Backtracking bruteforce Java password cracker

Question

I have this homework assignment to make a recursive method to crack a password of a given length, n (unlimited and unknown!) made of small English letters, a-z ONLY.

Here's the class "Password" that creates a random password:

import java.util.Random;

public class Password {
private String _password = "";

public Password(int length) {
    Random generator = new Random();
    for (int i = 0; i < length; ++i) {
        this._password = this._password + (char) (generator.nextInt(26) + 97);
    }
}

public boolean isPassword(String st) {
    return st.equals(this._password);
}

public String getPassword() {
    return this._password;
  }
}  

And here is the question in detail: "You must write a static recursive method, public static String findPassword(Password p, int length) that "cracks" the code. Here's an example of a main method:

public class Main {                                                                                                     
    public static void main(String[] args) {                                                                            
        Password p = new Password(5);                                                                                   
        System.out.println(p.getPassword());                                                                            
        System.out.println(Ex14.findPassword(p, 5));                                                                    
    }                                                                                                                   
}       

Important notes:

1. The method MUST be recursive, without using ANY loops.
2. You may not use the getPassword method.
3. If you would like to use a method of the String class, you may only use the following: charAt, substring, equals, length.
4. You MAY use overloading, but you MAY NOT use other methods. (You cannot use String.replace/String.replaceall)
5. You MAY NOT use static(global) variables.
6. You MAY NOT use any Array. "

Here's what I have until now, which clearly doesn't work; :\

public static String findPassword(Password p, int length) {
    return findPassword(p, length, "", 'a');
}

public static String findPassword(Password p, int length, String testPass, char charToChange) {
    int currDig = testPass.length() - 1;
    if (p.isPassword(testPass))
        return testPass;
    if (length == 0) // There is no password.
        return ""; // Returns null and not 0 because 0 is a password.
    if (length > testPass.length())
        return findPassword(p, length, testPass + charToChange, charToChange);
    if (testPass.length() == length) {
        //TODO if charToChange is 'z', then make it the one before it '++', and reset everything else to a.
        //if (charToChange == 'z') {
            // charToChange = 'a';
            // String newString = testPass.substring(0, currDig-1) +
            // (charToChange++)
            // +testPass.substring(currDig+1,testPass.length()-1);
            System.out.println("it's  z");
            // TODO currDig --;
            // String newerString = testPass.substring(0, currDig - 1)
            // + (char) (testPass.charAt(testPass.length() - 1) - 25);
            // currDig--;
        }
        return "";  
}

Thank you very much! much appreciated! - TripleS