Encapsulation causing crash

Question

I am trying to incorporate encapsulation into my app (for an explanation of what this code is supposed to do, see here)... this is the code in Class 'A':

.h file

@interface ExportBookData : NSObject {

@public NSArray *booksArray;
@public NSMutableDictionary *builtFileList;
@public NSMutableArray *exportData;
}

- (id)initWithCategory: (NSString*) booksellerID;

@end

This is the code for the .m file:

.m file

@implementation ExportBookData

-(id)initWithCategory: (NSString*) booksellerID  {

return (id)booksellerID;
}

@end

This is the beginning of the method in class 'B' (.m file) that uses the encapsulated data:

ExportBookData *abe = [[ExportBookData alloc] initWithCategory:@"ABE"];
abe->builtFileList = [NSMutableDictionary dictionary]; <- crash on this line
abe->exportData = [NSMutableArray arrayWithCapacity:abe->booksArray.count];

if(cbvABE.checked)  {

I'm getting the following error on the 2nd line of code as indicated:

Since I'm a noob using encapsulation, I don't see what I've done wrong. I have followed several examples which are similar to my code; what am I doing wrong to cause this crash?

Answer 1

So many issues here.

First off, do not declare public instance variables. Use properties and only for values that you want other classes to have access to.

@interface ExportBookData : NSObject

@property(nonatomic, strong) NSArray *booksArray;
@property(nonatomic, strong) NSMutableDictionary *builtFileList;
@property(nonatomic, strong) NSMutableArray *exportData;

- (id)initWithCategory: (NSString*) booksellerID;

@end

Now your ExportBooksData init method.

It needs to be:

-(id)initWithCategory: (NSString*) booksellerID  {
    self = [super init];
    if (self) {
        // do something with booksellerID
    }

    return self;
}

Every init method of a base class should following this general pattern.

And now your other code is needlessly using -> operator. Use the actual properties provided by the interface instead:

ExportBookData *abe = [[ExportBookData alloc] initWithCategory:@"ABE"];
abe.builtFileList = [NSMutableDictionary dictionary];
abe.exportData = [NSMutableArray arrayWithCapacity:booksArray.count];

But it makes no sense that external code is doing all of this. Let your class set itself up as needed. So now your init method should be:

-(id)initWithCategory: (NSString*) booksellerID  {
    self = [super init];
    if (self) {
        self.builtFileList = [NSMutableDictionary dictionary];
        self.exportData = [NSMutableArray arrayWithCapacity:booksArray.count];

        // do something with booksellerID
    }

    return self;
}

Now your other code simply becomes:

ExportBookData *abe = [[ExportBookData alloc] initWithCategory:@"ABE"];

without the need to set the other properties.

There's a lot more you should be doing here (like making use of the booksellerID and booksArray) but this will get you started.