Question

Here is a simple sorting program of an ArrayList:

ArrayList<String> list = new ArrayList<String>();

list.add("1_Update");
list.add("11_Add");
list.add("12_Delete");
list.add("2_Create");

Collections.sort(list);
for (String str : list) {
  System.out.println(str.toString());
}

I was expecting the output of this program as:

1_Update
2_Create
11_Add
12_Delete

But when I run this program I am getting output as:

11_Add
12_Delete
1_Update
2_Create

Why is this and how do I get the ArrayList to sort as shown in the expected output?

Answer 1

You could write a custom comparator:

Collections.sort(list, new Comparator<String>() {
    public int compare(String a, String b) {
        return Integer.signum(fixString(a) - fixString(b));
    }
    private int fixString(String in) {
        return Integer.parseInt(in.substring(0, in.indexOf('_')));
    }
});