浅拷贝的问题
先看浅拷贝代码:
class Writer implements Cloneable{
private int age;
private String name;
// getter/setter 、构造方法、toString()
}
其中,Cloneable 接口是一个标记接口,它肚子里面是空的,标记接口的作用其实很简单,用来表示某个功能在执行的时候是合法的。
public interface Cloneable {
}
如果一个类没有实现 Cloneable 接口,即便它重写了 clone()
方法,依然是无法调用该方法进行对象克隆的,程序在执行 clone()
方法的时候会抛出 CloneNotSupportedException 异常。
测试类代码:
class TestClone {
public static void main(String[] args) throws CloneNotSupportedException {
Writer writer1 = new Writer(18,"二哥");
Writer writer2 = (Writer) writer1.clone();
System.out.println("浅拷贝后:");
System.out.println("writer1:" + writer1);
System.out.println("writer2:" + writer2);
writer2.setName("三妹");
System.out.println("调整了 writer2 的 name 后:");
System.out.println("writer1:" + writer1);
System.out.println("writer2:" + writer2);
}
}
浅拷贝后:
writer1:Writer@68837a77{age=18, name='二哥'}
writer2:Writer@b97c004{age=18, name='二哥'}
调整了 writer2 的 name 后:
writer1:Writer@68837a77{age=18, name='二哥'}
writer2:Writer@b97c004{age=18, name='三妹'}
可以看得出,浅拷贝后,writer1 和 writer2 引用了不同的对象,但值是相同的,说明拷贝成功。之后,修改了 writer2 的 name 字段
我们再为 Writer 类增加一个自定义的引用类型字段 Book,先来看定义:
class Book {
private String bookName;
private int price;
// getter/setter 、构造方法、toString()
}
class Writer implements Cloneable{
private int age;
private String name;
private Book book;
// getter/setter 、构造方法、toString()
}
测试类:
class TestClone {
public static void main(String[] args) throws CloneNotSupportedException {
Writer writer1 = new Writer(18,"二哥");
Book book1 = new Book("编译原理",100);
writer1.setBook(book1);
Writer writer2 = (Writer) writer1.clone();
System.out.println("浅拷贝后:");
System.out.println("writer1:" + writer1);
System.out.println("writer2:" + writer2);
Book book2 = writer2.getBook();
book2.setBookName("永恒的图灵");
book2.setPrice(70);
System.out.println("writer2.book 变更后:");
System.out.println("writer1:" + writer1);
System.out.println("writer2:" + writer2);
}
}
浅拷贝后:
writer1:Writer@68837a77 age=18, name='二哥', book=Book@32e6e9c3 bookName='编译原理', price=100}}
writer2:Writer@6d00a15d age=18, name='二哥', book=Book@32e6e9c3 bookName='编译原理', price=100}}
writer2.book 变更后:
writer1:Writer@68837a77 age=18, name='二哥', book=Book@32e6e9c3 bookName='永恒的图灵', price=70}}
writer2:Writer@36d4b5c age=18, name='二哥', book=Book@32e6e9c3 bookName='永恒的图灵', price=70}}
与之前例子不同的是,writer2.book 变更后,writer1.book 也发生了改变。这是因为字符串 String 是不可变对象,一个新的值必须在字符串常量池中开辟一段新的内存空间,而自定义对象的内存地址并没有发生改变,只是对应的字段值发生了改变,见下图。
实现深拷贝
浅拷贝克隆的对象中,引用类型的字段指向的是同一个,当改变任何一个对象,另外一个对象也会随之改变,除去字符串的特殊性外(或者new一个新对象)。深拷贝和浅拷贝不同的,深拷贝中的引用类型字段也会克隆一份,当改变任何一个对象,另外一个对象不会随之改变。
class Book implements Cloneable{
private String bookName;
private int price;
// getter/setter 、构造方法、toString()
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
注意,此时的 Book 类和浅拷贝时不同,重写了 clone()
方法,并实现了 Cloneable 接口。为的就是深拷贝的时候也能够克隆该字段。因为Book内部都是基本的数据类型,所以不需要其他代码处理。
class Writer implements Cloneable{
private int age;
private String name;
private Book book;
// getter/setter 、构造方法、toString()
@Override
protected Object clone() throws CloneNotSupportedException {
Writer writer = (Writer) super.clone();
writer.setBook((Book) writer.getBook().clone());
return writer;
}
}
这样再经过测试类测试时,就不是同一个 Book 对象了:
深拷贝后:
writer1:Writer@6be46e8f age=18, name='二哥', book=Book@5056dfcb bookName='编译原理', price=100}}
writer2:Writer@6d00a15d age=18, name='二哥', book=Book@51efea79 bookName='编译原理', price=100}}
writer2.book 变更后:
writer1:Writer@6be46e8f age=18, name='二哥', book=Book@5056dfcb bookName='编译原理', price=100}}
writer2:Writer@6d00a15d age=18, name='二哥', book=Book@51efea79 bookName='永恒的图灵', price=70}}