Chapter 21 Exercise 7, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

21.7 (Revise Listing 21.9, CountOccurrenceOfWords.java)
Rewrite Listing 21.9 to display the words in ascending order
of occurrence counts. (Hint: Create a class named WordOccurrence
that implements the Comparable interface. The class contains two fields,
word and count. The compareTo method compares the counts. For each pair
in the hash set in Listing 21.9, create an instance of WordOccurrence and
store it in an array list. Sort the array list using the Collections.sort method.
What would be wrong if you stored the instances of WordOccurrence in a tree set?)

import java.util.ArrayList;
import java.util.Collections;
import java.util.Map;
import java.util.TreeMap;

public class Exercise_07 {

    public static void main(String[] args) {
        // Set text in a string
        String text = "z Good morning. Have a good class. " +
                "Have a good visit. Have fun!";

        // Create a TreeMap to hold words as key and count as value
        Map<String, Integer> map = new TreeMap<>();

        String[] words = text.split("[\\s+]");
        for (String word1 : words) {
            String key = word1.toLowerCase();

            if (key.length() > 0) {
                if (!map.containsKey(key)) {
                    map.put(key, 1);
                } else {
                    int value = map.get(key);
                    value++;
                    map.put(key, value);
                }
            }
        }

        ArrayList<WordOccurrence> list = new ArrayList<>();
        // Display key and value for each entry
        map.forEach((k, v) -> list.add(new WordOccurrence(k, v)));
        Collections.sort(list);
        list.forEach((word) -> System.out.println(word.word + "\t" + word.count));


    }

    private static class WordOccurrence implements Comparable<WordOccurrence> {

        int count;
        String word;

        public WordOccurrence(String word, int count) {
            this.word = word;
            this.count = count;
        }

        @Override
        public int compareTo(WordOccurrence o) {
            if (o.count > count)
                return -1;
            else if (o.count < count)
                return 1;
            else
                return 0;
        }
    }
}