Java program that does Permutation & Combination

Forms the unique target combinations, recursively, starting from source list that has all the positions.

import java.util.LinkedList;
import java.util.List;

import org.apache.commons.lang.SerializationUtils;

public class Combinations {
 
 private final static String characters = "abcd";
 
 private static int length = characters.length();
 
 public static void main(String[] args) {
  List source = new LinkedList();
  
  // form the source list that have all the possible positions
  for (int i = 0; i < length; i++) {
   source.add(i);
  }

  // create a target list for forming unique combinations
  List target = new LinkedList();

  combine(source, target);

 }
 
 public static void combine(List source, List target) {

  // break the recursion
  if (target.size() == length) {
   for (int i = 0; i < length; i++) {    
    System.out.print(characters.charAt(target.get(i)));    
   }
   System.out.println();
   return;
  }

  for (Integer position : source) {
   //form the target combination by selecting a position from the source
   List reducedSource = (List)SerializationUtils.clone((LinkedList)source);
   reducedSource.remove(position);
   List combinedTarget = (List)SerializationUtils.clone((LinkedList)target);
   combinedTarget.add(position);
   combine(reducedSource, combinedTarget);   
  }
 }
}