设置对象成员时出现NullReferenceException

Question

我运行以下程序时收到NullReferenceException.我认为问题来自于我正在创建一个Line包含Point类的事实.

using System;

class Driver
{
    static void Main()
    {
        Point pOne = new Point();
        Point pTwo = new Point(2, 1);

        Console.Write("Point pOne: ");
        PrintPoint(pOne);

        Console.Write("Point pthree: ");
        PrintPoint(pTwo);

        Line lOne = new Line(pOne, pTwo);

        Console.WriteLine("Line lOne: ");
        PrintLine(lOne);

        //Rectangle rOne = new Rectangle();
        Rectangle rOne = new Rectangle(lOne);

        Console.WriteLine("Rectangle rOne: ");
        PrintRectangle(rOne);

        Console.ReadLine();
    }

    // The PrintPoint method
    // purpose: display the coordinates of a Point
    // Parameters: a Point object
    // returns: none
    static void PrintPoint(Point p)
    {
        Console.Write("({0},{1})", p.GetXCoord(), p.GetYCoord());
    }

    // the PrintLine method
    // purpose: display the endpoints of a line
    // Parameters: a Line object
    // returns: none
    static void PrintLine(Line aline)
    {
        // notice how we get the point objects from the line
        // and then print their coordinates
        Point p1 = aline.GetStartPoint();
        Point p2 = aline.GetEndPoint();
        Console.Write("      \t");
        PrintPoint(p1);
        Console.Write(" - ");
        PrintPoint(p2);
    }

    static void PrintRectangle(Rectangle aRec)
    {
        Line Left = aRec.getLeft();
        Line Top = aRec.gettop();
        Line Right = aRec.getRight();
        Line Bottem = aRec.getBottem();

        Console.Write("\t Left: ");
        PrintLine(Left);
        Console.Write("\n\t Top: ");
        PrintLine(Top);
        Console.Write("\n\t Right: ");
        PrintLine(Right);
        Console.Write("\n\t Bottem: ");
        PrintLine(Bottem);

    }
}

class Rectangle
{
    private Line left;
    private Line top;
    private Line right;
    private Line bottem;


    public Rectangle()
  {
        Point zero = new Point();

        left.setEndPoint(zero);
        left.SetStartPoint(zero);

        top.setEndPoint(zero);
        top.SetStartPoint(zero);

        right.setEndPoint(zero);
        right.SetStartPoint(zero);

        bottem.setEndPoint(zero);
        bottem.SetStartPoint(zero);

    }

    public Rectangle(Line enter)
    {
        Point stDgl = new Point();
        Point endDgl = new Point();

        stDgl = enter.GetStartPoint();
        endDgl = enter.GetEndPoint();

        //stDgl
        int a = stDgl.GetXCoord();
        int b = stDgl.GetYCoord();

       //endDgl
        int c = endDgl.GetXCoord();
        int d = endDgl.GetYCoord();

        Point endright = new Point();

        endright.SetXCoord(c);
        endright.SetYCoord(b);

        Point endleft = new Point();

        endleft.SetXCoord(a);
        endleft.SetYCoord(d);

        //LEFT
        left.SetStartPoint(stDgl); // **NullReferenceException**
        left.setEndPoint(endleft); 

        //TOP
        top.SetStartPoint(endleft);
        top.setEndPoint(endDgl);

        //RIGHT
        right.SetStartPoint(endDgl);
        right.setEndPoint(endright);

        //BOTTEM
        bottem.SetStartPoint(endright);
        bottem.setEndPoint(stDgl);
    }

    public Line getLeft()
    {
        return left;
    }

    public Line gettop()
    {
        return top;
    }

    public Line getRight()
    {
        return right;
    }

    public Line getBottem()
    {
        return bottem;
    }

}

// the Line class  
class Line
{
    // data members - notice that they are Point objects
    private Point startPoint;
    private Point endPoint;

