设置vs列表java中的字母顺序

Question

Arent Lists a Ordered Collection, and Sets arent ordered? Then Why does this program sorts the String in Alphabetical order with Sets but not Lists? I understand the duplicates parts of the two.

    PrintStream out = System.out;

    List<String> set = new ArrayList<String>();
    String s = "ILLUSIONS";

    for(int i = 0; i< s.length(); i++)
    {
        set.add((new Character(s.charAt(i))).toString());

    }
    out.println(set);

outputs: ILLUSIONS

---

    PrintStream out = System.out;

    Set<String> set = new TreeSet<String>();
    String s = "ILLUSIONS";

    for(int i = 0; i< s.length(); i++)
    {
        set.add((new Character(s.charAt(i))).toString());

    }
    out.println(set);

outputs: ILNOSU

Answer 1

Lists are "ordered" by element index. That means they retain the order of insertion of elements. Sets (in general) do not retain such an order. Some exceptions:

• The TreeSet is a particular Set , that keeps its elements in a naturally "sorted" order.
• The LinkedHashSet is a particular Set , that does retain the insertion order.

If you want to "order" your list, you'll have to do that manually:

Collections.sort(list);

In fact, by "sorting" a list, you will re-arrange all list element indexes. See the relevant Javadoc on Collections.sort()

Answer 2

When you say a List is ordered, it really just means that the lists preserve the order in which the element were inserted and the order in which they can be retrieved is predictable.

A Set is not ordered, its focus is just to have unique elements. A TreeSet is a SortedSet which along with maintaining uniqueness, also maintains elements in a sorted order. And hence the result that you see above

Answer 3

Yes, Lists are ordered which means that the order in which items are returned by an iterator is well defined (it will return items in the order in which they were inserted). If you want the items to be returned in a different order (eg alphabetic) then you need to explicitly sort the list:

java.util.Collections.sort(myList);