Sort a List of Maps using a field from the value object

Question

Looking for a solution using Java version 7 and below.

I have the following list of maps.

List<Map.Entry<String, Person>> stuff = new ArrayList<>();

This is Person class

class Person{
    private String name;
    private int order;

    // get set
}

I want to sort the list "stuff" based on Person's order. How can I do this? I was planning on implementing Comparable on the Person class and write the logic for comparison.

But how do I sort it since the Person object is held as a value in a map which in turn is held in a list. For me to sort a Map, I was thinking maybe I could consider a TreeMap. Then again I am not sure if I am over complicating things.

I need to keep that structure of a List holding a Map of keys as Strings and values as Person. Appreciate any input on this. Thanks.

@shmosel Thanks good to know there is this solution in 8. Looking for another option in version 7 and less. — kang, Aug 22 '18 at 01:07
Oops, missed that. Btw, you keep mentioning maps, but there's no map in your code. — shmosel, Aug 22 '18 at 01:07
Hm guess I am using wrong terms here. I want to sort the list using the Entry's order's value. — kang, Aug 22 '18 at 01:11
Short answer - you'll need to write a `Comparator>` which does the required comparison on its enclosed `Person` object, then pass that to the `sort` method. — Dawood ibn Kareem, Aug 22 '18 at 01:14
@DawoodibnKareem Think I get your point. But is there a reason to go for ?mark instead of String for the key? — kang, Aug 22 '18 at 01:17
Just to make it slightly more useful. You're not using the `String`ness of the key, so why insist that the key has to be `String`? In fact, you should probably make it `Comparator>` for the same reason. — Dawood ibn Kareem, Aug 22 '18 at 01:20
@DawoodibnKareem Thanks make sense for some re-usability I suppose. — kang, Aug 22 '18 at 01:26

shmosel · Accepted Answer · 2018-08-22T01:55:13.953

9

Just create a Comparator that compares the desired fields:

Collections.sort(stuff, new Comparator<Map.Entry<String, Person>>() {
    @Override
    public int compare(Map.Entry<String, Person> entry1, Map.Entry<String, Person> entry2) {
        return Integer.compare(entry1.getValue().getOrder(), entry2.getValue().getOrder());
    }
});

Once you're on Java 8, this becomes much easier:

stuff.sort(Comparator.comparingInt(e -> e.getValue().getOrder()));

edited Aug 22 '18 at 01:55

answered Aug 22 '18 at 01:14

shmosel

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Got to say I love the Java 8 approach, but yes this is what I was looking for. – kang Aug 22 '18 at 01:26
@kang Are you using Guava by any chance? – shmosel Aug 22 '18 at 01:28
The project does use Guava. – kang Aug 22 '18 at 01:49

Sort a List of Maps using a field from the value object

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