Question

Let’s say I have two Classes like so:

Car
{
   NSInteger wheels;
   NSInteger bumpers;
}

+ (Car *)carWithData:(NSDictionary *)carData;


Lexus : Car
{
   GPS *navigation;
}

+ (Lexus *)carWithData:(NSDictionary *)carData;

carWithData: is a simple helper method that creates an instance of Car populated with variables from carData. Lexus’ version would also set the navigation data.

How would Lexus’ carWithData look like without duplicating code from Car?

Answer 1

This is accomplished by calling super‘s implementation of init… in the init method:

//Car.m:
- (id)initWithData:(NSDictionary *)carData {
    self = [super init];
    if (self) {
        //setup generic car properties:
        self.wheels = [carData objectForKey:@"wheels"]; //example
        self.bumpers = [carData objectForKey:@"bumpers"]; //example
    }
    return self;
}

+ (id)carWithData:(NSDictionary *)carData {
    return [[[self alloc] initWithData:carData] autorelease];
}


//Lexus.m:
- (id)initWithData:(NSDictionary *)carData {
    //this call to super is where the car's generic properties get initialized:
    self = [super initWithWithData:carData];
    if (self) {
        //setup lexus car properties:
        self.navigation = [carData objectForKey:@"navigation"]; //example
    }
    return self;
}

//there is no need to override super's [carWithData:] method as it's only a wrapper anyway.

Also note that both the initWith… and carWith… methods return id, not Car or Lexus.
The way your code is set up you end up with casting problems, where [Lexus carWithData:dataDict] does return an object of class Lexus, but the compiler doesn’t know about it, as it expects a Car.