Objective-C Fundamentals

Subclassing in your demo application
141
Listing 6.13
CTPeriodicLease.h
#import "CTLease.h"
@interface CTPeriodicLease : CTLease {
float weeklyRental;
}
@property(nonatomic) float weeklyRental;
+ (CTLease *)periodicLeaseWithWeeklyPrice:(float)weeklyPrice;
@end
Here a CTPeriodicLease is declared as a subclass of CTLease. A CTPeriodicLease
contains a float number representing the weeklyrentalCost and provides a class
method to create a CTLease object using the weekly price. Now all that’s left to do is
implement the method in your main file. Use the code shown in the following listing.
Listing 6.14
CTPeriodicLease.m
#import "CTPeriodicLease.h"
@implementation CTPeriodicLease
@synthesize weeklyRental;
+ (CTLease *)periodicLeaseWithWeeklyPrice:(float)weeklyPrice {
CTPeriodicLease *lease = [CTPeriodicLease alloc];
}
lease.weeklyRental = weeklyPrice;
return [lease autorelease];
- (NSString *)description {
return [NSString stringWithFormat:@"$%0.2f per week",
self.weeklyRental];
}
@end
The creation of an object occurs B where you allocate a CTPeriodicLease object. You
set the lease’s weekly rental price and return the autoreleased version of the object.
One point of interest here is that this method is designated as returning a CTLease
object, yet a CTPeriodicLease object is allocated and returned. The reason this is
valid is that CTPeriodicLease is a subclass of CTLease, so CTPeriodicLease is a
CTLease but a CTLease isn’t necessarily a CTPeriodicLease. Next, the NSObject method
description is overridden. You can do this because CTPeriodicLease is also a subclass
of NSObject. As you learned earlier, this technique is useful when you want
objects to print their attributes out of an NSLog.
6.7.3 Creating CTFixedLease as a subclass of CTLease
Alloc
CTPeriodicLease b
You now make another CTLease subclass called CTFixedLease so you can represent a
lease as having a different set of terms rather than some periodic set amount. This