Re: Where do Methods belong?

Lew <>
Wed, 4 Apr 2012 18:09:33 -0700 (PDT)
Davej wrote:

I've been looking at Java for six months and I realize I still have a
fundamental uncertainty regarding methods. Where do they belong?

They belong in a type, i.e., a class or interface.

Methods in an interface, and some methods in an abstract class, will be onl=
y declared, not defined. That means that they have no method body:

 public void nonAbstract()

 abstract public void notConcrete();

In that snippet, 'nonAbstract()' is declared and defined; it has a method b=
ody. Conversely, 'notConcrete()' is only declared, not defined; it has a me=
thod signature but no method body, hence it is 'abstract'.

Obviously when you create a "business" class you create plenty of
them, but do they belong anywhere else? When I first started and I was

Any type that defines attributes or behaviors.

writing tiny console programs I would throw them into the same file
with main(). Is that bad form? I've created them in servlets. Is that
bad form? Where should methods not be created? Thanks.

You're looking at it backwards. You don't decide from the code outward wher=
e to put methods, but from your logical model inward to the code.

There are two kinds of methods in Java, which I call "attributes" (really t=
hey're called "accessors" and "mutators", or "property methods") and "behav=
iors" (methods that implement logic).

Pretty much *every* type needs methods. You should almost never declare mem=
ber variables as 'public' or 'protected'; they should only be accessed (or =
mutated) via the "attribute" methods. Some types will also have behaviors -=
 methods that actually do some work, not just receive or emit values.

The real question is when the heck would you ever write a type /without/ me=

The methods will match your object model. When you model the business domai=
n, you will invent entities that imitate the business-domain distinctions. =
Those entities types are defined by their attributes and behaviors - that's=
 all there is to an entity. And each of those attributes and behaviors is r=
epresented by methods.

So the short, nearly universal answer to your question is - always put meth=
ods in every type.


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