char-dupes-1.java

import java.util.ArrayList;

// Let's develop our first solution
public class CountingDuplicates {
  public static int duplicateCount(String text) {
    // We need to count the amount of characters which have duplicates (not the amount of duplicates)
    int totalCharsWithDupes = 0;
    
    // Keep an array of characters which have already been counted towards the totalDupes counter
    // Since we want to count the amount of characters which have duplicates
    // Not the actual amount of total duplicates
    ArrayList<Character> charsCountedForDupes = new ArrayList();
    
    // Let's iterate over every character, and see whether it has dupes
    for (int x = 0; x < text.length(); x++) {
      // Get the lowercase character, to be case-insensitive
      Character currentChar = Character.toLowerCase(text.charAt(x));
      
      // Check if the current string has it as duplicate
      // We can start from our current position in the string (+ 1, for not counting the same character)
      // Previous characters have already been checked 
      for (int y = x + 1; y < text.length(); y++) {
        // Get the second character to lowercase for a fair case-insensitive comparison
        Character secondChar = Character.toLowerCase(text.charAt(y));
          // If the character is the same, it is a duplicate
          if (currentChar == secondChar) {
            // It's a duplicate
            // Did we already count this character towards the duplicates counter?
            // We don't want to count the same character twice
            if (!charsCountedForDupes.contains(currentChar)) {
              // New duplicate! Increment total and add to the counted list 
              // to avoid counting it again in the future
              charsCountedForDupes.add(currentChar);
              totalCharsWithDupes++;
            }
        }
      }
      
    }
    
    // Return the amount of characters which have duplicates
    return totalCharsWithDupes;
  }
}