As you are aware, Collection interfaces are 'interfaces'. You need concrete implementations of these interfaces to use. Standard JDK also provides some classes that implement these interfaces. This is what the question is trying to ask.
Different implementations solve different purposes. For eg. HashMap and TreeMap both implement Map interface but their internal data structure is different which affects their performance in different cases. HashMap is faster to store and retireve but TreeMap is faster for ordering the elements.
It's a good idea, IMHO, to read the description of these classes.
-Paul.
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