subclassing

I am trying to subclass str object, and add couple of methods to it. My main purpose is to learn how to do it. Where I am stuck is, am I supposed to subclass string in a metaclass, and create my class with that meta, or subclass str directly? And also, I guess I need to implement __new__() somehow, because, my custom methods will modify my string object, and will return new mystr obj. My class's methods, should be completely chainable with str methods, and should always return a new my class instance when custom methods modified it. I want to be able to do something like this: a = mystr(

This list shows what methods you need to implement for your class to be "regarded" as Sequence: __getitem__ , __len__ , __contains__ , __iter__ , __reversed__ , index , and count . So why does this minimal implementation does not work, i.e. why issubclass(S, Sequence) is False ? from collections import * class S(object): def __getitem__(self, item): raise IndexError def __len__(self): return 0 def __contains__(self, item): return False def __iter__(self): return iter(()) def __reversed__(self): return self def index(self, item): raise IndexError def count(self, item): return 0 issubclass(S,

I'm currently building a tabbed iPhone application where each tab's view controller is an instance of UINavigationController , and where every subcontroller of every one of the UINavigationController instances is an instance of UITableViewController . Ideally, I'd like to subclass UINavigationController so that the controller for each tab is a subclass of UINavigationController that (in addition to having all the standard UINavigationController functionality, obviously) serves as the datasource and the delegate for each of the table views associated with its subcontrollers. Trying to do this

I'm using a framework which defines and uses 'ClassA', a subclass of NSObject. I would like to add some variables and functionality so naturally I created 'ClassB', a subclass of 'ClassA' Now my problem is this. Many of the methods within this framework return instances of 'ClassA' which I would like to cast to my subclass. For example take this method: - (ClassA *)doSomethingCool:(int)howCool Now in my code I try this: ClassB * objB; objB = (ClassB *)doSomethingCool(10); NSLog(@"objB className = %@", [objB className]); This runs just fine. No compile or runtime errors or anything. But what is

Is it possible to narrow the allowed type of an ivar in a subclass. Something like this: @interface person: NSObject { NSArray *friendArray; } @interface mutablePerson: person { NSMutableArray *friendArray; } I just tried that exact code, and Xcode gave me a compile error. I'm wondering if there is a way around it. The project I am working on is going to have a lot of this sort of situation. I understand that I can use casts to make the code work. But I will be making an awful lot of casts if I do that, and I'm wondering if there is a better way. No, you can't redeclare ivars at all. However,

public class MyGeneric<T, E> {} public class Extend1<T, E> extends MyGeneric<T, E> {} public class Extend2 extends MyGeneric<String, Object> {} As far as I am aware, both of the subclasses in the above example are valid. I was wondering how Java knows when the types given in the superclass are going to be defined when the subclass is instantiated, and when they are actual class names (i.e. How does it know T, E are not class names)? A side note, is it permissible (even if uncommon) to use more than one letter for the generic types? What if (through some sever error of planning) The types