lingprash wrote:

India
Posted - 08/04/2004 : 6:54:18 PM

Dear Friends,
I cannot follow the Line class from Sir's Book.
IVOR HORTON : BEGINNING JAVA 2.
Can somebody explain me the nitty-gritty details
of the TryGeometry class too.
Honestly i tried going through the code but i am lost.
My sincere thanks to all you who will chip in with your advice.
Have A Nice Day !!!
__________________________________________________ ________

A Line is defined by two Points: the starting point and the end point. A Point is defined by two coordinates (x,y).
The x is the width measured from left to right.
The y is the height measured from the top to the bottom.

Every coordinate pair x,y should be measured in its own rendering context. This can be an area like a button or a frame or a panel, of which the top left is the (0,0) coordinate. A line could be drawn on a panel and you would supply the coordinates for the start and end points of this line measuring from the top left of the panel.

You can supply these coordinates as float or double type, creating a Line2D.Float or Line2D.Double object.

But what was your question, dear student of Sir Ivor's???

Francis from Holland

My question is:

Line(final Point start, final Point end)
  {
    this.start = new Point(start);
    this.end = new Point(end);
  }
what happens here???
************************************************** ****
Line(double xStart, double yStart, double xEnd, double yEnd)
  {
    start = new Point(xStart, yStart); // Create the start point
    end = new Point(xEnd, yEnd); // Create the end point
  }

what happens here...???

i presume here we get 8 coordinates...
That's what I think...

Have A Nice Day!!!

The arguments to the Line constructor are two Point objects.
These are called final so it is made impossible to change the value of the argument Point objects.
We use the two arguments to construct two new Point objects.
Which we store in the start point and end point to construct our new Line.
We use this to refer to the data variables of our Line, instead of the parameter values with the same name, so the compiler will know the difference.

Interesting is:
Here the compiler will not allow you to leave out the this qualifier, because the parameters are declared final.
Try it out!

An Object type variable is passed by reference. The data members of an Object can be changed when you have a reference to it. Qualifying it as final prevents changing the original object.

************************************************** ****
No, 4 coordinates, creating two new Point objects: the start & end data members that define our Line.

Another constructor for a Line here, asking for 4 double type arguments.

Interesting is:
The parameters need not be declared final here. Because basic type variables (like double, char, etc.) are always duplicated when passed to a constructor. So the original double variables won't be modified, passing them as arguments to the Line constructor.