How can I do a secondary sorting on a list of maps

Question

Lets suppose I have the following list of maps

[{id:1,count:2,name:xyz},
 {id:2,count:3,name:def},
 {id:3,count:2,name:abc},
 {id:4,count:5,name:ghj}
]

I first want to sort this map by count and then by name:

Desired Output :

[{id:3,count:2,name:abc},
 {id:1,count:2,name:xyz},
 {id:2,count:3,name:def},
 {id:4,count:5,name:ghj}
]

I tried the following to perform the first sorting,but unable to sort using name after sorting by count

Collections.sort(list, new Comparator() {
      public int compare(Object o1, Object o2) {
           return ((Comparable) ((Map.Entry) (o1)).getValue())
          .compareTo(((Map.Entry) (o2)).getValue());
      }

Answer 1

With Java 1.8, I would use the new Comparator methods (although the lack of Type inference makes it necessary to declare all types, reducing the lisibility):

    final Comparator<Map<String, Comparable<Object>>> nameThenCountComparator = Comparator.<Map<String, Comparable<Object>>, Comparable<Object>> comparing(
            m -> m.get("name")).thenComparing(Comparator.<Map<String, Comparable<Object>>, Comparable<Object>> comparing(
            m -> m.get("count")));

With Java 1.7, I would probably use a chainedComparator (see Apache's ComparatorUtils or Guava's Ordering) and a custom MapValueComparator (there are probably one in common libraries, but haven't found it). Then the wanted ordering get quite readable :

    class MapValueComparator implements Comparator<Map<String, Object>> {
        private final String key;

        public MapValueComparator(final String key) {
            this.key = key;
        }

        @Override
        public int compare(final Map<String, Object> o1, final Map<String, Object> o2) {
            return ((Comparable<Object>)o1.get(key)).compareTo(o2.get(key));
        }
    }

    Comparator<Object> nameThenCountComparator = ComparatorUtils.chainedComparator(
            new MapValueComparator("name"), 
            new MapValueComparator("count")
    );

And then use it (Java 7 or 8):

final List<Map<String, Comparable<Object>>> list = null;
Collections.sort(list, nameThenCountComparator);

Rq: you should, as stated in other answers, check for nulls and absent keys in the MapValueComparator.

Answer 2

Assuming list's type is List<Map<String,Object>> (it's not clear what's the type of the value of the Map, so I used Object), your Comparator should be able to compare two Map<String,Object> instances.

Collections.sort(list, new Comparator<Map<String,Object>>() {
      public int compare(Map<String,Object> o1, Map<String,Object> o2) {
           // first compare o1.get("count") to o2.get("count")
           // if they are equal, compare o1.get("name") to o2.get("name")
           // don't forget to handle nulls (for example if either o1 or o2 is null
           // or if any of the keys are not present in one or both of the maps)
      }

Answer 3

If I understand correctly, you have a List<Map<String, Object>>. You'll need to write a custom Comparator in order to sort it. There, you can compare each entry separately (error handling removed for bravity):

public class ListMapComparator implements Comparator<List<Map<String, Object>>> {

    @Override
    public in compare (List<Map<String, Object>> l1, List<Map<String, Object>> l2) {
        Integer count1 = (Integer)l1.get("count");
        Integer count2 = (Integer)l2.get("count");
        int comp = count1.compare(count2);
        if (comp != 0) {
            return comp;
        }

        String name1 = (String)l1.get("name");
        String name2 = (String)l2.get("name");
        return name1.compare(name2);
    }    
}