Cloneable 和 clone()的总结和使用

一.Cloneable 的用途 
  Cloneable和Serializable一样都是标记型接口，它们内部都没有方法和属性，implements Cloneable表示该对象能被克隆，能使用Object.clone()方法。如果没有implements Cloneable的类调用Object.clone()方法就会抛出CloneNotSupportedException。 

二.克隆的分类 
  (1)浅克隆（shallow clone），浅拷贝是指拷贝对象时仅仅拷贝对象本身和对象中的基本变量，而不拷贝对象包含的引用指向的对象。 
  (2)深克隆（deep clone），深拷贝不仅拷贝对象本身，而且拷贝对象包含的引用指向的所有对象。 

举例区别一下：对象A1中包含对B1的引用，B1中包含对C1的引用。浅拷贝A1得到A2，A2中依然包含对B1的引用，B1中依然包含对C1的引用。深拷贝则是对浅拷贝的递归，深拷贝A1得到A2，A2中包含对B2（B1的copy）的引用，B2中包含对C2（C1的copy）的引用。 

三.克隆的举例 
  要让一个对象进行克隆，其实就是两个步骤： 
  1. 让该类实现java.lang.Cloneable接口； 
  2. 重写（override）Object类的clone()方法。
Java代码:   

public class Wife implements Cloneable {
    private int id;
    private String name;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Wife(int id,String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public int hashCode() {//myeclipse自动生成的
        final int prime = 31;
        int result = 1;
        result = prime * result + id;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {//myeclipse自动生成的
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Wife other = (Wife) obj;
        if (id != other.id)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    /**
     * @param args
     * @throws CloneNotSupportedException
     */
    public static void main(String[] args) throws CloneNotSupportedException {
        Wife wife = new Wife(1,"wang");
        Wife wife2 = null;
        wife2 = (Wife) wife.clone();
        System.out.println("class same="+(wife.getClass()==wife2.getClass()));//true
        System.out.println("object same="+(wife==wife2));//false
        System.out.println("object equals="+(wife.equals(wife2)));//true
    }
}

四.浅克隆的举例 

Java代码:  

public class Husband implements Cloneable {
    private int id;
    private Wife wife;

    public Wife getWife() {
        return wife;
    }

    public void setWife(Wife wife) {
        this.wife = wife;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Husband(int id) {
        this.id = id;
    }

    @Override
    public int hashCode() {//myeclipse自动生成的
        final int prime = 31;
        int result = 1;
        result = prime * result + id;
        return result;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    @Override
    public boolean equals(Object obj) {//myeclipse自动生成的
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Husband other = (Husband) obj;
        if (id != other.id)
            return false;
        return true;
    }

    /**
     * @param args
     * @throws CloneNotSupportedException
    */
    public static void main(String[] args) throws CloneNotSupportedException {
        Wife wife = new Wife(1,"jin");
        Husband husband = new Husband(1);
        Husband husband2 = null;
        husband.setWife(wife);
        husband2 = (Husband) husband.clone();
        System.out.println("husband class same="+(husband.getClass()==husband2.getClass()));//true
        System.out.println("husband object same="+(husband==husband2));//false
        System.out.println("husband object equals="+(husband.equals(husband)));//true
        System.out.println("wife class same="+(husband.getWife().getClass()==husband2.getWife().getClass()));//true
        System.out.println("wife object same="+(husband.getWife()==husband2.getWife()));//true
        System.out.println("wife object equals="+(husband.getWife().equals(husband.getWife())));//true
    }
}

五.深克隆的举例 
  如果要深克隆，需要重写（override）Object类的clone()方法，并且在方法内部调用持有对象的clone()方法；注意如下代码的clone()方法

Java代码:  

public class Husband implements Cloneable {
    private int id;
    private Wife wife;

    public Wife getWife() {
        return wife;
    }

    public void setWife(Wife wife) {
        this.wife = wife;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Husband(int id) {
        this.id = id;
    }

    @Override
    public int hashCode() {//myeclipse自动生成的
        final int prime = 31;
        int result = 1;
        result = prime * result + id;
        return result;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        Husband husband = (Husband) super.clone();
        husband.wife = (Wife) husband.getWife().clone();
        return husband;
    }

    @Override
    public boolean equals(Object obj) {//myeclipse自动生成的
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Husband other = (Husband) obj;
        if (id != other.id)
            return false;
        return true;
    }

    /**
     * @param args
     * @throws CloneNotSupportedException
     */
    public static void main(String[] args) throws CloneNotSupportedException {
        Wife wife = new Wife(1,"jin");
        Husband husband = new Husband(1);
        Husband husband2 = null;
        husband.setWife(wife);
        husband2 = (Husband) husband.clone();
        System.out.println("husband class same="+(husband.getClass()==husband2.getClass()));//true
        System.out.println("husband object same="+(husband==husband2));//false
        System.out.println("husband object equals="+(husband.equals(husband)));//true
        System.out.println("wife class same="+(husband.getWife().getClass()==husband2.getWife().getClass()));//true
        System.out.println("wife object same="+(husband.getWife()==husband2.getWife()));//false
        System.out.println("wife object equals="+(husband.getWife().equals(husband.getWife())));//true
    }
}

但是也有不足之处，如果Husband内有N个对象属性，突然改变了类的结构，还要重新修改clone()方法。解决办法：可以使用Serializable运用反序列化手段，调用java.io.ObjectInputStream对象的 readObject()方法。

参考java创建对象的第四种方法：