How to alphabetize a list of hashmaps

Question

I have an ArrayList defined as so:

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

Each map only has one element in it company which is the name of a company. My question is how do I alphabetize the ArrayList. I am using this because later down the line (in other views) there will be more elements to the HashMap.

You can use `Collections.sort()` with a custom comparator. You can find [many examples on stackoverflow](http://stackoverflow.com/search?q=[java]%20collections%20sort%20comparator), for example: http://stackoverflow.com/questions/14154127/collections-sortlistt-comparator-super-t-method-example — assylias, Feb 22 '14 at 22:27
Please expand on this in the answers section. Im still a bit confused. — heinst, Feb 22 '14 at 22:28
Out of curiosity, why not just use a `List` or a `Set`? There is nothing unique about the word `company` such that you'd want to make it a hash key, and you could use a `Comparator` or a sorted implementation of one of those lists. — rpmartz, Feb 22 '14 at 23:00

Devavrata · Answer 1 · 2014-02-22T22:56:27.310

1

You can use Collections.sort() to sort it in lexicographically order but you have make the object comparable by using Comparator

class CmpHashMap implements Comparator<HashMap<String,String>>
{
    public int compare(HashMap<String,String> h1,HashMap<String,String> h2)//assuming second String as company name and key as "key"
    {
         return h1.get("key").compareTo(h2.get("key"));
     }
 }

then use collections

Collections.sort(recalllist,new CmpHashMap());

edited Feb 22 '14 at 22:56

answered Feb 22 '14 at 22:43

Devavrata

1,785
17
30

The comparator should be a `Comparator>` – JB Nizet Feb 22 '14 at 22:47
@Neuron Do you mean `compareTo`? – wolfrevo Feb 22 '14 at 22:53
oops making too much mistake today.. – Devavrata Feb 22 '14 at 22:57

score 1 · Accepted Answer · answered Feb 22 '14 at 22:49

Use the following:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;

public class MainClass {
  public static void main(String[] args) {
    ArrayList<HashMap<String, String>> recallList = new ArrayList<HashMap<String, String>>();
    HashMap<String, String> hashMap;
    hashMap = new HashMap<String, String>();
    hashMap.put("company", "a");
    recallList.add(hashMap);
    hashMap = new HashMap<String, String>();
    hashMap.put("company", "c");
    recallList.add(hashMap);
    hashMap = new HashMap<String, String>();
    hashMap.put("company", "b");
    recallList.add(hashMap);
    System.out.println(recallList);
    Collections.sort(recallList, new Comparator<HashMap<String, String>>() {
      @Override
      public int compare(HashMap<String, String> hashMap1, HashMap<String, String> hashMap2) {
        return hashMap1.get("company").compareTo(hashMap2.get("company"));
      }
    });
    System.out.println(recallList);
  }
}

The first and second output are:

[{company=a}, {company=c}, {company=b}]
[{company=a}, {company=b}, {company=c}]

How to alphabetize a list of hashmaps

2 Answers2