Why can we change the unmodifiable list if we have the original one?

Question

By looking at the code of Collections class, i got to know that when we are using the method unmodifiableList(List list) or unmodifiableCollection(Coll

Answer 1

(answer of the queston at the bottom)

When you create an unmodifiable list, the purpose is that it should not be modified by people other than you - i.e. clients of an API.

the method unmodifiableList(..) creates a new object of type UnmodifiableList (but this is not a public class), which gets the original list, and delegates all methods to it except the methods which would modify it.

The point is, as stated in the documentation:

Returns an unmodifiable view of the specified list. This method allows modules to provide users with "read-only" access to internal lists.

So, an example: You have a List of devices that your API has detected and can operate, and you want to give them a client of your API. But he is not supposed to change them. So you have two options:

• give him a deep copy of your List, so that even if he modifies it, this does not change your list
• give him an unmodifiable collection - he can't modify it, and you spare the creation of a new collection.

And now here comes the answer to the title of your question - the unmodifiable list is a view of the original collection. So if you need to add a new item to it - say, you have discovered a new device that was just plugged-in, the clients will be able to see it in their unmodifiable view.