Java 使用OOP实现我的点和线类-错误：null指针异常

我正在尝试编写自己的Point and Line类（尽管类似的代码可以在网上找到）。 我这样做是为了练习如何从头开始构建基本代码。 我得到了一个空指针异常错误，我想有人帮我指出我做错了什么，以及如何解决这个问题。谢谢大家!

这是我的重点课程：

public class Point {
    private double x;
    private double y;

public Point() {
    this.x = 0.0;
    this.y = 0.0;
}
public Point(double x, double y) {
    this.x = x;
    this.y = y;
}
public void setX(double x) {
    this.x = x;
}
public void setY(double y) {
    this.y = y;
}
public double getX() {
    return this.x;
}
public double getY() {
    return this.y;
}
public String toString() {
    return "(" + this.x + ", " + this.y + ")";
}
public static void main(String[] args) {
    Point p = new Point(1.5, 3);
        System.out.println(p.toString());
}

}

这是我的线路课：

public class Line {
    private Point p1;
    private Point p2;

public Line() {
    p1 = new Point(0,0);
    p2 = new Point(0,0);
}

public Line(double x1, double y1, double x2, double y2) {
    p1 = new Point(x1, y1);
    p2 = new Point(x2, y2);
}
public Line(Point p1, Point p2) {
    p1 = new Point(p1.getX(), p1.getY());
    p2 = new Point(p2.getX(), p2.getY());
}
public Point getP1() {
    return this.p1;
}
public Point getP2() {
    return this.p2;
}
public void setP1(double x, double y) {
    this.p1 = new Point (x, y);
}
public void setP2(double x, double y) {
    this.p2 = new Point(x, y);
}
public double getSlope() {
    return ((p2.getY() - p1.getY()) / (p2.getX() - p1.getX()));
}
public static void main(String[] args) {
    Point p1 = new Point(0, 3);
    Point p2 = new Point(5, 5);
    Line l = new Line(p1, p2);
    System.out.println(l.getSlope()); //Threw an error at this point!!
}

} 我的line类出现空指针异常错误，即使我的point is class是ok。

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有人能解释一下如何解决这个问题吗？

您需要调用this.p1/p2，因为这里您只更改构造函数的参数

public Line(Point p1, Point p2) {
  this.p1 = new Point(p1.getX(), p1.getY());
  this.p2 = new Point(p2.getX(), p2.getY());
}

---

无需更改Point类源代码

 public class Point { 
      private double x;
      private double y;

    public Point() {
        this.x = 0.0;
        this.y = 0.0;
    }
    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }
    public void setX(double x) {
        this.x = x;
    }
    public void setY(double y) {
        this.y = y;
    }
    public double getX() {
        return this.x;
    }
    public double getY() {
        return this.y;
    }
    public String toString() {
        return "(" + this.x + ", " + this.y + ")";
    }
    public static void main(String[] args) {
        Point p = new Point(1.5, 3);
        System.out.println(p.toString());
    }
}

请按以下方式更改Line类中的代码

public class Line {
     private Point p1;
     private Point p2;

    public Line() {
        p1 = new Point(0,0);
        p2 = new Point(0,0);
    }

    public Line(double x1, double y1, double x2, double y2) {
        p1 = new Point(x1, y1);
        p2 = new Point(x2, y2);
    }
    public Line(Point p1, Point p2) {
        this. p1 = new Point(p1.getX(), p1.getY());//add this.p1 instead of p1
        this.p2 = new Point(p2.getX(), p2.getY());//add this.p2 intead of p2
    }
    public Point getP1() {
        return this.p1;
    }
    public Point getP2() {
        return this.p2;
    }
    public void setP1(double x, double y) {
        this.p1 = new Point (x, y);
    }
    public void setP2(double x, double y) {
        this.p2 = new Point(x, y);
    }
    public double getSlope() {
        return ((p2.getY() - p1.getY()) / (p2.getX() - p1.getX()));
    }
    public static void main(String[] args) {
        Point p1 = new Point(0, 3);
        Point p2 = new Point(5, 5);
        Line l = new Line(p1, p2);
        System.out.println(l.getSlope()); //Threw an error at this point!!
    }
}

给定输入的输出：0.4

说明： 您正在尝试分配p1和p2引用变量（这是双参数重载行构造函数中的方法参数）类p1和p2的实例变量。但在这种情况下，方法还包含与类中的实例变量同名的方法参数，因此在这种情况下，方法参数将不会分配给类的实例变量。在新点（p1.getX（）、p1.getY（））和新点之后生成的对象（p2.getX（），p2.getY（））将再次分配给方法参数p1和p2

解决方案1：（我在上面的代码中这样做了） 如果在实例变量和方法参数中使用相同的名称，则在处理实例变量时使用此关键字

解决方案2:

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使用不同的变量名来定义方法参数和实例变量。

谢谢。但同样的问题仍然存在。