Java Reference
In-Depth Information
PolyLine
ListPoint end;
ListPoint start;
refers to
refers to
ListPoint
ListPoint
ListPoint
ListPoint
double x;
double y;
ListPoint next;
double x;
double y;
ListPoint next;
double x;
double y;
ListPoint next;
double x;
double y;
ListPoint next;
null
This illustrates the basic structure we might have for a linked list of points stored as a
PolyLine
. The points
are stored as members of
ListPoint
objects. As well as constructors the
PolyLine
class will need a
method to add points, but before we look into that, let's consider the
ListPoint
class in more detail.
There are at least three approaches you could take to define the
ListPoint
class, and there are
arguments in favor of all three.
You could define the
ListPoint
class with the x and y coordinates stored explicitly. The
main argument against this would be that we have already encapsulated the properties of a
point in the
Point
class, so why not use it.
❑
You could regard a
ListPoint
object as something that contains a reference to a
Point
object, plus members that refer to previous and following
ListPoint
objects in the list. This
is not an unreasonable approach. It is easy to implement and not inconsistent with an intuitive
idea of a
ListPoint
.
❑
You could view a
ListPoint
object as a specialized kind of
Point
so you would derive the
ListPoint
class from
Point
. Whether or not this is reasonable depends on whether you see this
as valid. To my mind this is stretching the usual notion of a point somewhat - I would not use this.
❑
The best option looks like the second approach. We could implement the
ListPoint
class with a data
member of type
Point
, which defines a basic point with its coordinates. A
ListPoint
object would
have an extra data member
next
of type
ListPoint
that is intended to contain a reference to the next
object in the list. With this arrangement, you can find all the points in a
Polyline
object by starting
with its
start
member that stores a reference to its first
ListPoint
object. This contains a reference
to the next
ListPoint
object in its
next
member, which in turn contains a reference to the next, and
so on through to the last
ListPoint
object. You will know it is the last one because its
next
member
that usually points to the next
ListPoint
object will be
null
. Let's try it.
