克隆出现NullPointerException
初学克隆，不懂。
下面代码、、
package 克隆矩形;

/**
*
* @author Administrator
*/
public class Rectangular {
int x1;
int x2;
int y1;
int y2;
int Sum;
Rectangular(int x1,int x2,int y1,int y2)
{
this.x1=x1;
this.x2=x2;
this.y1=y1;
this.y2=y2;
}
@Override
public Object clone()
{
Object o=null;
try
{
o=(Rectangular)super.clone();
}
catch(CloneNotSupportedException e)
{
System.out.println(e.getMessage());
}
return o;
}

public int Sum()
{
Sum=((x2-x1)+(y2-y1))*2;
return Sum;
}
}


package 克隆矩形;

/**
*
* @author Administrator
*/
public class test {

/**
* @param args the command line arguments
*/
public static void main(String[] args) {
Rectangular r1 = new Rectangular(3,13,5,17);
Rectangular r2 = (Rectangular)r1.clone();
r2.x1=1; //到这里开始出错。
r2.x2=2;
r2.y1=7;
r2.y2=8;

System.out.println(r1.Sum());
System.out.println(r2.Sum());
}
}
我不知道什么原因，希望能解答一下，谢谢了




[解决办法]

Java code

public class Rectangular implements Cloneable {
[解决办法]
你自己的Java类必须要声明实现Cloneable这个接口，然后对象执行clone方法的时候，才能给将属性正常克隆，不至于抛出CloneNotSupportedException
Java code
/** * * @author Administrator */public class Rectangular implements Cloneable{    int x1;    int x2;    int y1;    int y2;    int Sum;        public Rectangular(int x1,int x2,int y1,int y2)    {        this.x1=x1;        this.x2=x2;        this.y1=y1;        this.y2=y2;    }    @Override    public Object clone()    {        Object o=null;        try        {            o=super.clone();        }        catch(CloneNotSupportedException e)        {//            System.out.println(e.getMessage());            e.printStackTrace();        }        return o;    }    public int Sum()    {        Sum=((x2-x1)+(y2-y1))*2;        return Sum;    }    public static void main(String[] args) {        Rectangular r1 = new Rectangular(3,13,5,17);        System.out.println(""+r1.x1);        Rectangular r2 = (Rectangular)r1.clone();        System.out.println(""+r2.x1);        r2.x1=1; //到这里开始出错。        r2.x2=2;        r2.y1=7;        r2.y2=8;        System.out.println(r1.Sum());        System.out.println(r2.Sum());    }}