CODEWITHSUNDEEP
javacollectionsiterationhashset
Thor
Thor

Reputation: 10028

iterating through instance of HashSet class, expect random order but the results are always the same order

I'm trying to learn the concept of Set interface and HashSet class in java. I have read

when using HashSet class, that there is no guarantee of the sequence of elements when I iterate over.

I have came up with some code and run ten times and it always iterate in the order of insertion. So is this due to luck?

Set<Integer> set1 = new HashSet<>();
set1.add(1);
set1.add(2);
set1.add(3);
set1.add(2);

Iterator i1 = set1.iterator();
while (i1.hasNext()){
    System.out.println(i1.next());
}

Upvotes: 2

Views: 414

Answers (2)

marcolopes
marcolopes

Reputation: 9287

YES. It's due to "luck". It depends on the JRE implementation! I learned this the hard way many years ago when started implementing in Java, and the code was tested on different platforms.

If i recall, in windows a HashSet iteration was perfectly ordered, but in Mac OSX totally scrambled!

Bottom line: Always rely on the docs, and not on the apparent results!

Upvotes: 2

FallAndLearn
FallAndLearn

Reputation: 4135

As per the javadoc:

This class implements the Set interface, backed by a hash table (actually a HashMap instance). 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. [...] The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created

From Java4 we have LinkedHashSet which guarantees that the iteration order and the insertion order is same.

Upvotes: 2

Related Questions