Collections

Collections are any

Class	Generic Name	Capability	Purpose
NSArray	Array List / Linked List	Rapid Access, Slow Mutation	A reizable list of items. NB Each node of the the list is just a reference, the list need not be of a single type.
NSSet	Unordered List	Slow Access, Rapid Mutation
NSIndexedSet	Indexed Unordered List	Fast Access, Fast Mutation
NSDictionary	Map/Hashtable	Arbitrary Association,Serializable

Mutability

For each immutable class there is a mutable equivalent, to make a collection mutable use "mutableCopy:". Alternatly when creating the object create an instace of the mutable version instead.

Serialization

Dictionaries may only serialize known classes to plist files, they are:

• NSDictionary(must cointain only serializable classes)
• NSNumber
• NSArray(must cointain only serializable classes)
• NSString

see also NSKeyedArchiver.

Weak and Strong

Garbage collection and ARC use weak and strong, it gets complicated: read Apple on Garbage Collection Apple on ARC