Dá se použít například rekurze:

import java.util.ArrayList;
import java.util.List; 
public class CharacterCombinations {

	public static void main(String[] args) {
		char[] charArray = {'a', 'b', 'c', 'd'};
		int length = 4;
		List<String> Combinations = getCombinations(charArray, new ArrayList<String>(), length, "");
		System.out.println(Combinations);
	}

	private static List<String> getCombinations(char[] charArray, List<String> Combinations, int length, String prefix) {
		if(prefix.length()==length){
			Combinations.add(prefix);
		}else{
			for(int i=0; i<charArray.length; i++){
				getCombinations(charArray, Combinations, length, prefix+charArray[i]);
			}
		}
		return Combinations;
		
	}
}