Java: Sort ArrayList with nested HashMap by value

Question

I have a ArrayList<HashMap<String, String>>, which I want to sort by the value of the HashMap. What would be the quickest way to do this in Java (with the least amount of code)?

Which value of the HashMap? It may have many values, in an undefined iteration order. — Louis Wasserman
– Louis Wasserman, Commented Nov 26, 2015 at 19:25
@LouisWasserman Thanks for your quick response. My ArrayList looks like this: [{Name=Pete, Score=1}, {Name=Joe, Score=5}, {Name=Henry, Score=4}] and I want to sort by score. — TheLeonKing
– TheLeonKing, Commented Nov 26, 2015 at 19:29
It looks like you want a custom object with two fields name and score rather than a Map<String, String>. — Paul Boddington
– Paul Boddington, Commented Nov 26, 2015 at 19:35
what are your ideas so far? for example you could look at Collections.sort() or at a Comparator-based approach; you could then measure runtime performance for inputs of different sizes — jev
– jev, Commented Nov 26, 2015 at 21:22

Louis Wasserman · Accepted Answer · 2015-11-26 19:31:10Z

3

If you are using Java 8, you'd probably be best off writing

list.sort(Comparator.comparingInt(map -> Integer.parseInt(map.get("Score"))));

If not, you're probably best off writing

Collections.sort(list, new Comparator<Map<String, String>>() {
   @Override public int compare(Map<String, String> a, Map<String, String> b) {
      return Integer.compare(
         Integer.parseInt(a.get("Score")),
         Integer.parseInt(b.get("Score")));
   }
});

answered Nov 26, 2015 at 19:31

Louis Wasserman

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6 Comments

TheLeonKing Over a year ago

Thank you very much! What if I wanted to do this for the Name, which is a string?

Louis Wasserman Over a year ago

Then just write Comparator.comparing(map -> map.get("Name")).

TheLeonKing Over a year ago

I'm sorry, I'm using Android Studio, which doesn't have Java 8. What would be the non-Java 8 version?

Louis Wasserman Over a year ago

Them the body of your compare method is just return a.get("Name").compareTo(b.get("Name")).

Andreas Over a year ago

@Algorithm_NL This answer is great for your stated question, but you'd be better off following Paul Boddington's comment and replace the HashMap with a custom object with two fields name and score. You can then make the object implement Comparable and you won't need an anonymous Comparator.

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