Question

I have an ArrayList<HashMap<String,String>>.

Is there a way to change the order of the array of HashMaps? For example, if the String “firstName” is inside the HashMap, can I sort my ArrayList of HashMaps by that value? Just to be clear, I do not need to order keys and objects inside HashMaps. I need order HashMaps inside Arraylist.

What is a best way?

i tried to do from examples:

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

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

    public int compare(HashMap<String, String> first,
            HashMap<String, String> second)
    {
        // TODO: Null checking, both for maps and values
         String firstName1 = first.get(key);
            String firstName2 = second.get(key);
            if(firstName1 == null)
                if(firstName2 == null)
                    return 0;
                else
                    return -1; // treat null as less than any non-null
            else
                if(firstName2 == null)
                    return 1; // treat null as less than any non-null
                else
                    return firstName1.compareTo(firstName2);

    }
}

and using:

                Log.e(THIS_FILE, "before ->" + addedRows);
                Collections.sort(addedRows, new MapComparator("name"));
                Log.e(THIS_FILE, "after ->" + addedRows);

and nothing sorted (to be sure i renamed one contact to start by letter a, hashmap with name=as ice:) must be first):

> E/ContactsActivity(23571): before ->[{type=, contactID=4, name=Office
> O}, {type=, contactID=2912, name=Test Text Last}, {type=,
> contactID=2915, name=as ife:) Eng}, {type=, contactID=2914,
> name=life:) Rus}, {type=, contactID=2913, name=life:) Ukr}, {type=,
> contactID=2897, name=дима куплеватскиц}] E/ContactsActivity(23571):
> after ->[{type=, contactID=4, name=Office O}, {type=, contactID=2912,
> name=Test Text Last}, {type=, contactID=2915, name=as ife:) Eng},
> {type=, contactID=2914, name=life:) Rus}, {type=, contactID=2913,
> name=life:) Ukr}, {type=, contactID=2897, name=дима куплеватскиц}]

Answer 1

It sounds like what you want is:

java.util.Collections.sort
(
    list,
    new java.util.Comparator<Map<String,String>>()
    {
        public int compare(Map<String,String> map1, Map<String,String> map2)
        {
            String firstName1 = map1.get("firstName");
            String firstName2 = map2.get("firstName");
            if(firstName1 == null)
                if(firstName2 == null)
                    return 0;
                else
                    return -1; // treat null as less than any non-null
            else
                if(firstName2 == null)
                    return 1; // treat null as less than any non-null
                else
                    return firstName1.compareTo(firstName2);
        }
    }
);

(where list is the name of your variable of type ArrayList<HashMap<String,String>>).