Java的clone()：深复制与浅复制
Java的clone()：深复制与浅复制
Java中要想自定义类的对象可以被复制，自定义类就必须实现Cloneable中的clone()方法，如下：
1 public class Student implements Cloneable {
2
3 private String name;
4
5 private int age;
6
7 private Professor professor;
8
9 public String getName() {
10 return name;
11 }
12
13 public void setName(String name) {
14 this.name = name;
15 }
16
17 public int getAge() {
18 return age;
19 }
20
21 public void setAge(int age) {
22 this.age = age;
23 }
24
25 public Professor getProfessor() {
26 return professor;
27 }
28
29 public void setProfessor(Professor professor) {
30 this.professor = professor;
31 }
32
33 @Override
34 public String toString() {
35 return "Student [name=" + name + ", age=" + age + ", professor="
36 + professor + "]";
37 }
38
39 public Object clone() throws CloneNotSupportedException{
40 return super.clone();
41 }
42
43 }
其中，Professor类同样为自定义类：
public class Professor {
private String name;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Professor [name=" + name + ", age=" + age + "]";
}
}
然而，当自定义类的字段的类型不是基本数据类型时，上面实现了clone()方法会导致问题，不信看下面的代码：
【程序实例1】
1 public class ShadowCopy {
2
3 public static void main(String[] args) {
4 Professor p1 = new Professor();
5 p1.setName("Professor Zhang");
6 p1.setAge(30);
7
8 Student s1 = new Student();
9 s1.setName("xiao ming");
10 s1.setAge(18);
11 s1.setProfessor(p1);
12
13 System.out.println(s1);
14
15 try {
16 Student s2 = (Student) s1.clone();
17 Professor p2 = s2.getProfessor();
18 p2.setName("Professor Li");
19 p2.setAge(45);
20 s2.setProfessor(p2);
21 System.out.println("复制后的：s1 = " + s1);
22 System.out.println("复制后的：s2 = " + s2);
23 } catch (CloneNotSupportedException e) {
24 e.printStackTrace();
25 }
26
27 }
28
29 }
【运行结果1】
1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]] 2 复制后的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]] 3 复制后的：s2 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]
【结果分析】
学生s1的导师为30岁的Professor Zhang，恰好学生s2与学生s1同名同岁，但是s2的导师为45岁的Professor Li，于是我们顺理成章地复制复制s1并复制给s2，再修改下s2的导师的信息。可是，问题出现了，当我们修改了s2的导师后，s2的信息是对了，但是s1的导师信息也跟着修改了，这可不是我们期望的。
【问题分析】
程序实例1中的问题出在哪儿呢？我们已经对Student类实现了clone()方法，怎么还是出问题了呢？我们在看下面的代码：
【程序实例2】
1 public class ShadowCopy {
2
3 public static void main(String[] args) {
4 Professor p1 = new Professor();
5 p1.setName("Professor Zhang");
6 p1.setAge(30);
7
8 Student s1 = new Student();
9 s1.setName("xiao ming");
10 s1.setAge(18);
11 s1.setProfessor(p1);
12
13 System.out.println(s1);
14
15 try {
16 Student s2 = (Student) s1.clone();
17 s2.setName("xiao hong");
18 s2.setAge(17);
19 Professor p2 = s2.getProfessor();
20 p2.setName("Professor Li");
21 p2.setAge(45);
22 s2.setProfessor(p2);
23 System.out.println("复制后的：s1 = " + s1);
24 System.out.println("复制后的：s2 = " + s2);
25 } catch (CloneNotSupportedException e) {
26 e.printStackTrace();
27 }
28
29 }
30
31 }
【运行结果】
1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]] 2 复制后的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]] 3 复制后的：s2 = Student [name=xiao hong, age=17, professor=Professor [name=Professor Li, age=45]]
【结果分析】
这次，我们在clone后，又修改了s2的name和age，从结果可以看出，s1的name和age并没有因为s2的修改而改变。
结合程序实例1和程序实例2，我们发现Student的字段如果不是一个引用时，修改clone()得到对象的该字段(name, age)时并不会影响原来的对象，但是当字段为一个引用时，修改clone()得到对象的该字段(professor)时并会影响原来的对象。上面实现的clone()方法为浅复制（shadow copy）。
如果想要clone()得到的新对象的修改不会影响被复制的对象的字段时，我们就需要实现深复制（deep copy），代码修改如下：
1 public class Professor implements Cloneable {
2
3 private String name;
4
5 private int age;
6
7 public String getName() {
8 return name;
9 }
10
11 public void setName(String name) {
12 this.name = name;
13 }
14
15 public int getAge() {
16 return age;
17 }
18
19 public void setAge(int age) {
20 this.age = age;
21 }
22
23 @Override
24 public String toString() {
25 return "Professor [name=" + name + ", age=" + age + "]";
26 }
27
28 public Object clone() throws CloneNotSupportedException{
29 return super.clone();
30 }
31
32 }
public class Student implements Cloneable {
private String name;
private int age;
private Professor professor;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public Professor getProfessor() {
return professor;
}
public void setProfessor(Professor professor) {
this.professor = professor;
}
@Override
public String toString() {
return "Student [name=" + name + ", age=" + age + ", professor="
+ professor + "]";
}
public Object clone() throws CloneNotSupportedException{
Student newStudent = (Student) super.clone();
newStudent.professor = (Professor) professor.clone();
return newStudent;
}
}
再次运行【程序实例2】得到的结果为：
1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]] 2 复制后的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]] 3 复制后的：s2 = Student [name=xiao hong, age=17, professor=Professor [name=Professor Li, age=45]]
可以看到：修改clone()得到的s2的任何字段都不会影响s1的字段，这也就是深复制的作用
参考资料：
1、http://pengcqu.iteye.com/blog/493120
2、http://www.cnblogs.com/shuaiwhu/archive/2010/12/14/2065088.html
3、https://www.cnblogs.com/acode/p/6306887.html
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