code: public <T extends CharSequence> T validIndex(final T chars, final int index, final String message, final Object... values) {
          notNull(chars);
          if (index < 0 || index >= chars.length()) {
              failIndexOutOfBounds(String.format(message, values));
          }
          return chars;
      }
docs: <p>Validates that the index is within the bounds of the argument character sequence; otherwise throwing an exception with the specified message.</p>
  <pre>Validate.validIndex(myStr, 2, "The string index is invalid: ");</pre>
  <p>If the character sequence is {@code null}, then the message of the exception is &quot;The validated object is null&quot;.</p>
  
  @param <T>
  the character sequence type
  @param chars
  the character sequence to check, validated not null by this method
  @param index
  the index to check
  @param message
  the {@link String#format(String, Object...)} exception message if invalid, not null
  @param values
  the optional values for the formatted exception message, null array not recommended
  
  @return the validated character sequence (never {@code null} for method chaining)
  
  @throws NullPointerValidationException
  if the character sequence is {@code null}
  @throws IndexOutOfBoundsException
  if the index is invalid
  @see #validIndex(CharSequence, int)