Line object from two point objects and using objects as variables

skf001 · April 23rd, 2012, 04:03 PM

Reference example in chapter 5 pages 188 and 190.

If I strip down the code for the Point class to:

Code:

 
public class Point  {
  double x;
  double y;
  public Point(double xVal, double yVal)  {
    x = xVal;
    y = yVal;
  }
 public Point(Point oldPoint)  {
    x = oldPoint.x;    
    y = oldPoint.y;    
  }
}

And strip the code of the Line class down to:

Code:

public class Line {
  Point start;   
  Point end;     

  public Line(Point start, Point end) {
    this.start = new Point(start);
    this.end = new Point(end);
  }
}

And compile the Line.java file, both the Line and Point source files compile. But if I remove this method:

Code:

public Point(Point oldPoint) {
    x = oldPoint.x;    
    y = oldPoint.y;    
  }

from the Point class source file, the Point source file compiles but the Line source file won't. The compiler says:
Line.java:6: error: constructor Point in class Point cannot be applied to given
types;
this.start = new Point(start);
^
required: double,double
found: Point
reason: actual and formal argument lists differ in length
Line.java:7: error: constructor Point in class Point cannot be applied to given
types;
this.end = new Point(end);
^
required: double,double
found: Point
reason: actual and formal argument lists differ in length

I understand the error messages but not why there is are errors. Just wondering what it is I am missing that is causing me to not understand what is wrong with the code this way.

javaKid1337 · May 6th, 2012, 06:13 AM

This constructor is the constructor being calles when you write
this.start = new Point(start)

Code:

public Point(Point oldPoint) {
    x = oldPoint.x;    
    y = oldPoint.y;    
  }

If this is missing , how do you expect the compiler to compile ?
Its like writing address to the house that does not exist.

The compiler sees , OH! The only constructor in class Point is Point(double x,double y)
but who on earth is Point(Point oldPoint)??
so it says you need double,double instead of Point as an argument.

Thats why it says that actual arguments and formal arguments differ.
You deleted the constructor you are trying to call.

Hope that sums it up :)

skf001 · May 13th, 2012, 11:18 AM

I understand what you were trying to explain and I really appreciate the fact that you took the time to help. Thankyou.

The problem I'm having with my example is that the constructor I removed from the Point class was the constructor included only to make new point objects out of already existing point objects. Let's say that I was only planning to make Point objects using the constructor that takes the double x and double y as arguements; why should I have to have the contructor that can make new objects from existing ones in order for the compiler to compile the Line and Point classes?

You said that the constructor being called by the keyword 'this' is the constructor that creates new objects from existing ones but I don't see that. The 'this' keyword is refering to the Point variable 'start' and point variable 'end' of the Line class.

Like I said, thanks for taking the time to respond. Any help is appreciated.

javaKid1337 · May 14th, 2012, 01:43 AM

Line.java:6: error: constructor Point in class Point cannot be applied to given
types;
this.start = new Point(start);
^
required: double,double
found: Point
reason: actual and formal argument lists differ in length

Based on this , I gave my answer. Now you said that you wanted to create ENTIRELY NEW POINT OBJECTS then you will have to pass 4 co-ordinate points because initially , the Point objects start and end have no x,y values stored in them.
so you should make a constructor like this

Code:

public Line(double x1,double y1,double x2,double y2) {
start = new Point(x1,y1);
end = new Point(x2,y2)
}

and you do have such a constructor , just that the values passed to them are not right. Thats what your compiler is saying - actual and formal arguments differ.
regarding calling the constructor using this , you can do it.
this(start,end)

this will call a constructor IN THE SAME CLASS that needs two Point objects as arguments

skf001 · May 15th, 2012, 07:56 PM

I see now where my thinking went wrong. You were absolutely right in your responses (which I misunderstood, even though I thought I understood them when I read them).

I missed, no matter how many times I looked at it, that the Point constructor that's called in the Line constructor...

Code:

Line(final Point start, final Point end) {
    this.start = new Point(start);
    this.end = new Point(end);}

...is calling the Point constructor that makes Point objects from existing ones. For some reason I kept thinking it was making brand new Point objects. (I don't know why I was thinking this, but it was why I couldn't see my mistake).

Like you said, to make new objects, the Line constructor that takes the four type double arguements would have to be called.

Once my brain realized this, everything - including, obviously, your correct and insightful responses - clicked into place.

Thanks again.

Addendum:
I think what was making me go wrong was that I was seeing the words 'start' and 'end' in the Point constructors in the Line constructor (ref. above code example) as refering to the variables in the Line object instead of what they were actually refering to, which were the start and end objects, already created, being passed as arguments, to the Line constructor. The fact that 'start' and 'end' are used as labels for the Line constructor parameters, for the names of two objects created in the TryGeometry class, and for the names of Point class variables in the Line class threw me off.