In Java 5, If i have a Set, and i add two objects to the Set. When I retrieve the objects, will it give me back in the same order as I added? I am not able to find the answer to this. Does it depend on which Set implementation I use?
6 Answers
Yes, it depends on which implementation you use. For example, LinkedHashSet will preserve insertion order:
Hash table and linked list implementation of the Set interface, with predictable iteration order. 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. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)
... but HashSet won't:
It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.
Comments
Straight from the documentation:
The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).
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Depends on Set implementation. LinkedHashSet does exactly that.
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Yes it does depend on the implementation you choose. HashSet will not guarantee order but LinedHashSet will.
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JavaDocs is your best friend. It's implementation specific. For example:
java.util.Set:
Iterator<E> iterator();
Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).
and
java.util.TreeSet:
public Iterator<E> iterator();
Returns an iterator over the elements in this set in ascending order.
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The Set interface itself is for unordered container implementations. But there might be implementations of Set that do return the elements in a particular order.
Also see the documentation of Set.iterator:
Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee)
Set's documentation. See docs.oracle.com/javase/6/docs/api/java/util/Set.html