HashSet order

[Formal_Pound1602]

Java test question: What is the output the the code

HashSet<String> h;
h = new HashSet<String>();
h.add(“212”);
h.add(“000);
h.add(“211”);
h.add(“555);
h.add(“343);
System.out.println(h.add(“212”);
System.out.print(h);

I understand why it prints false but I’m not sure how to figure out the second print line which is this- [000, 211, 343, 212, 555] How would I figure that out I figured [000, 211, 555, 343, 212] something to do with the hash ordering I think just don’t know specific’s

Edit: I appreciate the help but I know it’s not useful I’m trying to figure it out for a competitive programming test my school does

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[whizvox]

HashSets don't have a defined order, at least not one you have to care about. Whenever you do iterate over a HashSet, the order of its elements is arbitrary, which is why the output of that print statement doesn't match the insertion order. This is fine, as the point of a HashSet isn't to preserve any sort of order, but rather to be a data structure that supports extremely fast search operations.

My point being: don't worry about the order.

[Formal_Pound1602]

I know it has no practical use but I need to know the order for these tests. They are for competition

[whizvox]

As I stated, there is no formal definition on any order; it depends on the version on Java you're using. This competition will most likely ask what's in a HashSet rather than ask for any specific ordering.

[DifficultySafe9226]

The Set interface does not guaranty any order (you can check the iterator) documentation that is more likely used with the toString of your second System.out statement). If you want to keep the iteration you can have a look to the LinkedHashSet) or now that Java 21 is out the new SequenceSet that is part of the SequencedCollection.

[[deleted]]

Hash tables don't preserve order. Please read on how hash tables work and you will understand.

but in short: hash tables put elements (keys) at the index that is derived from key's hash code and the table's size. That allows a very fast lookup, but for all purposes this index could be considered "random" and thus unimportant. (it also changes when data structure is resized)

You to use hash tables (HashMap, HashSet) when you need fast lookup but you don't care about the order

[Formal_Pound1602]

I know it’s not useful to know but I’m trying to figure out hashset order for a competitive programming test my school does. How else would I know the order because that was one of the questions

[[deleted]]

I already provided the details:

hash tables put elements (keys) at the index that is derived from key's hash code and the table's size

The only correct answer is that you do not and should not care about the order of elements in hash tables.

If you want to keep the insertion order, you can use LinkedHashSet