Java 深拷贝与浅拷贝的分析

时间:2021-05-20

在正式的进入主题之前,我们先来了解下深拷贝和前拷贝的概念:

浅拷贝:

会创建一个新对象,这个对象有着原始对象属性值的一份精确拷贝,如果属性是基本类型,拷贝的是基本类型的值;如果属性是内存地址,拷贝的就是内存地址,因此如果一个对象改变了这个地址就会影响到另一个对象;

深拷贝:

不仅要复制对象的所有非引用成员变量值,还要为引用类型的成员变量创建新的实例,并且初始化为形式参数实例值;

了解完概念之后,我们来测试下普通的对象赋值操作属于深拷贝还是浅拷贝:

测试代码:

public class DepthCopy { public static void main(String[] args) { Copy first = new Copy("hzw", 24); Copy second = first; second.name = "shanxi"; System.out.println(first.name);//输出shanxi } } class Copy { public String name; public int age; public Copy(String name,int age) { this.name = name; this.age = age; } }

可以发现,在second将name属性值修改为shanxi之后,first的name属性值也变成了shanxi,这点就可以看出普通的对象赋值属于浅拷贝;

明白了对象之间赋值是浅拷贝之后,接下来我们来看看克隆到底是深拷贝还是浅拷贝,测试代码是让上面的Copy对象实现Cloneable接口里面的clone方法:

public class DepthCopy { public static void main(String[] args) { Copy first = new Copy("hzw", 24); Copy second = null; try { second = (Copy) first.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } second.name = "shanxi"; System.out.println(first.name);//输出: hzw System.out.println(first);//输出: com.hzw.day33.Copy@7f39ebdb System.out.println(second);//输出: com.hzw.day33.Copy@33abb81e } } class Copy implements Cloneable { public String name; public int age; public Copy(String name,int age) { this.name = name; this.age = age; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } }

可以看出原先创建出的对象first和克隆创建出的对象second是两个实例,因此对于second中name属性的修改并不会影响first中的name属性;但是,我们并不能单纯的认为克隆就是深拷贝的,比如下面这个例子:

public class DepthCopy { public static void main(String[] args) { Student student = new Student(95); Copy first = new Copy("hzw", 24,student); Copy second = null; try { second = (Copy) first.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } second.name = "shanxi"; second.student.score = 60; System.out.println(first == second);//false System.out.println(first.student == second.student);//true System.out.println(first.student.score);//60 } } class Copy implements Cloneable { public String name; public int age; public Student student; public Copy(String name,int age,Student student) { this.name = name; this.age = age; this.student = student; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } } class Student { public int score; public Student(int score) { this.score = score; } }

看到没有呢?我们通过克隆的方式创建了second,很明显发现first和second是两个实例,因为first == second输出为false,但是first和second里面的student对象却是一样的,通过second修改了student的score值之后,first里面student的score也发生了改变,这也就是说first和second里面的student是相同的,这也就说明了克隆是浅拷贝的,我们要想实现克隆的深拷贝,必须让Copy对象里面的Student对象也要实现Cloneable接口里面的clone方法,并且在Copy里面的克隆方法返回Student的一个克隆即可,这样就可以保证Student的唯一啦,修改之后的代码如下:

public class DepthCopy { public static void main(String[] args) { Student student = new Student(95); Copy first = new Copy("hzw", 24,student); Copy second = null; try { second = (Copy) first.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } second.name = "shanxi"; second.student.score = 60; System.out.println(first == second);//false System.out.println(first.student == second.student);//false System.out.println(first.student.score);//95 System.out.println(second.student.score);//60 } } class Copy implements Cloneable { public String name; public int age; public Student student; public Copy(String name,int age,Student student) { this.name = name; this.age = age; this.student = student; } @Override protected Object clone() throws CloneNotSupportedException { Copy copy = (Copy)super.clone(); copy.student = (Student) student.clone(); return copy; } } class Student implements Cloneable { public int score; public Student(int score) { this.score = score; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } }

可以看到此时first和second和first.student和second.student都不是相同的,因此我们修改second的student的score之后并没有影响到first里的student的score值,达到了深拷贝的目的;

但是,仔细一想问题就出来了,假如我们上面例子的Student类中也存在引用类型的属性,比如College类,那么我们必须让College类实现Cloneable接口,然后在Student类里面的clone方法里面调用College类的clone方法,在Copy类的clone方法中调用Student类的clone方法,发现没有了,这个过程好复杂,必须让类中的有关引用类型全部实现Cloneable接口,感觉好麻烦是不是,好的,接下来就该牛人登场了;

解决深拷贝问题最好的方式就是采用序列化方式,这样各种类均不用实现Cloneable接口的,直接序列化反序列化就可以啦,我们来见识下吧。

import java.io.File; import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; public class DepthCopy { public static void main(String[] args) { College school = new College("nongda"); Student student = new Student(95, school); Copy copy = new Copy("hzw",23, student); Copy another = null;//表示反序列化出来的类实例 //进行序列化操作 try { FileOutputStream fos = new FileOutputStream(new File("d:/copy.txt")); ObjectOutputStream oos = new ObjectOutputStream(fos); oos.writeObject(copy); } catch (Exception e) { e.printStackTrace(); } //进行反序列化操作 FileInputStream fis; try { fis = new FileInputStream(new File("d:/copy.txt")); ObjectInputStream ois = new ObjectInputStream(fis); another = (Copy) ois.readObject(); } catch (Exception e) { e.printStackTrace(); } System.out.println(copy == another);//false System.out.println(copy.student == another.student);//false System.out.println(copy.student.school == another.student.school);//false another.student.school.schoolName = "wuda"; System.out.println(copy.student.school.schoolName);//nongda } } class Copy implements Serializable { public String name; public int age; public Student student; public Copy(String name,int age,Student student) { this.name = name; this.age = age; this.student = student; } } class Student implements Serializable { public int score; public College school; public Student(int score,College school) { this.score = score; this.school = school; } } class College implements Serializable { public String schoolName; public College(String schoolName) { this.schoolName = schoolName; } }

从输出就可以看出来,反序列化之后生成的对象完全就是对原对象的一份拷贝,除了属性值相同之外并不和原对象有任何关系,因此当我们修改反序列化生成对象的schoolName为"wuda"的时候并没有修改原来实例的schoolName值,还是输出"nongda",因此达到了真正的深拷贝效果,但是要想实现序列化,所有的有关类都必须实现Serializable接口,这总也比既实现Cloneable接口又实现clone方法更方便吧。

以上就是对Java 深拷贝和浅拷贝的详细讲解,有需要的可以参考下。

声明:本页内容来源网络,仅供用户参考;我单位不保证亦不表示资料全面及准确无误,也不保证亦不表示这些资料为最新信息,如因任何原因,本网内容或者用户因倚赖本网内容造成任何损失或损害,我单位将不会负任何法律责任。如涉及版权问题,请提交至online#300.cn邮箱联系删除。

相关文章