Sorting a list of triples within a map using one of the values of the triple

Question

I have a hashmap with a triplet like this:

HashMap<String, List<Triplet<String,Integer,Integer>>> output = new HashMap<String, List<Triplet<String,Integer,Integer>>>();

Where the key is a file name and triplet contains information of a word and its frequency information(tf and df).E.g.

{File1 = {coffee,23,1},{caffeine,12,2},{brew,9,1}; File2 = {}.......}

I want to sort the values of this list of triples according to the second value of the triplet i.e tf (term frequency).

for (Entry<String, List<Triplet<String, Integer, Integer>>> entry : output.entrySet()) {
    String key = entry.getKey();
    List<Triplet<String, Integer, Integer>> value = entry.getValue();

What will come next?

Here's what Triplet looks like:

public class Triplet<T, U, V> {

    private final T first;
    private final U second;
    private final V third;

    public Triplet(T first, U second, V third) {
        this.first = first;
        this.second = second;
        this.third = third;
    }

    public T getFirst() { 
        return first;
    }

    public U getSecond() { 
        return second; 
    }

    public V getThird() {
        return third; 
    }
}

Any help, guidance will be appreciated.

Possible duplicate of [How to sort List of objects by some property](https://stackoverflow.com/questions/5805602/how-to-sort-list-of-objects-by-some-property) — OH GOD SPIDERS, Dec 29 '17 at 12:17
instead of using triplets why not use a custom object with speaking and understandable names? — Lino, Dec 29 '17 at 12:21
thanks for the suggestion Lino. I am now using an object instead of a triple. Makes things less unwieldy. — serendipity, Dec 29 '17 at 13:57

score 0 · Answer 1 · answered Dec 29 '17 at 12:20

0

There are 2 ways than came into my mind:

make your Triplet Comparable
write a Comparator

Than you can either sort the List or use e.g. a TreeSet

answered Dec 29 '17 at 12:20

MrSmith42

9,961
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score 0 · Answer 2 · answered Dec 29 '17 at 12:34

If you can not change your Triplet class by implementing Comparable on it, this would be a solution, using the Collections.sort method:

for (Entry<String, List<Triplet<String, Integer, Integer>>> entry : output.entrySet()) {
        String key = entry.getKey();
        List<Triplet<String, Integer, Integer>> value = entry.getValue();

        // This sorts the list using the anonymous instantiated Comparator class
        Collections.sort(value, new Comparator<Triplet<String, Integer, Integer>>(){
            @Override
            public int compare(Triplet<String, Integer, Integer> object1, Triplet<String, Integer, Integer> object2) {
                return object1.getSecond().compareTo(object2.getSecond());
            }
        });

Or, in a more generic way, without using hard types:

Collections.sort(value, new Comparator<Triplet>(){
                @Override
                public int compare(Triplet object1, Triplet object2) {
                    return object1.getSecond().compareTo(object2.getSecond());
                }
            });

`new Comparator(){ @Override public int compare(){...}}`can be reduced by using java-8 lambdas: `(left, right) -> {...}` — Lino, Dec 29 '17 at 12:41
@HaroldH I am using your answer in my code but from the errors I am seeing and the Java implementation, value should be a List not a variable. Right? — serendipity, Dec 29 '17 at 13:37
never mind the question. The errors got resolved. However, one last question...how do I put the sorted list back in the map? — serendipity, Dec 29 '17 at 13:56
@serendipity: Sorting a list will change the list itself, there is no need to reassign it. — OH GOD SPIDERS, Dec 29 '17 at 16:18

Dr. Debasish Jana · Answer 3 · 2017-12-29T13:16:29.763

-1

What about this?

public class Triplet<T, U, V> implements Comparable<Triplet<T, U, V> > {

    private final T first;
    private final U second;
    private final V third;

    public Triplet(T first, U second, V third) {
        this.first = first;
        this.second = second;
        this.third = third;
    }

    public T getFirst() { 
        return first;
        }
    public U getSecond() { 
        return second; 
        }
    public V getThird() {
        return third; 
        }
    public int compareTo(Triplet<T, U, V> comparetrpl) {
        T a = comparetrpl.getFirst();
        //ascending order
        return (this.first > a) ? 1 : 0;

        //descending order
        //return (a > this.first) ? 1 : 0;

    }
}

And the use sort method?

edited Dec 29 '17 at 13:16

answered Dec 29 '17 at 12:21

Dr. Debasish Jana

6,980
4
30
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`return this.first > a;` returns a boolean and not the prefered `int` – Lino Dec 29 '17 at 12:42
also the return type of `getFirst()` is `T` which not always must be `Integer` – Lino Dec 29 '17 at 12:43

Sorting a list of triples within a map using one of the values of the triple

3 Answers3