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
作者: Acode
出处: http://www.cnblogs.com/acode/
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