克隆出现NullPointerException

Question

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

Answer 1

public class Rectangular implements Cloneable {
即可

Answer 2

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
/**
 *
 * @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());
    }
}