How to generate all permutations of length n given String of length k

Question

I am trying to use recursion to find all n length permutations of a 6-letter string.

For example: given the string "abcd" and n = 3 it'd generate abc, abd, acd, acb, adb, adc, bcd, bca, bac, bad, bda, bdc, etc.

My code currently only generates all string length permutations (6 digit permutations)

 public static void printPermutn(String str, String ans) { 

    if (str.length() == 0)
    { 
        System.out.println(ans); 
        return; 
    } 

    for (int i = 0; i < str.length(); i++) 
    { 
        char ch = str.charAt(i); 
        String ros = str.substring(0, i) +  
                     str.substring(i + 1); 
        printPermutn(ros, ans + ch); 
    } 
}