max = 4

for a1 := 1 to max do
   for a2 := 1 to max do
      for a2 := 1 to max do
        for a4 := 1 to max do
             write( a1,a2,a3,a4 )

import java.math.BigInteger;

public class PermutationGenerator {
  private int[] a;
  private BigInteger numLeft;
  private BigInteger total;

  public PermutationGenerator (int n) {
    if (n < 1) {
      throw new IllegalArgumentException ("Min 1");
    }
    a = new int[n];
    total = getFactorial (n);
    reset ();
  }

  public void reset () {
    for (int i = 0; i < a.length; i++) {
      a[i] = i;
    }
    numLeft = new BigInteger (total.toString ());
  }

  public BigInteger getNumLeft () {
    return numLeft;
  }

  public BigInteger getTotal () {
    return total;
  }

  public boolean hasMore () {
    return numLeft.compareTo (BigInteger.ZERO) == 1;
  }

  private static BigInteger getFactorial (int n) {
    BigInteger fact = BigInteger.ONE;
    for (int i = n; i > 1; i--) {
      fact = fact.multiply (new BigInteger (Integer.toString (i)));
    }
    return fact;
  }

  public int[] getNext () {
    if (numLeft.equals (total)) {
      numLeft = numLeft.subtract (BigInteger.ONE);
      return a;
    }
    int temp;
    int j = a.length - 2;
    while (a[j] > a[j+1]) {
      j--;
    }
    int k = a.length - 1;
    while (a[j] > a[k]) {
      k--;
    }
    temp = a[k];
    a[k] = a[j];
    a[j] = temp;
    int r = a.length - 1;
    int s = j + 1;
    while (r > s) {
      temp = a[s];
      a[s] = a[r];
      a[r] = temp;
      r--;
      s++;
    }
    numLeft = numLeft.subtract (BigInteger.ONE);
    return a;
  }
}

    int[] indices;
    String[] elements = {"a", "b", "c", "d"};
    PermutationGenerator x = new PermutationGenerator (elements.length);
    StringBuffer permutation;
    while (x.hasMore ()) {
      permutation = new StringBuffer ();
      indices = x.getNext ();
      for (int i = 0; i < indices.length; i++) {
        permutation.append (elements[indices[i]]);
      }
      System.out.println (permutation.toString ());
    }