为什么HashSet中的元素顺序是随机的？

Question

I am wondering how does the place of the element in a HashSet is determined. Is it through some built-in hash function?

Set <String>set=new HashSet<String>();
    set.add("January");
    set.add("February");
    set.add("July");
    set.add("August");
    set.add("September");
    set.add("October");
    set.add("Novermber");
    set.add("December");
    set.add("March");
    set.add("April");
    set.add("May");
    set.add("June");


    Iterator <String>it=set.iterator();
    while(it.hasNext()){
        System.out.println(it.next());
    }

I get this result

June
October
December
September
May
March
Novermber
July
January
February
April
August

What is the explanation for this order or the output?

Answer 1

HashSet is based on array. Item index in array is calculated based on hashCode() function.

Iterator in HashSet moves over array elements, skipping null elements. That's why your order is 'strange'. If you need to preserve insertion order, use LinkedHashSet .

Answer 2

You have to pay for ordering (in most cases in form of computational time). And the documentation states explicitly, that " [...]. 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. "

Therefore I think the order is disregarded for the sake of performance.