Chapter 21 Exercise 8, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

21.8 (Count the occurrences of words in a text file)
Rewrite Listing 21.9 to read the text from a text file.
The text file is passed as a command-line argument. Words are
delimited by whitespace characters, punctuation marks (,;.:?),
quotation marks ('"), and parentheses. Count words in case-insensitive
fashion (e.g., consider Good and good to be the same word). The words must
start with a letter. Display the output in alphabetical order of words,
with each word preceded by its occurrence count.

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.*;

public class Exercise_08 {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the file path: ");
        File file = new File(scanner.next());
        if (!file.isFile()) {
            System.out.println(file + " is not a file.");
        }
        String[] words;
        try (BufferedReader in = new BufferedReader(new FileReader(file))) {

            StringBuilder buffer = new StringBuilder(10000);
            String s;
            while ((s = in.readLine()) != null)
                buffer.append(s).append("\n");

            words = buffer.toString().split("[0-9]+|\\W+");


        } catch (IOException ex) {
            words = new String[1];
            System.out.println("Error opening file...");
            System.exit(0);
        }

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

        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);
                }
            }
        }

        // Display key and value for each entry
        map.forEach((k, v) -> System.out.println(k + "\t" + v));


    }

}