I'm originally a Java programmer who now works with Objective-C. I'd like to create an abstract class, but that doesn't appear to be possible in Objective-C. Is this possible?
If not, how close to an abstract class can I get in Objective-C?
Typically, Objective-C class are abstract by convention only—if the author documents a class as abstract, just don't use it without subclassing it. There is no compile-time enforcement that prevents instantiation of an abstract class, however. In fact, there is nothing to stop a user from providing implementations of abstract methods via a category (i.e. at runtime). You can force a user to at least override certain methods by raising an exception in those methods implementation in your abstract class:
[NSException raise:NSInternalInconsistencyException format:@"You must override %@ in a subclass", NSStringFromSelector(_cmd)];
If your method returns a value, it's a bit easier to use
@throw [NSException exceptionWithName:NSInternalInconsistencyException reason:[NSString stringWithFormat:@"You must override %@ in a subclass", NSStringFromSelector(_cmd)] userInfo:nil];
as then you don't need to add a return statement from the method.
If the abstract class is really an interface (i.e. has no concrete method implementations), using an Objective-C protocol is the more appropriate option.