克隆的理解

一、Cloneable 的用途 
Cloneable和Serializable一樣都是標記型接口，它們內部都沒有方法和屬性，implements Cloneable表示該對象能被克隆，能使用Object.clone()方法。如果沒有implements Cloneable的類調用Object.clone()方法就會拋出CloneNotSupportedException。

二、克隆的分類

1、淺克隆（shallow clone）

     拷貝對象的時候僅僅拷貝對象本身和對象中的基本變量，而不拷貝對象中包含的引用所指向的對象。

2、深克隆（deep clone）

     拷貝對象的時候不僅僅拷貝對象本身，同時拷貝對象所包含的的引用所指向的對象。

舉例說明：

對象A1中包含對B1的引用，B1中包含對C1的引用。

淺拷貝A1得到A2，A2中依然包含對B1的引用，B1中依然包含對C1的引用。深拷貝則是對淺拷貝的遞歸，深拷貝A1得到A2，A2中包含對B2（B1的copy）的引用，B2中包含對C2（C1的copy）的引用。 

三、克隆的舉例

要讓一個對象進行克隆，其實就是兩個步驟： 

      1、讓類實現java.lang.Cloneable接口

      2、重寫(override) Object類的clone()方法

代碼實現：

package com.tyxh.create;

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() {

           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) {

           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();

     }

     public static void main(String[] args) throws CloneNotSupportedException {

          Wife wife = new Wife(1, "wang");

          Wife wife2 = null;

          wife2 = (Wife) wife.clone();

          System. out.println( "wife  : " + wife);

          System. out.println( "wife2 : " + wife2);

          wife.setName( "tyxh");

          System. out.println( "wife  : " + wife);

          System. out.println( "wife2 : " + wife2);

          wife2.setName( "sotaof");

          System. out.println( "wife  : " + wife);

          System. out.println( "wife2 : " + wife2);

           //上面兩個輸出的地址相同，克隆的只是引用

          System. out.println( "class same="+(wife.getClass()==wife2.getClass())); //true

          System. out.println( "object same="+(wife==wife2));//false 說明兩個對象的地址不同

          System. out.println( "object equals="+(wife.equals (wife2)));//false

     }

}

輸入結果：

wife  : com.tyxh.create.Wife@3795e3

wife2 : com.tyxh.create.Wife@3795e3

wife  : com.tyxh.create.Wife@369415

wife2 : com.tyxh.create.Wife@3795e3

wife  : com.tyxh.create.Wife@369415

wife2 : com.tyxh.create.Wife@ca8fb940

class same=true

object same=false

object equals=false

第一行和第二行相同的原因不是因爲克隆後的wife和wife2地址相同，而是因爲hashcode的緣故，我們注意hashcode的返回值result = prime * result + (( name == null) ? 0 : name.hashCode());name變化的話，返回值就會發生改變。如果你在hashcode方法中返回1，那麼結果就會是@1；

當對wife的name值進行修改之後，wife的值變化了，而wife2的值沒有變化；

當對wife2的name值進行修改之後，wife2的值變化了，而wife的值沒有變化。

但是這並不代表wife和wife2的地址發生了改變，這僅僅能說明wife和wife2的值發生了改變。當我們註釋掉hashCode()和equals()方法之後，再看一下輸出結果：

wife  : com.tyxh.create.Wife@3771ed5e

wife2 : com.tyxh.create.Wife@1896d2c2

wife  : com.tyxh.create.Wife@3771ed5e

wife2 : com.tyxh.create.Wife@1896d2c2

wife  : com.tyxh.create.Wife@3771ed5e

wife2 : com.tyxh.create.Wife@1896d2c2

class same=true

object same=false

object equals=false

通過結果可以看出，不管是對wife還是對wife2進行值的修改，它們的地址都沒有改變。

四.淺克隆的舉例 

package com.tyxh.create;

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() { 

           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) {

           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  : " + husband);

          System. out.println( "husband2 : " + husband2);

          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 說明引用是同一個引用

          System. out.println( "wife object equals="+(husband.getWife().equals(husband.getWife()))); //true

     }

}

輸入結果如下：

husband  : com.tyxh.create.Husband@20

husband2 : com.tyxh.create.Husband@20

husband class same=true

husband object same=false

husband object equals=true

wife class same=true

wife object same=true

wife object equals=true

四.深克隆的舉例 

package com.tyxh.create;

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 自動生成的

           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 自動生成的

           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  : " + husband);

          System. out.println( "husband2 : " + husband2);

          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 說明引用不是同一個引用

          System. out.println( "wife object equals="+(husband.getWife().equals(husband.getWife()))); //true

     }

}

輸出結果如下：

husband  : com.tyxh.create.Husband@20

husband2 : com.tyxh.create.Husband@20

husband class same=true

husband object same=false

husband object equals=true

wife class same=true

wife object same=false

wife object equals=true

五、通過反序列化克隆 （也是深克隆）

package com.tyxh.create;

import java.io.*;

public class BeanUtil {

     @SuppressWarnings( "unchecked")

     public static <T> T cloneTo(T src) {

          ByteArrayOutputStream memoryBuffer = new ByteArrayOutputStream();

          ObjectOutputStream out = null;

          ObjectInputStream in = null;

          T dist = null;

          

           try {

              out = new ObjectOutputStream(memoryBuffer);

              out.writeObject(src);

              out.flush();

              

              in = new ObjectInputStream( new ByteArrayInputStream(memoryBuffer.toByteArray()));

              dist = (T) in.readObject();

              

          } catch (Exception e) {

              e.printStackTrace();

          } finally {

               if(out != null) {

                    try {

                        out.close();

                   } catch (IOException e) {

                        e.printStackTrace();

                   }

              }

               if(in != null) {

                    try {

                        in.close();

                   } catch (IOException e) {

                        e.printStackTrace();

                   }

              }

          }

                   

           return dist;

     }

     

     public static void main(String[] args) {

          Husband husband = new Husband(1);

          Wife wife = new Wife(1, "jin");

          husband.setWife(wife);

          Husband husband2 = cloneTo(husband);

          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 說明引用不是同一個引用

          System. out.println("wife object equals="+(husband.getWife().equals(husband.getWife()))); //true

     }

}

輸出結果如下：

husband class same=true

husband object same=false

husband object equals=true

wife class same=true

wife object same=false

wife object equals=true

通過第二行可以看出，反序列化之後的husband2與husband不是同一個對象

通過第五行可以看出，反序列化屬於一種深克隆，因爲husband和husband2的wife對象不同