在NSArray中有个方法
arrayWithArray:
Creates and returns an array containing the objects in another given array.
+ (id)arrayWithArray:(NSArray *)anArray
Parameters
anArray
An array.
Return Value
An array containing the objects in anArray.
你可以重新定义一个NSMutableArray *array = [NSMutableArray arrayWithArray:NSArray].
[NSMutablearray copy]就得到了NSArray.