是什么决定了Lists包含非字母数字字符的Collections.sort中的排序顺序？

Question

I have code that sorts an ArrayList of elements based on one attribute called 'title' which is of type String. The code uses Collator like this:

Collator( Collator collator = Collator.getInstance(); ).

I have two objects with title "@a" and the other object has title "#a"

I pass these objects as a List and call

Collections.sort(list,comparator)

This gives the order as

"@a" "#a"

Why is "#a" appearing last even though its ASCII value is less than "@a" ?

Answer 1

Based on one of your comments, you're using a collator to sort your titles. Why you didn't say that in your question is beyond me.

Anyway, the collator sorts Strings according to locale preferences. It doesn't sort in lexicographic order. And the collator you're using considers that the right order is the one you observe. If you want lexicographical order, you should not use a collator.

Also note that a collator is always associated to a locale. The javadoc of Collator.getInstance() method says:

Gets the Collator for the current default locale.

Answer 2

No, the output is "#a","@a". Which is absolutely right.

Answer 3

Why is # appearing last even though its ASCII value is less than @ ?

My clean-room implementation:

final List<String> list = Arrays.asList("@a", "#a");
Collections.sort(list);
System.out.println(list);

Output:

[#a, @a]

This code doesn't reproduce your problem.

For reference:
'#' is 0x23
'@' is 0x40

Everything looks normal.

---

EDIT: new code following your comment "The code uses Collator but its used as Collator collator = Collator.getInstance(); not specific to any locale." :

final List<String> list = Arrays.asList("@a", "#a");
final Collator c = Collator.getInstance();

Collections.sort(list, c);
System.out.println(list);

Output:

[@a, #a]

This reproduces your problem.

If I use Collator.getInstance() to sort the ASCII table, this is the output I get:

-, _, ,, ;, :, !, ?, /, ., `, ^, ', ", (, ), [, ], {, }, @, $, *, \\, &, #, %, +, <, =, >, |, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, A, b, B, c, C, d, D, e, E, f, F, g, G, h, H, i, I, j, J, k, K, l, L, m, M, n, N, o, O, p, P, q, Q, r, R, s, S, t, T, u, U, v, V, w, W, x, X, y, Y, z, Z

You can see this is quite different from the ASCII collating order:

", #, $, %, &, ', (, ), *, +, ,, -, ., /, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, :, ;, <, =, >, ?, @, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z, [, \\, ], ^, _, `, a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, {, |, }

For OP's interest, this is the code used to create this output:

final List<String> list = new ArrayList<String>();
final Collator col = Collator.getInstance();

for (char c = '!'; c < '~'; c++)
{
  list.add(c+"");
}

Collections.sort(list, col);
System.out.println(list);

Answer 4

You normally provide a Comparator when you want the order to be different to normal.

List<String> words = new ArrayList<>();
words.add("#a");
words.add("@a");
Collections.sort(words);
System.out.println("Natural order: " + words);

Collections.sort(words, Collections.reverseOrder());
System.out.println("Reverse natural order: " + words);

prints

Natural order: [#a, @a]
Reverse natural order: [@a, #a]

So if the order is the reverse of ASCII, its because that what you defined in your Comparator to be the order (whether you are aware that you did this or not)