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New Message on BDOTNET
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From: LovedJohnySmith
Message 4 in Discussion
THE FACTORY PATTERN
One
type of pattern that we see again and again in OO programs is
the Factory pattern or class. A Factory
pattern is one that returns an instance
of one of several possible classes depending
on the data provided to it.
Usually all of the classes it returns
have a common parent class and common
methods, but each of them performs a
task differently and is optimized for
different kinds of data.
How a Factory Works
To
understand a Factory pattern, let�s look at the Factory diagram
below.
Factory
In this figure, x is a base class and
classes xy and xz are derived from
it. The Factory is a class that decides
which of these subclasses to return
depending on the arguments you give
it. On the right, we define a getClass
method to be one that passes in some
value abc, and that returns some
instance of the class x. Which one it
returns doesn't matter to the programmer
since they all have the same methods,
but different implementations. How it
decides which one to return is entirely
up to the factory. It could be some very
complex function but it is often quite
simple.
Sample Code
Let's consider a simple case where we
could use a Factory class.
Suppose we have an entry form and we
want to allow the user to enter his
name either as �firstname lastname�
or as �lastname, firstname�. We�ll make
the further simplifying assumption that
we will always be able to decide the
name order by whether there is a comma
between the last and first name.
This is a pretty simple sort of decision
to make, and you could make
it with a simple if statement in a single
class, but let�s use it here to illustrate
how a factory works and what it can
produce. We�ll start by defining a simple
base class that takes a String and splits
it (somehow) into two names:
class Namer {
//a simple class to take a string apart
into two names
protected String last; //store last
name here
protected String first; //store first
name here
public String getFirst() {
return first; //return first name
}
public String getLast() {
return last; //return last name
}
}
In this base class we don�t actually
do anything, but we do provide
implementations of the getFirst and
getLast methods. We�ll store the split
first and last names in the Strings
first and last, and, since the derived classes
will need access to these variables,
we�ll make them protected.
The Two Derived Classes
Now we can write two very simple derived
classes that split the name
into two parts in the constructor. In
the FirstFirst class, we assume that
everything before the last space is
part of the first name:
class FirstFirst extends Namer { //split
first last
public FirstFirst(String s) {
int i = s.lastIndexOf(" ");
//find sep space
if (i > 0) {
//left is first name
first = s.substring(0, i).trim();
//right is last name
last =s.substring(i+1).trim();
}
else {
first = ��; // put all in last name
last = s; // if no space
}
}
And, in the LastFirst class, we assume
that a comma delimits the last
name. In both classes, we also provide
error recovery in case the space or
comma does not exist.
class LastFirst extends Namer { //split
last, first
public LastFirst(String s) {
int i = s.indexOf(","); //find
comma
if (i > 0) {
//left is last name
last = s.substring(0, i).trim();
//right is first name
first = s.substring(i + 1).trim();
}
else {
last = s; // put all in last name
first = ""; // if no comma
}
}
}
Building the Factory
Now our Factory class is extremely simple.
We just test for the
existence of a comma and then return
an instance of one class or the other:
class NameFactory {
//returns an instance of LastFirst or
FirstFirst
//depending on whether a comma is found
public Namer getNamer(String entry)
{
int i = entry.indexOf(",");
//comma determines name
order
if (i>0)
return new LastFirst(entry); //return
one class
else
return new FirstFirst(entry); //or the
other
}
}
I hope this will quite clear and simpler,
thanx and regards,
smith
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