A lot of what is being discussed here is based on patterns from a book
called "Effective Java" by Joshua Bloch. It's basically "design
patterns for Java" and it's very good. You should pick it up. Whether
you use its ideas or not is up to you, but I think it would extend your
understanding of Java quite a bit.
The basics of the Factory pattern are:
1. A static method that is the Factory.
2. A private-ish class that is produced by the Factory method.
public class StringUtils {
private StringUtils() {}
public StringUtils getInstance() {
return new StringUtils();
}
}
That's the idea, the full pattern can be more complicated. The
reasoning behind it is that there are some things you can't do in a
constructor, so the method gives you a change to add additional code
after (or before) the constructor is invoked. It's basically an
additional layer of encapsulation on top of the constructor. You should
look at the Java API and notice that a lot of the new-ish classes use
this kind of Factory pattern. Calendar, for example, and some of the
new Collections, like EnumSet.
I don't own a copy of Effective Java but I read the article from Dr.
Dobbs Journal that someone else cited and read that. Curiously, Bloch
says that this technique does not correspond exactly with the GoF
Factory design pattern (first paragraph after the Boolean example) but
perhaps that's nitpicking. In any case, I read Bloch's discussion of
this technique and found a few different examples of the Factory
design pattern as well but for the life of me, I'm still not clear on
exactly what you are proposing for my specific case.
The methods in my StringUtils class are all static now and I have made
the two constructors private. I've created a static factory method
along the lines you describe above. In fact, I've created two of them,
one that takes a Locale as a parameter and one that uses the desfault
Locale. The compiler is not complaining about what I've done, which is
a one hurdle overcome. But I'm not clear yet on how to invoke this
static factory method, which makes me wonder if I'm fully
understanding what you're saying.
Here's an excerpt of my code, showing the constructors and the static
factory methods and a few relevant fields:
----------------------------------------------------
public class StringUtils {
/**
* An instance of a Msg Resource Bundle.
*/
private static ResourceBundle locMsg = null;
/**
* The MessageFormat being used by this class.
*/
private static MessageFormat msgFmt = new MessageFormat(""); //
$NON-NLS-1$
/**
* The locale used by this class.
*/
private static Locale locale = null;
/**
* This constructor creates an instance of the class that uses the
default locale.
*/
private StringUtils() {
locale = Locale.getDefault();
locMsg = LocalizationUtils.getResources(locale, MSG_PREFIX);
msgFmt.setLocale(locale);
}
/**
* This constructor creates an instance of the class that uses a
specified locale.
*
* @param myLocale the locale that should be used by this instance
of the class.
*/
private StringUtils(Locale myLocale) {
locale = myLocale;
locMsg = LocalizationUtils.getResources(locale, MSG_PREFIX);
msgFmt.setLocale(locale);
}
/**
* This is a static factory method.
*
*/
public StringUtils getInstance() {
return new StringUtils();
}
/**
* This is a static factory method.
*
* @param myLocale the Locale that should be used by this class.
*/
public StringUtils getInstance(Locale myLocale) {
return new StringUtils(myLocale);
}
//Other methods omitted (all of them are static).
}
-------------------------------------------------------------------
Is this what you have in mind?
If yes, how do I invoke StringUtils.getInstance()? I've tried this
(from another class) in order to get an instance that uses the default
Locale:
StringUtils stringUtils = StringUtils.getInstance();
but it generates a compile error because it doesn't like the static
reference to getInstance(). And this gives the exact same error:
StringUtils stringUtils = new StringUtils.getInstance();
Can you (or anyone else reading this thread) kindly clarify what I've
got wrong and how to correct it?
