一、为什么要克隆?
答案是:克隆的对象可能包含一些已经修改过的属性,保留着你想克隆对象的值,而new出来的对象的属性全是一个新的对象,对应的属性没有值,所以我们还要重新给这个对象赋值。即当需要一个新的对象来保存当前对象的“状态”就靠clone方法了。那么我把这个对象的临时属性一个一个的赋值给我新new的对象不也行嘛?可以是可以,但是一来麻烦不说,二来,大家通过上面的源码都发现了clone是一个native方法,就是快啊,在底层实现的。
二、如何实现克隆
分三步:
- 对象的类实现Cloneable接口;
- 覆盖Object类的clone()方法 (覆盖clone()方法,访问修饰符设为public,默认是protected);
- 在clone()方法中调用super.clone();
三、两种不同的克隆方法,浅克隆(ShallowClone)和深克隆(DeepClone)。
浅克隆是指拷贝对象时仅仅拷贝对象本身(包括对象中的基本变量),而不拷贝对象包含的引用指向的对象。
深克隆不仅拷贝对象本身,而且拷贝对象包含的引用指向的所有对象。举例来说更加清楚:
浅克隆
package cn.zhm.day4; public class Student implements Cloneable { private int age; private String name; public Student(int age, String name) { this.age = age; this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Student [age=" + age + ", name=" + name + "]"; } @Override public Object clone() throws CloneNotSupportedException { // TODO Auto-generated method stub return super.clone(); } /** * @param args * @throws CloneNotSupportedException */ public static void main(String[] args) throws CloneNotSupportedException { Student student1 = new Student(20, "张三"); Student student2 = (Student) student1.clone(); student2.setAge(22);// 注意修改student2的age值 但是没有影响 student1的值 System.out.println("student1:" + student1.getName() + "-->"+ student1.getAge()); System.out.println("student2:" + student2.getName() + "-->"+ student2.getAge()); } } 运行结果: student1:张三-->20 student2:张三-->22
引入深克隆,浅入克隆导致的问题
package cn.zhm.day4; class Teacher implements Cloneable { private String name; private Student student; public String getName() { return name; } public void setName(String name) { this.name = name; } public Student getStudent() { return student; } public void setStudent(Student student) { this.student = student; } @Override public String toString() { return "Teacher [name=" + name + ", student=" + student + "]"; } @Override public Object clone() throws CloneNotSupportedException { // TODO Auto-generated method stub return super.clone(); } public static void main(String[] args) throws CloneNotSupportedException { Student s1 = new Student(); s1.setAge(20); s1.setName("张三"); Teacher teacher1 = new Teacher(); teacher1.setName("小赵老师"); teacher1.setStudent(s1); //为什么会出现以下结果, Teacher中的clone方法 Teacher teacher2 = (Teacher)teacher1.clone(); Student s2 = teacher2.getStudent(); s2.setName("李四"); s2.setAge(30); System.out.println("teacher1:"+teacher1); System.out.println("teacher2:"+teacher2); } }运行结果:
teacher1:Teacher [name=小赵老师, student=Student [age=30, name=李四]]
teacher2:Teacher [name=小赵老师, student=Student [age=30, name=李四]
深克隆
要克隆的类和类中所有非基本数据类型的属性对应的类
必需在clone()方法注意点
package cn.zhm.day4; class Teacher implements Cloneable { private String name; private Student student; public String getName() { return name; } public void setName(String name) { this.name = name; } public Student getStudent() { return student; } public void setStudent(Student student) { this.student = student; } @Override public String toString() { return "Teacher [name=" + name + ", student=" + student + "]"; } @Override public Object clone() throws CloneNotSupportedException { // TODO Auto-generated method stub //注意以下代码 Teacher teacher = (Teacher)super.clone(); teacher.setStudent((Student)teacher.getStudent().clone()); return teacher; } public static void main(String[] args) throws CloneNotSupportedException { Student s1 = new Student(); s1.setAge(20); s1.setName("张三"); Teacher teacher1 = new Teacher(); teacher1.setName("小赵老师"); teacher1.setStudent(s1); Teacher teacher2 = (Teacher)teacher1.clone(); teacher2.setName("小明老师"); Student s2 = teacher2.getStudent(); s2.setName("李四"); s2.setAge(30); System.out.println("teacher1:"+teacher1); System.out.println("teacher2:"+teacher2); } }
运行结果:
teacher1:Teacher [name=小赵老师, student=Student [age=20, name=张三]]
teacher2:Teacher [name=小明老师, student=Student [age=30, name=李四]]
好好理解上面代码一定可以掌握 浅克隆(ShallowClone)和深克隆(DeepClone)。