Lower.java

package strings;

public class Lower {

  /** Driver Code */
  public static void main(String[] args) {
    String[] strings = {"ABC", "ABC123", "abcABC", "abc123ABC"};
    for (String s : strings) {
      assert toLowerCase(s).equals(s.toLowerCase());
    }
  }

  /**
   * Converts all of the characters in this {@code String} to lower case
   *
   * @param s the string to convert
   * @return the {@code String}, converted to lowercase.
   */
  public static String toLowerCase(String s) {
    char[] values = s.toCharArray();
    for (int i = 0; i < values.length; ++i) {
      if (Character.isLetter(values[i]) && Character.isUpperCase(values[i])) {
        values[i] = Character.toLowerCase(values[i]);
      }
    }
    return new String(values);
  }
}