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

posted @ 2017-01-19 15:40 Acode 阅读(...) 评论(...) 编辑 收藏
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