Return instance of subclass

2 Message(s) by 2 Author(s) originally posted in java language

From: Andrew Brampton

Date: Saturday, March 31, 2007

Hi,
I have encoutered a design

<process> The approach that engineering (and some other)
	disciplines use to specify how to create or do something.  A
	successful design must satisfies a (perhaps informal)
	{functional specification} (do what it was designed to do);
	conforms to the limitations of the target medium (it is
	possible to implement); meets implicit or explicit
	requirements on performance and resource usage (it is
	efficient enough).

	A design may also have to satisfy restrictions on the design
	process itself, such as its length or cost, or the tools
	available for doing the design.

	In the {software life-cycle}, design follows {requirements
	analysis} and is followed by implementation.

	["Object-Oriented Analysis and Design with Applications", 2nd
	ed., Grady Booch].

	(1996-12-08)

problem that I'm unsure how to solve in a neat way.

I have a class

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

called Ranges, which represents a set of numerical range

1. <data, graphics> Data representing a two-dimensional scene.
	A digital image is composed of {pixels} arranged in a
	rectangular array with a certain height and width.  Each pixel
	may consist of one or more {bits} of information, representing
	the brightness of the image at that point and possibly
	including colour information encoded as {RGB} triples.

	{Images} are usually taken from the real world via a {digital
	camera}, {frame grabber}, or {scanner}; or they may be
	generated by computer, e.g. by {ray tracing} software.

	See also {image formats}, {image processing}.

	(1994-10-21)

	2. <mathematics> The image (or range) of a {function} is the
	set of values obtained by applying the function to all
	elements of its {domain}.  So, if f : D -> C then the set f(D)
	= \{ f(d) | d in D \} is the image of D under f.  The image is
	a subset of C, the {codomain}.

	(2000-01-19)

s,
such as [0-10] [30-50], etc. And I can OR different Range instance

<programming> An individual {object} of a certain {class}.
	While a class is just the type definition, an actual usage of
	a class is called "instance".  Each instance of a class can
	have different values for its {instance variables}, i.e. its
	{state}.

	(1998-03-06)

together
to work out the overlaps.

class Ranges {
// Returns the overlapping regions of the two ranges
public Ranges overlap ( Ranges r ) {
Ranges ret = new Ranges();
//do something
return ret;
}
}

Now I have a RangeCounts class, which does a similar thing, but assigns a
value to these ranges., such that [0-10 val 1] [30-40 val 2] [40-50 val 1].
Now this works in a very similar way to Ranges, and infact extend

<programming> To add {features} to a program, especially
	through the use of {hooks}.

	"Extend" is very often used in the phrase "extend the
	{functionality} of a program."

	{Plug-ins} are one form of extension.

	(1997-06-21)

s Ranges.

class RangeCounts extend Ranges {
// Does exactly the same as the parent, thus not defined
//public Ranges overlap ( Ranges r ) {}
}

Now the problem is, the overlap method

<programming> The name given in {Smalltalk} and other
	{object-oriented languages} to a procedure or routine
	associated with one or more {classes}.  An {object} of a
	certain class knows how to perform actions, e.g. printing
	itself or creating a new instance of itself, rather than the
	function (e.g. printing) knowing how to handle different types
	of object.

	Different classes may define methods with the same name
	(i.e. methods may be {polymorphic}).  The term "method" is used
	both for a named operation, e.g. "PRINT" and also for the code
	which a specific class provides to perform tha
	t operation.

	Most methods operate on objects that are instances of a
	certain class.  Some object-oriented languages call these
	"object methods" to distinguish then from "{class methods}".

	In {Smalltalk}, a method is defined by giving its name,
	documentation, temporary local variables and a sequence of
	expressions separated by "."s.

	(2000-03-22)

in RangeCounts is the exact same code

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

as Ranges, thus I do not overload it. BUT I want the overlap method to return
a new RangeCounts object, not a Ranges object. Now I can make this happen by
overloading overlap, and duplicating the code. OR I can create another
method that returns a new instance of that class, and make sure both Ranges
and RangesCount implement this method. That way in Ranges.overlap, it'll
call this method instead of doing "new Ranges()".

But both ways seem ugly to me.

How do people normally solve this problem? Hopefully I have explained it well
enough.

Thanks
Andrew

From: Lew

Date: Saturday, March 31, 2007

wrote in message:

Hi,
I have encoutered a design problem that I'm unsure how to solve in a neat way.
I have a class called Ranges, which represents a set of numerical ranges,
such as [0-10] [30-50], etc. And I can OR different Range instance together
to work out the overlaps.
class Ranges {
// Returns the overlapping regions of the two ranges
public Ranges overlap ( Ranges r ) {
Ranges ret = new Ranges();
//do something
return ret;
}
}
Now I have a RangeCounts class, which does a similar thing, but assigns a
value to these ranges., such that [0-10 val 1] [30-40 val 2] [40-50 val 1].
Now this works in a very similar way to Ranges, and infact extends Ranges.
class RangeCounts extend Ranges {
// Does exactly the same as the parent, thus not defined
//public Ranges overlap ( Ranges r ) {}
}
Now the problem is, the overlap method in RangeCounts is the exact same code
as Ranges, thus I do not overload it. BUT I want the overlap method to return
a new RangeCounts object, not a Ranges object. Now I can make this happen by
overloading overlap, and duplicating the code. OR I can create another
method that returns a new instance of that class, and make sure both Ranges
and RangesCount implement this method. That way in Ranges.overlap, it'll
call this method instead of doing "new Ranges()".
But both ways seem ugly to me.
How do people normally solve this problem? Hopefully I have explained it well
enough.

Why not use a Map< Ranges, Integer > instead of extending Ranges?

-- Lew

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