What is the logic behind having a mutable and immutable versions of classes like NSArray, NSDictionary etc in Objective C?

Question

Why do common collection classes in Objective C like NSString, NSArray, NSDictionary etc have a mutable as well as an immutable version. What is the logic behind defining them s

Answer 1

Apart from the answers mentioned above one difference is : Immutable objects are generally thread safe. Whereas, a Mutable objects are not thread safe.

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