new entry in the hashset

Is there any way to know what was the last new entries that were added to a hashset ? In my program the first cycle adds [Emmy, Carl] and than on my second cycle it adds [Emmy, Dan, Carl] is there anyway I can just use dan and not the rest of them fo开发者_如何学编程r cycle three ?

java.util.HashSet does not preserve order, but java.util.LinkedHashSet does. Can you use that instead? From the Javadoc:

This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set.

HashSets do not carry information about the order in which you add elements. You need to replace it with a Collection that does (e.g. ArrayList).

Hashset are backed by hash tables and there is no guarantee on the order of retrieval. The order of retrieval will not be the same as the order of insertion. So, no it's not possible to know which item was added last.

Workarounds - may be use two hashsets, compare old with new and get the new entries or have some sort of indicators to distinguish the perticular iteration it was added or use ArrayList or anything that fits in your design.

HashSet<String> oldpeople = new HashSet<String>();P
HashSet<String> newPeople;
for (Set<String> cycle : input)
{ 
    newPeople = new HashSet<String>();
    newPeople.addAll(cycle);
    newPeople.removeAll(oldPeople);
    oldPeople.addAll(cycle);
}

now you have the last new one always contained in newPeople.

Well if I understand your post and the comments correctly (well that's quite hard, try to be bit more precise :) ) what you actually want is: a) not add any items several times to the HashSet and b) see if the set already contains the given item when trying to add it.

a) is trivially true for every set and for b) you can just use the return value of add: If it returns false is already contained in the set.