It means that you type stuff like NSArray* array = [NSArray new]; And the implementation changes to suit what you're doing with the array rather than Col

It means that you type stuff like NSArray* array = [NSArray new]; And the implementation changes to suit what you're doing with the array rather than CollectionFactory colFactory = new CollectionFactory(); ICollection array = colFactory.getArrayOptimizedFor(100); Java: It's factories, all the way down.