HashMap.put method will return
oldValue: if key is already present, then it will overwrite the value and returns old value
null: if key is not present, then it will add this key,value pair and then returns null.
In case of HashSet, internally it has a hashmap. When we call add method it will call 'put' method of the hashmap and if put method returns null means new entry is added into hashmap and hashset
more at: http://javahungry.blogspot.com/2013/08/how-sets-are-implemented-internally-in.html
oldValue: if key is already present, then it will overwrite the value and returns old value
null: if key is not present, then it will add this key,value pair and then returns null.
In case of HashSet, internally it has a hashmap. When we call add method it will call 'put' method of the hashmap and if put method returns null means new entry is added into hashmap and hashset
more at: http://javahungry.blogspot.com/2013/08/how-sets-are-implemented-internally-in.html
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