Help in converting a certain recurtion

[transgalactic]

here is a question and i have a very similar code but i need to convert it
some how in order to fit this question

question:
In order to make their phone numbers more memorable, service providers like to find numbers that spell out some word (called a mnemonic) appropriate to their business that makes that phone number easier to remember. For example, the phone number for a recorded time-of-day message in some localities is 637-8687 (NERVOUS). Imagine that you have just been hired by a local telephone company to write a method listMnemonics that will generate all possible letter combinations that correspond to a given number, represented as a string of digits. For example, if you call
recursiveObject.listMnemonics("723")
your method shall generate and print out (using recursion) the following 27 possible letter combinations that correspond to that prefix:
PAD PBD PCD RAD RBD RCD SAD SBD SCD
PAE PBE PCE RAE RBE RCE SAE SBE SCE
PAF PBF PCF RAF RBF RCF SAF SBF SCF

my efforts in solving it:
i know that in order to solve this question is need to use a recursive methos that
prints out all the subsets of a given string letters with a given resolt string size

i have built this code

ublic class subsets {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		String word="abcd";
		
		
subsets(word.length(),3,"",word);
	}
	public static void phone(String phone){
		
	}
	public static void subsets(int n,int k,String also,String word){
		if (n==0){
			if (also.length()==k){
			System.out.println(also);
			}
		}
		else{
			subsets(n-1,k,also+word.charAt(n-1),word);
			subsets(n-1,k,also,word);
			
				
			
		}
	}

}

 

[transgalactic]

i had a theoretical idea the we put an inner counter for each cell
and then count them like numbers
001
002
003
but for example in order to have a combination for a number with length 4
we need to have 4 "for" loops one inside another

for 5 digit number we need to have 5 "for" loop one inside of the other

how do i solve its for every given length