    // default constructor
    // purpose: initialize data members to zero
    // Parameters: none
    // returns: none
    public Line()
    {
        // notice how we call methods in the Point class
        **startPoint.SetXCoord(0);**         ***NullReferenceException***
        startPoint.SetYCoord(0);
        endPoint.SetXCoord(0);
        endPoint.SetYCoord(0);
    }

    // parameterized constructor
    // purpose: initialize data members to p1 and p2
    // Parameters: Point objects p1 and p2
    // returns: none
    public Line(Point p1, Point p2)
    {
        startPoint = p1;
        endPoint = p2;
    }

    /*
            //LEFT
            Point endleft = new Point();

            endleft.SetXCoord(a);
            endleft.SetYCoord(d);

            left.SetStartPoint(stDgl);
            left.setEndPoint(endleft);
     * */


    // the GetStartPoint method
    // purpose: return the value of the starting point
    // Parameters: none
    // returns: the value of the starting point as a Point object
    public Point GetStartPoint()
    {
        return startPoint;
    }

    // the GetEndPoint method
    // purpose: return the value of the ending point
    // Parameters: none
    // returns: the value of the ending point as a Point object
    public Point GetEndPoint()
    {
        return endPoint;
    }

    // the SetStartPoint method
    // purpose: store the value of the starting point
    // Parameters: the value of the starting point as a Point object
    // returns: none
    public void SetStartPoint(Point p1)
    {
        startPoint = p1;
    }

    // the SetEndPoint method
    // purpose: store the value of the ending point
    // Parameters: the value of the ending point as a Point object
    // returns: none
    public void setEndPoint(Point p2)
    {
        endPoint = p2;
    }
}

// The Point class
class Point
{
    // data members
    private int xCoord;
    private int yCoord;

    // default constructor
    // purpose: initialize data members to zero
    // Parameters: none
    // returns: none
    public Point()
    {
        xCoord = 0;
        yCoord = 0;
    }

    // parameterized constructor
    // purpose: initialize data members to x an y
    // Parameters: two integers x and y
    // returns: none
    public Point(int x, int y)
    {
        xCoord = x;
        yCoord = y;
    }

    // the GetXCoord method
    // purpose: return the value of the x-coordinate
    // Parameters: none
    // returns: the value of the x-coordinate as an int
    public int GetXCoord()
    {
        return xCoord;
    }

    // the GetYCoord method
    // purpose: return the value of the y-coordinate
    // Parameters: none
    // returns: the value of the y-coordinate as an int
    public int GetYCoord()
    {
        return yCoord;
    }

    // the SetXCoord method
    // purpose: stores the value of the x-coordinate
    // Parameters: the value of the x-coordinate as an int
    // returns: none
    public void SetXCoord(int x)
    {
        xCoord = x;
    }

    // the SetYCoord method
    // purpose: stores the value of the y-coordinate
    // Parameters: the value of the y-coordinate as an int
    // returns: none
    public void SetYCoord(int y)
    {
        yCoord = y;
    }
}

Answer 1

您在创建对象之前尝试设置对象的成员.例如:

class Line
{
    private Point startPoint;
    private Point endPoint;

    public Line()
    {
        startPoint.SetXCoord(0);
        ...

在构造函数的开头,startPoint将是一个null引用.您需要创建一个新Point对象并将引用分配给startPoint,如下所示:

startPoint = new Point();
startPoint.SetXCoord(0); // etc

我还建议你在得到它之后立即将get/set方法更改为属性 - 这样你的代码看起来就像惯用的C#而不是Java.

Answer 2

当你这样做:

public Rectangle(Line enter)
{ 
   // ...

您实际上正在创建一个新的构造函数.这个将运行,但您的默认构造函数将不会运行.

我猜你也期望默认构造函数public Rectangle()也能运行.您可以使用构造函数链接轻松完成此操作.

public Rectangle(Line enter) : this() // Call the default constructor as well
{ 
   // ...

这将消除您的空引用异常,并为您提供我认为您期望的行为.

但是,我还建议阅读其他帖子中的一些建议,特别是Tony the Pony's - 提供一些很好的建议,比如使用属性.