Java sort a Hashmap on Value

问题

Learning Java as I go (Python background). Simple word count program in Java 7 code (can not use J8!).

I have a hash map of word:count pairs. Now I need to sort on count (decreasing order), and break ties with using word in alphabetical order.

Have read s/o and I tried a treemap but it does not seem to handle ties very well so I don't think that is right.

I have seen a lot of solutions posted that define a new class sortbyvalue and define a comparator. These will not work for me as I need to keep the solution all contained in the existing class.

I am looking for feedback on this idea:

iterate over the map entries (me) in the hashmap
use me.getKey = K and me.getValue = V
new Map.Entry reverse_me = (V,K) {not sure about this syntax}
add reverse_me to a List
repeat for all me in map
List.sort { this is where I am unsure, on how this will sort and no idea how to write a comparator. At this point each List element would be a (count, word) pair and the sort() should sort by count in decreasing and then by word in case of same counts in alphabetical order)

This would be the final output.

Is this a logical progression? I can tell from the many posts that there are many opinions on how to do this, but this is the one I can wrap my head around.

Also, can not use Guava.

回答1:

You can create an List of Entry set from the map. Sort the List using Collections.sort(). You can pass the custom Comparator for sorting by Key when Value(s) are same.

Set<Entry<String, Integer>> set = map.entrySet();
List<Entry<String, Integer>> list = new ArrayList<Entry<String, Integer>>(set);
Collections.sort( list, new Comparator<Map.Entry<String, Integer>>()
{
  public int compare( Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2 )
  {
    int result = (o2.getValue()).compareTo( o1.getValue() );
    if (result != 0) {
      return result;
    } else {
      return o1.getKey().compareTo(o2.getKey());
    }
  }
} );

回答2:

This collection reflects the correct order only as long as the map entries are not changed

        HashMap<String, Integer> map = new HashMap<>();
        TreeSet<Map.Entry<String, Integer>> entriesSet = new TreeSet<>(new Comparator<Map.Entry<String, Integer>>(){
            @Override
            public int compare(Map.Entry<String, Integer> me1, Map.Entry<String, Integer> me2) {
                return me1.getValue().compareTo(me2.getValue());  
            }
        });
        entriesSet.addAll(map.entrySet());

来源：https://stackoverflow.com/questions/32496409/java-sort-a-hashmap-on-value

标签

java

sorting

collections

hashmap