java - Why some Collections extends Abstract also Implements interface? -


this question has answer here:

i curious why collection classes has extended abstract class , @ same time implementing interfaces there corresponding collection interface list,set etc.?. why not extending abstract class. can understand implementing other interface adding functionality cloneble, serializable, etc. extending abstract class , implementing interface abstract class has implemented . think there design principle missing. please can elaborate design reason?

the collection interfaces exist people can consume collections without knowing anything type of collection – if doesn't extend of abstract classes.
example, of concurrent collections not extend abstract base classes.

the abstract classes exist provide common base functionality implementations.


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