Java必備技能：clone淺克隆與深克隆
摘要：
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...
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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