使用相同的顺序java复制arraylist的元素

Question

I've sorted an arraylist of int in ascending order, but when I copy it in a set, the elements are not sorted anymore. I'm using this :

HashSet<Integer> set = new HashSet<Integer>(sortedArray);  

why is like that?

Answer 1

LinkedHashSet will keep the order. TreeSet will sort based either on an external Comparator or natural ordering through Comparable .

A general point of a Set is that order is irrelevant. Hashing is intended to put the elements in as random an order as possible. LinkedHashSet maintains a linked-list between references to the elements, so can maintain an order.

BitSet (which is not a Set ) may, or may not, provide a more efficient data structure.

Answer 2

HashSet's don't sort or maintain order, and the API will tell you this:

it does not guarantee that the order will remain constant over time.

Consider using another type of Set such as a TreeSet.

Answer 3

If you just care about uniqueness, use the HashSet. If you're after sorting, then consider the TreeSet.

Answer 4

you need to use TreeSet and implement a Comparator object or Comparable interface for your data. you can read about Object ordering here hash set is designed for quick access to unique data, not for maintaining a particular order.