Java中的深拷贝和浅拷贝

[TOC]

一、什么是深拷贝、浅拷贝

浅拷贝：将被复制的对象的成员变量拷贝到新创建的对象的成员变量中，即
- 对于基本类型来说，会直接进行值传递，也就是将该属性值复制一份给新的对象，也就是说新老对象的数据是不共享的，只是值相同而已；
- 对于引用类型来说，拷贝时也是把老对象的引用对象的地址拷贝到新对象中，实际上新老两个对象引用的是用一个对象；
深拷贝：不仅对基本类型变量进行值赋值，而且对于引用类型会为新对象创建相应的新的引用类型，即新老对象引用的是不同的对象；
示意图：

二、如何实现浅拷贝

String类型属于引用数据类型，不属于基本数据类型，但是String类型的数据是存放在常量池中的，也就是无法修改的；

2.1通过构造方法实现

通过创建一个使用该类对象作为参数的构造方法，能实现浅拷贝；

class Student{
    public int age;
    public String name;
    public Teacher teacher;
      
    public Student(int age, String name, Teacher teacher){
        this.age=age;
        this.name=name;
        this.teacher=teacher;
    }
    // 拷贝构造方法
    public Student(Student s){
        this.age=s.age;
        this.name=s.name;
        this.teacher=s.teacher;
    }
      
}
  
class Teacher{
    public int age;
    public String name;
      
    public Teacher(int age, String name){
        this.age=age;
        this.name=name;
    }
}
  
public class ShallowCopy {
  
    public static void main(String[] args) {
        Teacher t1 = new Teacher(30,"zyn");
        Student s1 = new Student(20,"kevin",t1);
        Student s2 = new Student(s1);
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
        s1.age=25;
        s1.name="kevin11";
        s1.teacher.age=40;
        System.out.println("修改s1的name、age、teacher后");
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
    }
}
  

运行结果:

s1是kevin, 20,s1的老师为 zyn, 30
s2是kevin, 20,s2的老师为 zyn, 30
修改s1的name、age、teacher后
s1是kevin11, 25,s1的老师为 zyn, 40
s2是kevin, 20,s2的老师为 zyn, 40

2.2重写Object中的clone方法实现

在Object类中有一个方法为clone()就是用来进行浅拷贝的，使用时需要注意：
- 在Object类中，该方法是protected的，重写时要改为public;
- 使用clone方法的类必须要实现Cloneable接口；

代码：

class Student implements Cloneable{
    public int age;
    public String name;
    public Teacher teacher;
      
    public Student(int age, String name, Teacher teacher){
        this.age=age;
        this.name=name;
        this.teacher=teacher;
    }
    @Override
    public Object clone(){
        Object obj = null;
        try{
            obj = super.clone();
        }catch(CloneNotSupportedException e){
            e.printStackTrace();
        }
        return obj;
    }
      
}
  
class Teacher{
    public int age;
    public String name;
      
    public Teacher(int age, String name){
        this.age=age;
        this.name=name;
    }
}
  
public class ShallowCopy {
  
    public static void main(String[] args) throws CloneNotSupportedException {
        Teacher t1 = new Teacher(30,"zyn");
        Student s1 = new Student(20,"kevin",t1);
        Student s2 = (Student) s1.clone();
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
        s1.age=25;
        s1.name="kevin11";
        s1.teacher.age=40;
        System.out.println("修改s1的name、age、teacher后");
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
	}
}
  

运行结果：

s1是kevin, 20,s1的老师为 zyn, 30
s2是kevin, 20,s2的老师为 zyn, 30
修改s1的name、age、teacher后
s1是kevin11, 25,s1的老师为 zyn, 40
s2是kevin, 20,s2的老师为 zyn, 40

三、如何实现深拷贝

3.1重写Object中的clone方法实现

基本思路和浅拷贝的思路差不多，主要是要对每个引用对象创建相应的对象；

代码：

class Student implements Cloneable{
    public int age;
    public String name;
    public Teacher teacher;
      
    public Student(int age, String name, Teacher teacher){
        this.age=age;
        this.name=name;
        this.teacher=teacher;
    }
    @Override
    public Object clone(){
        Object obj = null;
        try{
            obj = super.clone();
        }catch(CloneNotSupportedException e){
            e.printStackTrace();
        }
        Student s = (Student)obj;
        s.teacher = (Teacher)this.teacher.clone();
        return obj;
    }
      
}
  
class Teacher implements Cloneable{
    public int age;
    public String name;
      
    public Teacher(int age, String name){
        this.age=age;
        this.name=name;
    }
      
    @Override
    public Object clone(){
        Object obj = null;
        try{
            obj = super.clone();
        }catch(CloneNotSupportedException e){
            e.printStackTrace();
        }
        return obj;
    }
}
  
public class DeepCopy {
  
    public static void main(String[] args) throws CloneNotSupportedException {
        Teacher t1 = new Teacher(30,"zyn");
        Student s1 = new Student(20,"kevin",t1);
        Student s2 = (Student) s1.clone();
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
        s1.age=25;
        s1.name="kevin11";
        s1.teacher.age=40;
        System.out.println("修改s1的name、age、teacher后");
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
    }
}

运行结果：

s1是kevin, 20,s1的老师为 zyn, 30
s2是kevin, 20,s2的老师为 zyn, 30
修改s1的name、age、teacher后
s1是kevin11, 25,s1的老师为 zyn, 40
s2是kevin, 20,s2的老师为 zyn, 30

3.2通过序列化和反序列化来实现

进行序列化时要将对象加入流中，该过程就是对象的深拷贝，原对象任在JVM运行时数据区域中，再通过反序列化从流中读取数据，可以获得新的拷贝对象
- 进行序列化的类要实现Serializable接口

代码：

import java.io.*;
  
public class DeepCopyBySerialization {
  
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Teacher t1 = new Teacher(30,"zyn");
        Student s1 = new Student(20,"kevin",t1);
  
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(s1);
        oos.flush();
  
        ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bos.toByteArray()));
        Student s2 = (Student) ois.readObject();
  
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
        s1.age=25;
        s1.name="kevin11";
        s1.teacher.age=40;
        System.out.println("修改s1的name、age、teacher后");
  
        System.out.println("s1是"+s1.name+", "+s1.age+",s1的老师为 "+s1.teacher.name+", "+s1.teacher.age);
        System.out.println("s2是"+s2.name+", "+s2.age+",s2的老师为 "+s2.teacher.name+", "+s2.teacher.age);
  
  
  
    }
}
  
  
class Student implements Serializable {
    public int age;
    public String name;
    public Teacher teacher;
  
    public Student(int age, String name, Teacher teacher){
        this.age=age;
        this.name=name;
        this.teacher=teacher;
    }
  
}
  
class Teacher implements Serializable{
    public int age;
    public String name;
  
    public Teacher(int age, String name){
        this.age=age;
        this.name=name;
    }
  
}

运行结果：

s1是kevin, 20,s1的老师为 zyn, 30
s2是kevin, 20,s2的老师为 zyn, 30
修改s1的name、age、teacher后
s1是kevin11, 25,s1的老师为 zyn, 40
s2是kevin, 20,s2的老师为 zyn, 30

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