constructor or getInstance( )?

[hannes]

Hey,

If i want to make a new instance of MyClass, then i do

MyClass m =  new MyClass( );

But in some classes in java and real programs, to make an instance of MyClass, they do

MyClass m = MyClass.getInstance( );

When do i have to use getInstance, and when an normal constructor call?

Hannes

[gcalvin]

getInstance() is typically used in the Singleton pattern, where there is only one instance of the class, and all code should refer to that instance. For example, you might have a Lexicon class that implements a quick lookup of every word in the English language. There's no need for individual objects to get their own copy of the Lexicon, because there's nothing dynamic about it -- every instance just needs to check if a word is in there, or maybe get a random word from it. So rather than

    Lexicon lex = new Lexicon();

you would say

    Lexicon lex = Lexicon.getInstance();

and you would get a reference to the one Lexicon object that is out there. (And if by chance it hadn't been instantiated yet, it would be instantiated behind the scenes, and you'd get a reference to it.)

Hope that helps,

-Gary-

[Steve11235]

getInstance() (the static method of Class) is part of Java Reflection. The only time I see it used is when a class has been loaded by name:

Class<?> clazz = Class.forName("package.ClassName");
ClassName className = clazz.getInstance();

I used to load JDBC drivers this way, until the driver providers made that impossible.

Having said that, I suggest staying away from Reflection unless you have a good reason to use it.

[hannes]

ok, thanks guys. Steve, you told me about Reflection and i better stay away of it, but when is a good reason to use it?

[Steve11235]

Yes, but Reflection is used for advanced applications. It allows you to look into and even modify and create classes. Frameworks and tools use Reflection to work with classes in generic manner. Reflection also has a lot of overhead, it can make your code difficult to understand, it opens the door for for errors that you could not otherwise produce, and it "breaks" the OO concept.