How sort an ArrayList of HashMaps holding several key-value pairs each?

Question

I need to call an external API with an ArrayList of HashMaps holding several predefined key-value pairs each. An example:

ArrayList<HashMap<String, String>> arrayListHashMap = new ArrayList<HashMap<String, String>>();

    {
        HashMap hashMap = new HashMap<String, String>();
        hashMap.put("key", "A key");
        hashMap.put("value", "B value");
        arrayListHashMap.add(hashMap);
    }

    {
        HashMap hashMap = new HashMap<String, String>();
        hashMap.put("key", "B key");
        hashMap.put("value", "A value");
        arrayListHashMap.add(hashMap);
    }

Now I need to sort this construct on the contents of the "value" key. This sort would result in the "key=B key/value=A value" entry as the first one in the arrayListHashMap.

Any help is highly appreciated.

HJW

Answer 1

You need to implement a Comparator<HashMap<String, String>> or more generally Comparator<Map<String, String>> which just extracts the value assocated with the value key, then use Collections.sort. Sample code (with generalization for whatever key you want to sort on):

class MapComparator implements Comparator<Map<String, String>>
{
    private final String key;

    public MapComparator(String key)
    {
        this.key = key;
    }

    public int compare(Map<String, String> first,
                       Map<String, String> second)
    {
        // TODO: Null checking, both for maps and values
        String firstValue = first.get(key);
        String secondValue = second.get(key);
        return firstValue.compareTo(secondValue);
    }
}

...
Collections.sort(arrayListHashMap, new MapComparator("value"));

Answer 2

You can use the below solution to achieve it:

arrayListHashMap.sort(Comparator.comparing(m -> m.get("value"), Comparator.nullsLast(Comparator.naturalOrder())));