Java之浅拷贝与深拷贝
----?浅拷贝
--- 概念
被复制对象的所有变量都含有与原来的对象相同的值,而所有的对其他对象的引用仍然指向原来的对象。简单说,浅拷贝就是只复制所考虑的对象,而不复制它所引用的对象
--- 实现方式
继承自java.lang.Object类的Cloneable接口,实现clone()方法
--- 举个栗子
package com.xxg; //浅拷贝
public class ShallowCopy {
public static void main(String[] args) throws CloneNotSupportedException {
Teacher teacher = new Teacher();
teacher.setName("Delacey");
teacher.setAge(29); Student student1 = new Student();
student1.setName("Dream");
student1.setAge(18);
student1.setTeacher(teacher); Student student2 = (Student) student1.clone();
System.out.println("拷贝后");
System.out.println("student2: "+student2.getName());
System.out.println("student2: "+student2.getAge());
student1.setAge(20);
System.out.println("属性值修改后");
System.out.println("student1: "+student1.getAge());
System.out.println("student2: "+student2.getAge());
System.out.println("student2.teacher_name: "+student2.getTeacher().getName());
System.out.println("student2.teacher_age: "+student2.getTeacher().getAge());
System.out.println("修改老师的信息后-------------"); // 修改老师的信息
teacher.setName("Jam");
System.out.println("student1: "+student1.getTeacher().getName());
System.out.println("student2: "+student2.getTeacher().getName());
}
} class Teacher implements Cloneable{
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
protected Object clone() throws CloneNotSupportedException { return super.clone();
}
} class Student implements Cloneable{ private String name;
private int age;
private Teacher teacher;
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 Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
protected Object clone() throws CloneNotSupportedException {
// TODO Auto-generated method stub
return super.clone();
} }
--- 结果
--- 内存图
----?深拷贝
--- 概念
深拷贝是一个整个独立的对象拷贝,深拷贝会拷贝所有的属性,并拷贝属性指向的动态分配的内存。当对象和它所引用的对象一起拷贝时即发生深拷贝。深拷贝相比于浅拷贝速度较慢并且花销较大。简而言之,深拷贝把要复制的对象所引用的对象都复制了一遍。
--- 实现方式
一种是通过继承java.lang.Object类的Cloneable接口,重写clone方法;另外一种是通过实现对象序列化的方式,使对象实现Serializable接口,然后把对象(实际上只是对象的一个拷贝,原先的对象依然在JVM里面)写到一个流里,再从流里读出来,便可以重建对象。
--- 举个栗子
CloneTest2 深复制 public class CloneTest2
{
public static void main(String[] args) throws Exception
{
Teacher teacher = new Teacher();
teacher.setName("Teacher Zhang");
teacher.setAge(40); Student2 student1 = new Student2();
student1.setName("ZhangSan");
student1.setAge(20);
student1.setTeacher(teacher); Student2 student2 = (Student2) student1.clone();
System.out.println("拷贝得到的信息");
System.out.println(student2.getName());
System.out.println(student2.getAge());
System.out.println(student2.getTeacher().getName());
System.out.println(student2.getTeacher().getAge());
System.out.println("-------------"); // 修改老师的信息
teacher.setName("Teacher Zhang has changed");
System.out.println(student1.getTeacher().getName());
System.out.println(student2.getTeacher().getName()); // 两个引用student1和student2指向不同的两个对象
// 但是两个引用student1和student2中的两个teacher引用指向的是同一个对象
// 所以说明是浅拷贝 // 改为深复制之后,对teacher对象的修改只能影响第一个对象
}
} class Teacher implements Cloneable
{
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 Object clone() throws CloneNotSupportedException
{
return super.clone();
} } class Student2 implements Cloneable
{
private String name;
private int age;
private Teacher teacher; 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 Teacher getTeacher()
{
return teacher;
} public void setTeacher(Teacher teacher)
{
this.teacher = teacher;
} @Override
public Object clone() throws CloneNotSupportedException
{
// 浅复制时:
// Object object = super.clone();
// return object; // 改为深复制:
Student2 student = (Student2) super.clone();
// 本来是浅复制,现在将Teacher对象复制一份并重新set进来
student.setTeacher((Teacher) student.getTeacher().clone());
return student;
} }
--- 结果
拷贝得到的信息
ZhangSan
20
Teacher Zhang
40
-------------
Teacher Zhang has changed
Teacher Zhang
--- 内存图
--- 举个栗子(序列化实现)
package com.xxg; import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable; //序列化实现深拷贝
public class DeepCopyServiable { public static void main(String[] args) throws Exception { Teacher3 t = new Teacher3();
t.setName("Teacher Wang");
t.setAge(50); Student3 s1 = new Student3();
s1.setAge(20);
s1.setName("ZhangSan");
s1.setTeacher(t); Student3 s2 = (Student3) s1.deepClone(); System.out.println("拷贝得到的信息:");
System.out.println(s2.getName());
System.out.println(s2.getAge());
System.out.println(s2.getTeacher().getName());
System.out.println(s2.getTeacher().getAge());
System.out.println("---------------------------"); // 将复制后的对象的老师信息修改一下:
s2.getTeacher().setName("New Teacher Wang");
s2.getTeacher().setAge(28); System.out.println("修改了拷贝对象的教师后:");
System.out.println("拷贝对象的教师:");
System.out.println(s2.getTeacher().getName());
System.out.println(s2.getTeacher().getAge());
System.out.println("原来对象的教师:");
System.out.println(s1.getTeacher().getName());
System.out.println(s1.getTeacher().getAge());
} }
class Teacher3 implements Serializable
{
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;
} } class Student3 implements Serializable
{
private String name;
private int age;
private Teacher3 teacher; 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 Teacher3 getTeacher()
{
return teacher;
} public void setTeacher(Teacher3 teacher)
{
this.teacher = teacher;
} public Object deepClone() throws Exception
{
// 序列化
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos); oos.writeObject(this); // 反序列化
ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
ObjectInputStream ois = new ObjectInputStream(bis); return ois.readObject();
} }
--- 结果
----?其他
?clone方法是Object的方法,为什么还要实现cloneable接口?
Cloneable接口是不包含任何方法的具体实现,仅仅作为一个识别标志,而且这个标志也仅仅是针对 Object类中clone()方法的,但是如果没有实现该接口,Object的clone()方法就会抛出ClassNotSupportedException异常
?为什么实现Serializable接口,同时需要一个serialVersionUID?
当一个类被序列化时,并且没有定义字段serialVersionUID,编译器eclipse会提示如下
在Eclipse中有两种生成方式:
一个是默认的1L;
private static final long serialVersionUID = 1L;
一个是根据类名、接口名、成员方法及属性等来生成一个64位的哈希字段,比如:
private static final long serialVersionUID = -932183802511122207L;
如果你没有考虑到兼容性的问题,就把它关掉,不过有这个功能是好的,只要任何类别实现了Serializable接口,如果没有加入serialVersionUID,Eclipse都会给你提示,这个serialVersionUID为了让该类别Serializable向后兼容。
如果你的对象序列化后存到硬盘上面后,你却更改了类的field(增加或减少或改名),当你反序列化时,就会出现异常,这样就会造成不兼容性的问题。
但当serialVersionUID相同时,它就会将不一样的field以type的缺省值Deserialize,这个可以避开不兼容性的问题。
参考资料:
java如果clone为什么要必须实现Cloneable接口?
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