In Java: Get sorted array of keys of a map

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".

It is worth following up answers which are not acceptable because it may be that the question is unclear, could not have been reasonably answered or missing important details. Sometimes answers are not accepted because the questioner didn't like the answers given, e.g. "you can't do that" type answers. — Peter Lawrey
– Peter Lawrey, Commented Nov 8, 2012 at 13:32

Peter Lawrey · Accepted Answer · 2012-11-08 12:16:48Z

5

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]

edited Nov 8, 2012 at 12:16

answered Nov 8, 2012 at 11:31

Peter Lawrey

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2 Comments

Peter Lawrey Over a year ago

An important detail to include. ;) In that case you need a custom Comparator. I assume you want "2AB" < "10A" ??

Peter Lawrey Over a year ago

@mudit See above for an example

Alex Lynch · Accepted Answer · 2012-11-08 11:38:26Z

2

if converting your code to a naturally sorted Map is not an option for some reason, there are built in tools to return the key set and sort it manually -

    Object[] keys = yourMap.keySet().toArray();
    Arrays.sort(keys);

    System.out.println(Arrays.toString(keys));

answered Nov 8, 2012 at 11:38

Alex Lynch

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