public class CamelCaseUtils {
 
    private static final char SEPARATOR = '_';
 
    public static String toUnderlineName(String s) {
        if (s == null) {
            return null;
        }
 
        StringBuilder sb = new StringBuilder();
        boolean upperCase = false;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
 
            boolean nextUpperCase = true;
 
            if (i < (s.length() - 1)) {
                nextUpperCase = Character.isUpperCase(s.charAt(i + 1));
            }
 
            if ((i >= 0) && Character.isUpperCase(c)) {
                if (!upperCase || !nextUpperCase) {
                    if (i > 0) sb.append(SEPARATOR);
                }
                upperCase = true;
            } else {
                upperCase = false;
            }
 
            sb.append(Character.toLowerCase(c));
        }
 
        return sb.toString();
    }
 
  public static String toCamelCase(String value, final boolean capitalizeFirstLetter) {
    if (value == null) {
      return null;
    }
    value = value.toLowerCase();
 
    StringBuilder sb = new StringBuilder(value.length());
    boolean upperCase = false;
    for (int i = 0; i < value.length(); i++) {
      char c = value.charAt(i);
 
      if (c == SEPARATOR) {
        upperCase = true;
      } else if (upperCase) {
        sb.append(Character.toUpperCase(c));
        upperCase = false;
      } else {
        sb.append(c);
      }
    }
 
    if (capitalizeFirstLetter) {
      return sb.substring(0, 1).toUpperCase() + sb.substring(1);
    } else {
      return sb.toString();
    }
  }
 
    public static void main(String[] args) {
        System.out.println(CamelCaseUtils.toUnderlineName("ISOCertifiedStaff"));
        System.out.println(CamelCaseUtils.toUnderlineName("CertifiedStaff"));
        System.out.println(CamelCaseUtils.toUnderlineName("UserID"));
        System.out.println(CamelCaseUtils.toCamelCase("iso_certified_staff", false));
        System.out.println(CamelCaseUtils.toCamelCase("certified_staff", false));
        System.out.println(CamelCaseUtils.toCamelCase("user_id", false));
    }
}