获取空指针异常错误传递空值

Question

我想创建一个三角形对象并获取三角形的面积和周长。 我在Triangle.area和Demo.main收到了NullPointerException Demo.main 。 似乎没有值传递给area方法和perimeter方法，但我看不出我的代码出了什么问题。 这是我的代码。

//POINTCLASS

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

    public Point(int X, int Y){
        this.x = X;
        this.y = Y;           
    }
    public int getX(){
        return x;
    }
    public int getY(){
        return y;
    }
    public double distance(Point p){
        double distance,dx,dy;
        dx=x-p.x;
        dy=y-p.y;

        distance = Math.sqrt((dx*dx)+(dy*dy));
        return distance;
    }
    public void translate(int dx, int dy){
        x+=dx;
        y+=dy;

    }
    public void scale(int factor){
        x*=factor;
        y*=factor;
    }
    public String toString(){
        String attributes = "x = " + x + "," + " y = " + y;
        return attributes;

    }

}

//TRIANGL CLASS

public class Triangle {
    private int sides = 3;
    private Point point1,point2,point3;
    private Point[] Points;
    public Triangle(Point[] vertices){
        Points = vertices;
    }
    public Triangle(int x1,int y1,int x2,int y2,int x3, int y3){
        point1 = new Point(x1,y1);
        point2 = new Point(x2,y2);
        point3 = new Point(x3,y3);
        Points[0]=point1;
        Points[1]=point2;
        Points[2]=point3;
    }
    public double perimeter(){
        double side1=point1.distance(point2);
        double side2=point2.distance(point3);
        double side3=point1.distance(point3); 

        return side1+side2+side3;
    }
    //issue with code seems to be here
    public double area(){
        double side1=point1.distance(point2);
        double side2=point2.distance(point3);
        double side3=point1.distance(point3);
        double s = perimeter()/2;
        double area = Math.sqrt(s*(s-side1)*(s-side2)*(s-side3));
        return area;
    }
    public String toString(){
        String attributes = "Sides " + sides;
        return attributes;
    }
    public void translate(int dx, int dy){
        Points[0].translate(dx,dy);
        Points[1].translate(dx, dy);
        Points[2].translate(dx, dy);
    }
    public void scale(int factor){
        vertices*=factor;

    }
    public Point getVertex(int i){
        return vertices[i];

    }
}

//MAIN CLASS

public class GDemo {
    public static void main(String[] args) {
        Point p1 = new Point(0,20);
        Point p2 = new Point(20,0);
        Point p3 = new Point(30,30);

        Point[] vertices = {p1,p2,p3};

        Triangle t = new Triangle(vertices);


        System.out.println("Triangle" + t.toString() + " area:" + t.area() + 
                           " perimeter: " + t.perimeter());
        t.translate(10, 15);
        System.out.println("Triangle" + t.toString() + " area:" + t.area() + 
        " perimeter: " + t.perimeter());
        t.scale(2);
        System.out.println("Triangle" + t.toString() + " area:" + t.area() + 
        "perimeter: " + t.perimeter());   
    }
}

Answer 1

您将构造函数称为public Triangle(Point[] vertices) ，但没有设置point1，point2和point3值-它们保持为空。 因此，在方法区域中尝试计算side1 = point1.distance(point2); 它会抛出NPE。

Answer 2

您在创建Triangle Object时正在调用构造函数Triangle(Point[] vertices) 。 因此，您的这些字段将保留为未初始化的private Point point1,point2,point3; 这就是为什么要获取NullPointerException的原因。

固定的构造函数应如下所示：

public Triangle(Point[] vertices){
    Points = vertices;

    // initialize point1, point2, point3 here.
    if(vertices.length > 2){
       this.point1 = vertices[0];
       this.point2 = vertices[1];
       this.point3 = vertices[2];
    }
}

Answer 3

尝试将整个代码放入try-catch块中，并捕获NullPointerException并编写

 e.printStackTrace(); 

例如：

try{

    //your block of code

}catch(NullPointerException e){

    e.printStackTrace();
}

它会显示错误代码的行号