Object对象详解（一）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

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