Object对象详解（二）之clone

clone方法会返回该实例对象的一个副本，通常情况下x.clone() != x || x.clone().getClass() == x.getClass() || x.clone().equals(x)也为真，但不严格要求，我们可以通过重写该方法来覆盖。

protected native Object clone() throws CloneNotSupportedException;

可以看到，clone是一个本地方法，可能会抛出CloneNotSupportedException异常，什么情况下会抛出呢？

/** * A class implements the <code>Cloneable</code> interface to * indicate to the {@link java.lang.Object#clone()} method that it * is legal for that method to make a * field-for-field copy of instances of that class. * <p> * Invoking Object's clone method on an instance that does not implement the * <code>Cloneable</code> interface results in the exception * <code>CloneNotSupportedException</code> being thrown. * <p> * By convention, classes that implement this interface should override * <tt>Object.clone</tt> (which is protected) with a public method. * See {@link java.lang.Object#clone()} for details on overriding this * method. * <p> * Note that this interface does <i>not</i> contain the <tt>clone</tt> method. * Therefore, it is not possible to clone an object merely by virtue of the * fact that it implements this interface.  Even if the clone method is invoked * reflectively, there is no guarantee that it will succeed. * * @author  unascribed * @see     java.lang.CloneNotSupportedException * @see     java.lang.Object#clone() * @since   JDK1.0 */public interface Cloneable {}

说明中写到，如果该对象未实现Cloneable 接口，那么当实例调用clone方法时，就会抛出该异常。
下面看Object中对于clone方法的描述。

    /**     * Creates and returns a copy of this object.  The precise meaning     * of "copy" may depend on the class of the object. The general     * intent is that, for any object {@code x}, the expression:     * <blockquote>     * <pre>     * x.clone() != x</pre></blockquote>     * will be true, and that the expression:     * <blockquote>     * <pre>     * x.clone().getClass() == x.getClass()</pre></blockquote>     * will be {@code true}, but these are not absolute requirements.     * While it is typically the case that:     * <blockquote>     * <pre>     * x.clone().equals(x)</pre></blockquote>     * will be {@code true}, this is not an absolute requirement.     * <p>     * By convention, the returned object should be obtained by calling     * {@code super.clone}.  If a class and all of its superclasses (except     * {@code Object}) obey this convention, it will be the case that     * {@code x.clone().getClass() == x.getClass()}.     * <p>     * By convention, the object returned by this method should be independent     * of this object (which is being cloned).  To achieve this independence,     * it may be necessary to modify one or more fields of the object returned     * by {@code super.clone} before returning it.  Typically, this means     * copying any mutable objects that comprise the internal "deep structure"     * of the object being cloned and replacing the references to these     * objects with references to the copies.  If a class contains only     * primitive fields or references to immutable objects, then it is usually     * the case that no fields in the object returned by {@code super.clone}     * need to be modified.     * <p>     * The method {@code clone} for class {@code Object} performs a     * specific cloning operation. First, if the class of this object does     * not implement the interface {@code Cloneable}, then a     * {@code CloneNotSupportedException} is thrown. Note that all arrays     * are considered to implement the interface {@code Cloneable} and that     * the return type of the {@code clone} method of an array type {@code T[]}     * is {@code T[]} where T is any reference or primitive type.     * Otherwise, this method creates a new instance of the class of this     * object and initializes all its fields with exactly the contents of     * the corresponding fields of this object, as if by assignment; the     * contents of the fields are not themselves cloned. Thus, this method     * performs a "shallow copy" of this object, not a "deep copy" operation.     * <p>     * The class {@code Object} does not itself implement the interface     * {@code Cloneable}, so calling the {@code clone} method on an object     * whose class is {@code Object} will result in throwing an     * exception at run time.     *     * @return     a clone of this instance.     * @exception  CloneNotSupportedException  if the object's class does not     *               support the {@code Cloneable} interface. Subclasses     *               that override the {@code clone} method can also     *               throw this exception to indicate that an instance cannot     *               be cloned.     * @see java.lang.Cloneable     */    protected native Object clone() throws CloneNotSupportedException;

其中提到的重写clone方法的几个注意点
1. 数组视为自动实现了Cloneable接口；
2. 非数组类型，使用clone方法，需要实现Cloneable接口，否则会抛出异常；
3. 非数组类型，克隆时，会新创建一个该类型的实例，并将被克隆对象实例的状态复制给新创建对象，并且这是一个浅克隆-（影子克隆——shallow copy），而不是deep copy；
4. 重写clone方法时，首先首先首先需要调用父类的clone方法。

那么问题来了，什么是shallow copy？而deep copy又是什么？

上例子：

public class Test {    public static void main(String[] args) throws CloneNotSupportedException {          People people = new People("zjh", '男', 21, new Cloth(COLOR.WHITE  , "XXL"));               People clone = (People) people.clone();          System.out.println("people == clone : " + (people == clone));          System.out.println("people.getCloth() == clone.getCloth() : "+ (people.getCloth() == clone.getCloth()));          System.out.println("people.getAge() == clone.getAge() : "+(people.getAge() == clone.getAge()));          System.out.println("people.getName() == clone.getName() : "+(people.getName() == clone.getName()));    }}class People implements Cloneable{    private String name;    private char sex;    private int age;    private Cloth cloth;    /**      * {@inheritDoc}.     */    @Override    protected Object clone() throws CloneNotSupportedException {        // TODO Auto-generated method stub        return super.clone();    }    ...若干getter/setter方法  }class Cloth{    private COLOR color;    private String size;    /**     * 构造函数.     *      * @param color     * @param size     */    public Cloth(COLOR color, String size) {        super();        this.color = color;        this.size = size;    }       enum COLOR {        RED,WHITE,BLACK,GREEN,BLUE    }}

运行结果：

people == clone : false
people.getCloth() == clone.getCloth() : true
people.getSex() == clone.getSex() : true
people.getName() == clone.getName() : true

age，sex，name比较为真还能理解，为什么people.getCloth() == clone.getCloth() 也是true呢？又做了下面的测试。

 people.getCloth().setColor(COLOR.BLACK);          System.out.println(clone.getCloth().getColor());

运行结果：

BLACK

现在已经能确定，people和它的克隆对象clone中的cloth引用指向了同一个Cloth实例。这就是“shallow copy”。那么想要“deep copy”，那么就需要在重写方法的时候，同时调用对象属性的克隆方法（要求该属性对象也需要实现Cloneable）。

clone方法修改如下：

    protected Object clone() throws CloneNotSupportedException {        // TODO Auto-generated method stub        People clone =    (People) super.clone();        clone.setCloth((Cloth)cloth.clone());        return clone;    }

再运行上面的测试程序：

people == clone : false
people.getCloth() == clone.getCloth() : false
people.getAge() == clone.getAge() : true
people.getSex() == clone.getSex() : true
people.getName() == clone.getName() : true