher clone() {
 Teacher clone = null;
 try {
 clone = (Teacher) super.clone();
 } catch (CloneNotSupportedException e) {
 e.printStackTrace();
 }
 return clone;
 }
}

要實現深克隆的話，克隆物件裡面的物件型別也必須實現Cloneable介面並呼叫clone()。

測試：

public static void main(String args[]) throws IOException, ClassNotFoundException {
 Teacher teacher = new Teacher("王老師", "英語");
 Student student = new Student("小明", 11, teacher);
 Student clone = student.clone();
 clone.setName("小強");
 clone.setAge(20);
 clone.getTeacher().setName("李老師");
 System.out.println(student.getName() + " " + student.getAge());
 System.out.println(clone.getName() + " " + clone.getAge());
 System.out.println(clone.getTeacher() == student.getTeacher());
}

輸出：

小明 11
小強 20

false複製程式碼這時，兩個物件的中的Teacher就不是同一個物件了，實現了深克隆，但是如果要克隆的物件繼承鏈比較長的話要實現深克隆，就必須逐層地實現Cloneable，這個過程是比較麻煩的，不過還有一種方法可以簡便地實現深克隆。

serializable克隆

大家知道，Java可以把物件序列化寫進一個流裡面，反之也可以把物件從序列化流裡面讀取出來，但這一進一出，這個物件就不再是原來的物件了，就達到了克隆的要求。

public class Student implements Serializable {
 private String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
 public Student serializableClone() throws IOException, ClassNotFoundException {
 Student clone;
 ByteArrayOutputStream bo = new ByteArrayOutputStream();
 ObjectOutputStream oo = new ObjectOutputStream(bo);
 oo.writeObject(this);
 ByteArrayInputStream bi = new ByteArrayInputStream(bo.toByteArray());
 ObjectInputStream oi = new ObjectInputStream(bi);
 clone = (Student) oi.readObject();
 return clone;
 }
}

通過把物件寫進ByteArrayOutputStream裡，再把它讀取出來。注意這個過程中所有涉及的物件都必須實現Serializable介面，由於涉及IO操作，這種方式的效率會比前面的低。

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