Totals the unique words found

WordCount.java

import java.io.*;
import java.util.*;

/*
 * Given the input string, count the number of unique words found
 */

public class WordCount {
    private static final String letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'-";
    private static String input1 = "After beating the eggs, Dana read the next step:";
    private static String input2 = "Add milk and eggs, then add flour and sugar.";
    private static String input3 = "We came, we saw, we conquered...then we ate Bill's (Mille-Feuille) cake.";
  public static void main(String[] args) {
      System.out.println("RUN 1 **********************");
      System.out.println("Unique words: " + countWords(input1).size());
      System.out.println("RUN 2 **********************");
      System.out.println("Unique words: " + countWords(input2).size());
      System.out.println("RUN 3 **********************");
      System.out.println("Unique words: " + countWords(input3).size());
  }
    
    private static Map<String, Integer> countWords(String input) {
        Map<String, Integer> foundWords = new HashMap<>();
        StringBuilder word = new StringBuilder();
        for(int i = 0; i < input.length(); i++) {
            if (isLetter(input.charAt(i))) {
                word.append(input.charAt(i));
            } else {
                // Not a word if first char isn't a letter
                if(word.toString().equals("")){
                    continue;
                }
                if (foundWords.get(word.toString()) == null) {
                    foundWords.put(word.toString(), 0);
                }
                
                int count = foundWords.get(word.toString()) + 1;
                foundWords.put(word.toString(), count);
                System.out.println("Word: " + word.toString() + " found " + count + " times!");    
                word = new StringBuilder();
            }
            
        }
        
        return foundWords;
    }
    
    private static boolean isLetter(char character) {
        return letters.contains(""+character);
    }
}