Question

I am a Java programmer, learning Objective-C and I have a problem with implementation of variables, similar to static final class variables in Java. In class PolygonShape, I would like to have NSDictionary with polygon types, which can be accessed from within and outside of the class. I already tried the following:

PolygonShape.h:

...
extern NSDictionary *polygonTypes;

@interface PolygonShape
...

PolygonShape.m:

...    
NSDictionary *polygonTypes = nil;

@implementation PolygonShape

- (id)init {
    self = [super init];
    if (self) {
        if(!polygonTypes) {
            polygonTypes = [NSDictionary dictionaryWithObjectsAndKeys:
                            @"triangle", [NSNumber numberWithInt: 3], @"quadrilateral", [NSNumber numberWithInt: 4],
                            @"pentagon", [NSNumber numberWithInt: 5], @"hexagon", [NSNumber numberWithInt: 6],
                            @"heptagon", [NSNumber numberWithInt: 7], @"octagon", [NSNumber numberWithInt: 8],  
                            @"enneagon", [NSNumber numberWithInt: 9], @"decagon", [NSNumber numberWithInt: 10],
                            @"hendecagon", [NSNumber numberWithInt: 11], @"dodecagon", [NSNumber numberWithInt: 12], nil];
        }
    }
...

But this is not good enough, because if I want to access polygon types from elsewhere (e.g. main.m) without initializing instance of PolygonShape, variable polygonTypes is nil. So I used static function which works fine:

PolygonShape.m:

static NSDictionary *polygonTypes = nil;

@implementation PolygonShape

...

+ (NSDictionary *) polygonTypesDicionary {
    if(!polygonTypes) {
        polygonTypes = [NSDictionary dictionaryWithObjectsAndKeys:
                        @"triangle", [NSNumber numberWithInt: 3], @"quadrilateral", [NSNumber numberWithInt: 4],
                        @"pentagon", [NSNumber numberWithInt: 5], @"hexagon", [NSNumber numberWithInt: 6],
                        @"heptagon", [NSNumber numberWithInt: 7], @"octagon", [NSNumber numberWithInt: 8],  
                        @"enneagon", [NSNumber numberWithInt: 9], @"decagon", [NSNumber numberWithInt: 10],
                        @"hendecagon", [NSNumber numberWithInt: 11], @"dodecagon", [NSNumber numberWithInt: 12], nil];
    }
    return polygonTypes;
}

Now this is ok, but I wonder, what is the best way to do this and is it possible to use extern for NSDictionary without having to initialize it in a class method? (and I know about singelton classes but I would really like to have constant array of polygon types inside PolygonShape class).

Answer 1

I am a Java programmer, […]

There’s your problem right there.

Objective-C is a quite different language from Java in a lot of ways, and a lot of idioms may be quite foreign to you.

As an example; why do you want, or need, to know what kind of polygon shapes the Polygon class can handle?

If you are trying to create a polygon with a certain number of corners, but can’t, then yes, you need to know.

Apart from that? Not very useful info, at least as far as I can see.

In fact, why would a polygon shape even need to know what other shapes exist?

Shed for a moment the very idea of static, and reconsider your problem. Enlightenment is sure to follow.