深拷贝与浅拷贝
浅拷贝
public class Student implements Cloneable{
Integer a;
Integer b;
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
String c;
Student child;
}
public class Main {
public static void main(String[] args) throws CloneNotSupportedException {
Student s = new Student();
Student s2 = (Student) s.clone();
System.out.println(s == s2);
System.out.println(s.child == s2.child);
}
}
/**********************************************************************/
false
true
由上述代码及运行结果我们可以看出，调用clone方法之后，确实s2是一个新的对象，内存地址已经发生了改变，但s和s2的child属性仍然指向相同的地址，这便是浅拷贝，当然8种基本数据类型是深拷贝，String则是例外。
深拷贝
public class Teacher implements Cloneable{
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
public class Student implements Cloneable{
Integer a;
Integer b;
public Student(Teacher t) {
this.t = t;
}
@Override
protected Object clone() throws CloneNotSupportedException {
Student s = (Student) super.clone();
s.t = (Teacher)s.t.clone();//彻底深拷贝关键
return s;
}
String c;
Teacher t;
}
public class IntersectionofTwoArraysII {
public static void main(String[] args) throws CloneNotSupportedException {
Teacher t = new Teacher();
Student s = new Student(t);
Student s2 = (Student) s.clone();
System.out.println(s == s2);
System.out.println(s.t == s2.t);
}
}
/**********************************************************************/
false
false
如上代码运行结果所示为深拷贝，想要实现深拷贝，就需要在重写Cloneable接口的clone()方法，并调用属性的clone()方法，因此关联类也要实现Cloneable接口，从而形成拷贝链，实现彻底深拷贝。
实现彻底的深拷贝并不容易，因为它要求涉及的所有类都实现了cloneable接口，但是如StringBuffer由于其没有实现Cloneable接口，想要实现深拷贝效果，则需要做特殊处理从而实现深拷贝的效果。可以在clone方法中实现
sb=new StringBuffer(sb.toString());
Cloneable接口
Cloneable其实就是一个标记接口，只有实现这个接口后，然后在类中重写Object中的clone方法，然后通过类调用clone方法才能克隆成功，如果不实现Cloneable其实就是一个标记接口接口，调用clone方法则会抛出CloneNotSupportedException(克隆不被支持)异常。
如何判断类是否实现了cloneable接口呢？
/**
* Creates and returns a copy of this {@code Object}. The default
* implementation returns a so-called "shallow" copy: It creates a new
* instance of the same class and then copies the field values (including
* object references) from this instance to the new instance. A "deep" copy,
* in contrast, would also recursively clone nested objects. A subclass that
* needs to implement this kind of cloning should call {@code super.clone()}
* to create the new instance and then create deep copies of the nested,
* mutable objects.
*
* @return a copy of this object.
* @throws CloneNotSupportedException
* if this object's class does not implement the {@code
* Cloneable} interface.
*/
protected Object clone() throws CloneNotSupportedException {
if (!(this instanceof Cloneable)) {
throw new CloneNotSupportedException("Class doesn't implement Cloneable");
}
return internalClone((Cloneable) this);
}
/*
* Native helper method for cloning.
*/
private native Object internalClone(Cloneable o);
clone方法首先会判对象是否实现了Cloneable接口，若无则抛出CloneNotSupportedException, 最后会调用internalClone. intervalClone是一个native方法，一般来说native方法的执行效率高于非native方法。