Problem with instantiating

Question

I'm having a bit of a problem understanding instantiating objects. The way I understand it, an object needs to be declared, instantiated and initialised befoore it can be used.
For example, I have a class Point.
So I can have a statement like :
Point P1 = new Point(0,0);
The way I understand it, Point P1+ is the declaration, new+ is the instantiation and Point(0,0)+ is the initialisation.
If my understanding is correct, there are two cases that confuse me.
Case 1:_
I already have a previously declared, instantiated and initialised object PArg of class Point.
then I can state:
Point p0 = PArg;
Now in this case I dont use the new +keyword. I can see where I decare the Object p0 and where I initialise it, but I don't see any instantiation. But the program still works. I don't understand how.
Case 2:+
Point PIn[] = new Point[5];
In this case I am declaring the array of objects and instantiating then with the new +keyword. But I still cannot initialse any elements of my array without intantiating them again. I need to have something like:
PIn[i] = new Point(x,y);
where it is instantiated again with the new +keyword and initialised. I on't understand why it is required or even possible to instantiate the objects in the array twice.
I hope my questions are appropriate for this forum. I would greatly appreciate any answers or links to answers.
Thanks.
Edited by: JulianPatel on Aug 29, 2008 1:13 PM

Answer 1

I have a follow-up question to the ones that have been answered here.
In your example, you assign the value of variable PArg (i.e. the value of the reference) to p0. After the assignment, p0 has the same value as PArg, meaning it refers to the same object as PArg (or both are null).
So if I'm understanding correctly, both p0 and PArg now point to the same physical object, that is to say the same memory location correct? Does that mean if I now change the value of some element in p0, it will also be changed in PArg? What about the other way around?
Eg In my class Point, store two integer values x and y. So suppose I decalre and initialise:
Point PArg = new Point(3,4);This will set PArg.x = 3 and PArg.y =4
Now if I have:
Point p0 = PArg;and then I change the value of p0.x and p0.y using :
p0.reset(1,2);This will change p0.x from 3 to 1. But will PArg.x also change from 3 to 1?
Similarly if it set :
PArg.reset(1,2);will this change the values of p0.x and p0.y?
The relevant potions of code for my class is:
class Point
int x,y;
Point(int xIn, int yIn)
x = xIn;
y = yIn;
void reset(int xIn, int yIn)
x = xIn;
y = yIn;
}And what if the object is created from a method ? Suppose I have a class Geometry containing methods like:
static double sqr(double d)
//Returns the square of the argument
return(d*d);
static double distance(Point p1, Point p2)
//Returns the distance between two Points and/or Floating Points
double d,x1,x2,y1,y2;
x1 = p1.x; x2 = p2.x;
y1 = p1.y; y2 = p2.y;
d = sqrt(sqr(x1-x2) + sqr(y1-y2));
return (d);
static Point integerAltitudeFoot(Point p0,Point pLine1,Point pLine2)
//Returns the Point of intersection of a perpendicular from p0 onto the line passing through pLine1 and pLine2
double d01,d02,d12,d1,d2;
int midX,midY;
d01 = distance(p0,pLine1);
d02 = distance(p0,pLine2);
d12 = distance(pLine2,pLine1);
d1 = (sqr(d12) + sqr(d01) - sqr(d02))/(2*d12);
d2 = (sqr(d12) + sqr(d02) - sqr(d01))/(2*d12);
midX = (int)((pLine1.x*d2 + pLine2.x*d1)/d12);
midY = (int)((pLine1.y*d2 + pLine2.y*d1)/d12);
Point footPrint = new Point(midX,midY);
return(footPrint);
}and then I have:
Point p = Geometry.altitudeFoot(p0,pLine1,pLine2);In this case the object is created by the line:
Point footPrint = new Point(midX,midY);within the altitude function, and then its reference is passed outside through the line:
return(footPrint);and passed onto p by
Point p = Geometry.altitudeFoot(p0,pLine1,pLine2);correct?
But wouldn't the life cycle of Point footPrint be terminated as soon as the method execution is complete? But Point p persists beyond the life cycle of footPrint.
I hope I have expressed my questions properly. As always, thanks in advance for the answers.
Edited by: JulianPatel on Aug 31, 2008 6:10 PM