[COLLECTIONS-881] MultiMapUtils.getXXX ensure safe copy

[clapppp]

This PR modifies getXXX method to return new Collection.

Updated getValuesAsSet, getValuesAsList, and getValuesAsBag to always return a new copy.
Added unit tests to verify that returned collections are independent of the backing map.
Updated Javadocs to reflect that a new collection is returned.

[garydgregory]

@clapppp

Thank you for your PR.

Please see my comments.

[clapppp]

@garydgregory

Thank you for the feedback!

I reviewed my test cases and found the issues.
I updated the test for getValuesAsSet accordingly.

However, I couldn't find any MultiValuedMapimplementation whose get(key)returns a Bag(Only Collections/List/Set).
So I'm not sure how to write a failing test case.
If it's okay to make test case with Mockito I'll make it!

[garydgregory]

Hi @clapppp
WRT bag you have 2 options:

• add a concrete implementation in the test sources
• mocking as you mentioned

[clapppp]

Hi @garydgregory

I made the changes as you suggested.
Could you please review the latest changes?