Objective-C newbie: Does anyone know of diagrams that explain class, objects and methods?

Question

Hi,

As you may have guessed from the question - I am right at the beginning of the Obj-C journey.

I'm hoping that someone out there knows of some diagrams that depict the relationship between classes, objects and methods - and that they're willing to share.

The problem I'm having is that just looking at code in a textbook doesn't completely explain it - for me at least.

Thanks for reading!

Regards, Spencer.

Answer 1

Classes are just like classes in any language. They are descriptions.

Objects are like nouns. They are an instance of a class. That is, if you had a description of a generic book (the class) and you made a thesaurus based on that description, the thesaurus would be the object.

Methods are more or less functions. If the objects are nouns, then the messages are verbs.

[ScienceBook getTableOfContents]; //this would like return a table of contents.

Here, the object ScienceBook is being sent a getTableOfContents message (method). So now, the science book would theoretically find, format and return the table of contents to whom ever sent the message.

Answer 2

To some extent, diagrams may not be that helpful to answer the questions you present.

It may help to think of things like this:

A "class" provides the prototype or definition for some thing. For example, a "Person" or a "Car". A common synonym for "class" is "type".

An "object" is a concrete example or instance of a class. For example, you are an instance of "Person", and your car is an instance of "Car".

A "method" is a behavior, action or property of a class. However, a method is normally only meaningful in the context of an object. "Person" -> "Eat" is not meaningful, but "you" -> "Eat" is.

These are fundamental Object-Oriented concepts that are not specific to Objective-C. If you are interested in a general overview that is language-agnostic, I recommend "Object Thinking" by David West. Even though it's from Microsoft Press, it covers the concepts rather than any specific language.

Answer 3

No diagrams, but this is the tutorial I wish I'd read before I started: http://www.cocoadevcentral.com/d/learn_objectivec/

Simple English, all the basic concepts.

Answer 4

I come from a fairly strong C++ background, but I can definitely remember when I started, I had a hard time grasping at the concept until I found a way to associate it with physical objects.

The word class and object you can use almost interchangeably. Think of an object as a container, like a bucket. The word bucket would be your "class". It is the name you give to the type of object you have.

A bucket has a certain purpose...to carry something. It might be water...or perhaps sand. So perhaps you want to fill the bucket. This would be something you do to the bucket, so in objective-c, this would be your method. You might write something like:

- (void) fillWith:(elementType)something;

So in this case, "something" might be something that represents and object you wish to fill your bucket with.

Your class might look like the following:

typedef enum items {
   CRAYONS,
   MARKERS,
   SAND,
   WATER } elementType;


@class Bucket {
   elementType item;
}
- (void) fillWith:(elementType)something;

@end

Here's one link to some objective-c samples. Also try the apple development center.

Answer 5

If you're after information on Object Orientated Programming (ie the meaning of classes, objects, methods etc) then I'd advise against Objective-C. Objective-C on the Mac relies heavily on the Cocoa framework. The Cocoa framework is vast and performs a lot of 'magic' which will make it harder to understand the fundamentals of OOP.

An easier place to start would be a language used for web development. It's easier to get to the nuts and bolts of OOP with these languages.