在Java中访问公共构造函数中的变量

Question

If you have

class Triangle
{
  private double x1,x2,x3,y1,y2,y3;
  public Triangle(Point point1, Point point2, Point point3) 
  { 
    x1=point1.getX();
    y1=point1.getY();
    x2=point2.getX();
    y2=point2.getY();
    x3=point3.getX();
    y3=point3.getY();

    //Trying to get x and y values of point1-point3

  }

  double width=x1-x2;
  double length=y3-y2;

  public double area() 
  { 
    return (length * width)/2; 
  }
}

EDIT: I'm also only getting values for everything but length. length prints out 0 but y3 and y2 are still able to print values. I'm using a random number generator for doubles:

double randomValue = Math.random() * 100;

Could this be an issue with subtraction for doubles?

I basically have points defined to take two variables x and y and I'm trying to calculate this area of a triangle. When someone gives 3 points to make this triangle, I'm trying to get those values from the points and I do have getters for my points but I'm just ending up with nothing for length and width.

Answer 1

width and length should both be local variables inside of your area() method or inside of your constructor. It can be calculated in your constructor if this object is immutable, but I would say it is best to calculate them in the area() method if you expose getters/setters to the objects.

public double area() 
{ 
    double width=x1-x2;
    double length=y3-y2;
    return (length * width)/2;
}

As you have it right now, they are package private members of your class, and are being set when you instantiate the class.

Answer 2

This code

double width=x1-x2;
double length=y3-y2;

Should be in your constructor.

As is it now, it is executed prior to the constructor, when x1,x2,y3,y2 are still 0.

  double width;
  double length;

  public Triangle(Point point1, Point point2, Point point3) 
  { 
    x1=point1.getX();
    y1=point1.getY();
    x2=point2.getX();
    y2=point2.getY();
    x3=point3.getX();
    y3=point3.getY();

    // init width and length after x1,x2,y3,y2 are intialized
    width=x1-x2;
    length=y3-y2;
  }

The alternative is to calculate width and length only when you need them :

  public double area() 
  { 
    double width=x1-x2;
    double length=y3-y2;
    return (length * width)/2; 
  }