Question

I have a hashmap, I wish to get an array of the keys in this hashmap, but I wish for the array to be sorted.

For instance if the map looks like this:

<"2",obj1>
<"4",obj2>
<"6",obj3>
<"10",obj4>
<"5",obj5>
<"1",obj6>
<"15",obj7>
<"3",obj8>

I want the array to be: [“1”, “2”, “3”, “4”, “5”, “6”, “10”, “15”]

The faster the better. Is there a built in way to do this?

Update 1

When i am using TreeMap, keys are sorted in following order:

“1”, “10”, “2” and so on.

but i want them to be like: “1”,”2″,”3″.. “10”,”15″.

Answer 1

I have some keys like “10A”, “2AB”

In that case you need a custom comparator like

public static void main(String... args) {
    NavigableMap<String, Object> map = new TreeMap<String, Object>(new Comparator<String>() {
        final Pattern pattern = Pattern.compile("(\\d+)(.*)");

        @Override
        public int compare(String o1, String o2) {
            Matcher matcher1 = pattern.matcher(o1);
            Matcher matcher2 = pattern.matcher(o2);
            matcher1.find();
            matcher2.find();
            int cmp = Long.compare(Long.parseLong(matcher1.group(1)), Long.parseLong(matcher2.group(1)));
            if (cmp != 0)
                return cmp;
            return matcher1.group(2).compareTo(matcher2.group(2));
        }
    });

    map.put("1", "one");
    map.put("1A", "oneA");
    map.put("10", "ten");
    map.put("10AB", "tenAB");
    map.put("15", "fifteen");
    map.put("2", "two");
    map.put("2AB", "twoAB");
    map.put("2", "three");

    System.out.println(map.keySet());
}

prints

[1, 1A, 2, 2AB, 10, 10AB, 15]