Issue with Sort native method Java

Question

I'm trying to order elements using native sort method.

Code:

List<String> list = new ArrayList();
Collections.sort(list);

Input 1:

Before order: 65 31 37 37 72 76 61 35 57 37
After order:  31 35 37 37 37 57 61 65 72 76
Expected:     Ok.

Input 2:

Before order: 45 186 185 55 51 51 22 78 64 26 49 21
After order:  185 186 21 22 26 45 49 51 51 55 64 78
Expected:     21 22 26 45 49 51 51 55 64 78 185 186

The problem is that the method is sorting wrong in some cases, how can I solve it?

Can you tell us your expected result and the actual result ? — Brian
– Brian, Commented Mar 22, 2016 at 4:09

Elliott Frisch · Accepted Answer · 2016-03-22 04:12:36Z

5

You have a List<String>, so Collections.sort is ordering the String(s) lexicographically. You could use a List<Integer> like

List<Integer> al = Arrays.asList(45, 186, 185, 55, 51, 51, 22, 78, 64, 26, 49, 21);
Collections.sort(al);
System.out.println(al);

But, if you must use String(s) then you'll need to provide a custom Comparator (because the default String ordering isn't what you want). Something like,

List<String> al = Arrays.asList("45", "186", "185", "55", "51", "51", //
        "22", "78", "64", "26", "49", "21");
Collections.sort(al, new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        return Integer.compare(Integer.parseInt(o1), Integer.parseInt(o2));
    }
});
System.out.println(al);

which outputs (like the first example)

[21, 22, 26, 45, 49, 51, 51, 55, 64, 78, 185, 186]

answered Mar 22, 2016 at 4:12

Elliott Frisch

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2 Comments

Paul Boddington Over a year ago

It's probably worth pointing out that in Java 8 you can do it particularly succinctly: list.sort(Comparator.comparingInt(Integer::parseInt));

developer033 Over a year ago

Thanks. And, yes I have to use it as String. Nice contribution @PaulBoddington. By the way, when I wrote that code of @ElliottFricsh, Netbeans shows me another option to use lambda: (String o1, String o2) -> Integer.compare(Integer.parseInt(o1), Integer.parseInt(o2)));. Just to contribute also.

Raghu K Nair · Accepted Answer · 2016-03-22 04:33:01Z

0

You should be using Integer comparator.

List<Integer> list = new ArrayList();
Collections.sort(list);

or

List<String> list = Arrays.asList(new String[]{"45", "186", "185", "55", "51", "51", "22", "78", "64", "26", "49", "21"});
Comparator<String> cmp = (String o1, String o2) -> Integer.valueOf(o1).compareTo(Integer.valueOf(o2));
Collections.sort(list,cmp);
System.out.println(list);

edited Mar 22, 2016 at 4:33

answered Mar 22, 2016 at 4:24

Raghu K Nair

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Comments

user4910279 · Accepted Answer · 2018-04-24 22:24:50Z

0

You should sort by

integer value of string

That is

Collections.sort(list,
    Comparator.comparing(Integer::parseInt));

Or sort by

length of string
string itself

That is

Collections.sort(list,
    Comparator.comparing(String::length)
              .thenComparing(Function.identity()));

answered Apr 24, 2018 at 22:24

user4910279

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