Sort list of maps using multiple keys in Java8

Question

I have list of maps and I want to sort the maps inside the list using the keys. As of now I am achieving this using the below Collections.sort method..

Collections.sort(listOfMaps, new Comparator<Map<String, String>>() {
        @Override
        public int compare(Map<String, String> o1, Map<String, String> o2) {
            //return o1.get("cm_order_x").compareTo(o2.get("cm_order_x"));
            String x1 = o1.get(Key1);
            String x2 = o2.get(Key1);
            String x3 = o1.get(Key2);
            String x4 = o2.get(Key2);
            int sComp = x1.compareTo(x2);
            int sComp1 = x3.compareTo(x4);
            if (sComp != 0) {
               return sComp;
            }
            else if(sComp1 != 0) {

                //return x3.compareTo(x4);
                return sComp1;
            }


                else
                {
                    String x5 = o1.get(Key3);
                    String x6 = o2.get(Key3);
                    return x5.compareTo(x6);
                }
             }

    });

Is there any other better way to sort the list of maps in Java 8 ?

As a style thing, those variables aren't really great names. There's nothing in them to suggest that x1 and x2 are related, but x2 and x3 are not. I'd encourage you to not be afraid of "long" names: String key1FromO1 = o1.get(Key1), for instance. — yshavit
– yshavit, Commented Oct 12, 2017 at 16:49
Thanks for the suggestion @yshavit. I will keep this in mind. — sparkey
– sparkey, Commented Oct 12, 2017 at 16:50

Holger · Accepted Answer · 2017-10-12 20:04:19Z

4

Since Java8, the Comparator interface offers factory methods and chaining methods:

Comparator<Map<String, String>> c
    = Comparator.comparing((Map<String, String> m) -> m.get(Key1))
                .thenComparing(m -> m.get(Key2))
                .thenComparing(m -> m.get(Key3))
                .thenComparing(m -> m.get(Key4));

listOfMaps.sort(c);

answered Oct 12, 2017 at 20:04

Holger

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Comments

broxhouse · Accepted Answer · 2017-10-12 16:43:16Z

0

I believe you can accomplish this task by using code like this:

listOfMaps.sort(Comparator.comparing(
                m -> m.get("yourKey"), 
                Comparator.nullsLast(Comparator.naturalOrder()))
           )

See: How can I sort a list of maps using Java 8?

answered Oct 12, 2017 at 16:43

broxhouse

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2 Comments

sparkey Over a year ago

I saw this thread and it is for single keys. What if have multiple keys to sort ?

broxhouse Over a year ago

Let me make sure I understand your question. Do you want to iterate through the ArrayList and sort each individual HashMap by key? Or do you want to make one HashMap containing all the keys and values from each HashMap, sorted by keys?

Shadov · Accepted Answer · 2017-10-12 17:20:52Z

0

There is no possibility of this, since you are sorting depending of something only you know.

What I mean is you are first comparing by Key1, then Key2 etc. Keys are not numbered in a map, here Key1 doesn't mean first key, but some key that you want compared first. There is no way to tell that to java, this behaviour is too specific.

You can come up with something for a generic solution (like for a map with more keys, so you don't have to list them all like that), but you are not gonna avoid the main problem of this implementation.

answered Oct 12, 2017 at 17:20

Shadov

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