How do I get the most frequent word in a Map and it's corresponding frequency of occurrence using Java 8 streams?

Question

I have a class IndexEntry which looks like this:

public class IndexEntry implements Comparable<IndexEntry>
{
    private String word;
    private int frequency;
    private int documentId;
    ...
    //Simple getters for all properties
    public int getFrequency()
    {
        return frequency;
    }
    ...
}

I am storing objects of this class in a Guava SortedSetMultimap (which allows for multiple values per key) where I am mapping a String word to some IndexEntrys. Behind the scenes, it maps each word to a SortedSet<IndexEntry>.

I am trying to implement a sort of indexed structure of words to documents and their occurrence frequencies inside the documents.

I know how to get the count of the most common word, but I can't seem to get the word itself.

Here is what I have to get the count of the most common term, where entries is the SortedSetMultimap, along with helper methods:

public int mostFrequentWordFrequency()
{
    return entries
            .keySet()
            .stream()
            .map(this::totalFrequencyOfWord)
            .max(Comparator.naturalOrder()).orElse(0);
}

public int totalFrequencyOfWord(String word)
{
    return getEntriesOfWord(word)
            .stream()
            .mapToInt(IndexEntry::getFrequency)
            .sum();
}

public SortedSet<IndexEntry> getEntriesOfWord(String word)
{
    return entries.get(word);
}

I am trying to learn Java 8 features because they seem really useful. However, I can't seem to get the stream working the way I want. I want to be able to have both the word and it's frequency at the end of the stream, but barring that, if I have the word, I can very easily get the total occurrences of that word.

Currently, I keep ending up with a Stream<SortedSet<IndexEntry>>, which I can't do anything with. I don't know how to get the most frequent word without the frequencies, but if I have the frequency, I can't seem to keep track of the corresponding word. I tried creating a WordFrequencyPair POJO class to store both, but then I just had a Stream<SortedSet<WordFrequencyPair>>, and I couldn't figure out how to map that into something useful.

What am I missing?

Answer 1

I think it would be a better design to use the documentId as the key of the TreeMultimap rather than the word:

import com.google.common.collect.*;

public class Main {

    TreeMultimap<Integer, IndexEntry> entries = TreeMultimap.<Integer, IndexEntry>create(Ordering.arbitrary(), Ordering.natural().reverse());

    public static void main(String[] args) {
        // Add elements to `entries`

        // Get the most frequent word in document #1
        String mostFrequentWord = entries.get(1).first().getWord();
    }

}

class IndexEntry implements Comparable<IndexEntry> {

    private String word;

    private int frequency;

    private int documentId;

    public String getWord() {
        return word;
    }

    public int getFrequency() {
        return frequency;
    }

    public int getDocumentId() {
        return documentId;
    }

    @Override
    public int compareTo(IndexEntry i) {
        return Integer.compare(frequency, i.frequency);
    }

}

You can then implement the methods that you had before with the following:

public static int totalFrequencyOfWord(String word) {
    return entries.values()
                   .stream()
                   .filter(i -> word.equals(i.getWord()))
                   .mapToInt(IndexEntry::getFrequency)
                   .sum();
}

/**
 * This method iterates through the values of the {@link TreeMultimap},
 * searching for {@link IndexEntry} objects which have their {@code word}
 * field equal to the parameter, word.
 *
 * @param word
 *     The word to search for in every document.
 * @return
 *     A {@link List<Pair<Integer, Integer>>} where each {@link Pair<>}
 *     will hold the document's ID as its first element and the frequency
 *     of the word in the document as its second element.
 *
 * Note that the {@link Pair} object is defined in javafx.util.Pair
 */  
public static List<Pair<Integer, Integer>> totalWordUses(String word) {
    return entries.values()
                   .stream()
                   .filter(i -> word.equals(i.getWord()))
                   .map(i -> new Pair<>(i.getDocumentId(), i.getFrequency()))
                   .collect(Collectors.toList());
}

Answer 2

Native solution by JDK:

entries.keySet().stream()
     .collect(groupingBy(IndexEntry::getWord, summingInt(IndexEntry::getFrequency)))
     .values().stream().max(Comparator.naturalOrder()).orElse(0L);

Or By StreamEx

StreamEx.of(entries.keySet())
    .groupingBy(IndexEntry::getWord, summingInt(IndexEntry::getFrequency))
    .values().stream().max(Comparator.naturalOrder()).orElse(0L);