This class does not provide a secure way to generate passwords. The password is limited to a range of letter, which can not be modified without changing the code of the class. A limitation of character is always a bad idea to generate a random password string. The limitation allows the use of rainbow tables limited to lower case letters, digits and a subset of special characters.
A strong and secure method must be able to generate passwords in any language (charset), which indicates that the character range has to be unicode.
At least the generator should be able to use lower and upper case letters.