classes.rb

class Point
  # attributes (like @x and @y here) are "protected", 
  # meaning they can only be directly accessed within
  # the class or a subclass
  def initialize(x, y)
    @x, @y = x, y
  end
  
  # define "reader" methods (getters)
  def getX() @x end     #one way to define a getter...
  def getY() @y end
  
  # we can also define "writer" methods (setters)
  def setX(x) @x = x end
  def setY(y) @y = y end
    
  def to_s
    "(#{@x}, #{@y})"
  end

end

class Line
  
  def initialize(p1, p2)
    @start, @end = p1, p2
    @dx = @start.getX - @end.getX     #using the reader methods
    @dy = @start.getY - @end.getY     
  end
  
  def length
    Math.sqrt(@dx**2 + @dy**2)
  end
  
  def slope
    @dy / @dx rescue puts "slope is undefined"
  end
end

#make two points:
point1 = Point.new(3, 2)
point2 = Point.new(3, 0)

#use the two points to define a line
line = Line.new(point1, point2)
puts "New line from #{point1.to_s} to #{point2.to_s}"

#get the length and slope of the line, and print
puts "length is #{line.length}"
puts "slope is #{line.slope}"

#change point1:
point1.setX(4)
point1.setY(5)
#but the line object is unchanged...
puts "length is #{line.length}"
puts "slope is #{line.slope}"

#define a new line with the new value of point1:
line2 = Line.new(point1, point2)
puts "length is #{line2.length}"
puts "slope is #{line2.slope}"