Recursion and Tail Recursion in Java and Erlang

Typical Recursion Example (hanoi problem)

public void move(int n, String strFrom, String strTemp, String strTo) {
        if (n == 1) {
            show(1, strFrom, strTo);
        } else {
            move(n - 1, strFrom, strTo, strTemp);
            move(n - 1, strTemp, strFrom, strTo);
        }
    }

Then I come cross the term recursion and tail recursion in Erlang.

%% recursion
sum([H|T]) ->
    H + sum(T);
sum([]) ->
    0.

%% tail recurion
sum(X) ->
    sum(X, 0).
sum([H|T], Acc) ->
   sum(T, H + Acc);
sum([], Acc) ->
    Acc.

What is A Tail Call?

A tail call is a fancy term that refers to a situation in which a method or function call is the last instruction inside of another method or function.

Recursion Code with Java.

     /**
     * sum from 1 to n. recursion
     * @param i
     * @return sum 
     */
    public int recur_head(int i){
        if(i==1)
            return 1;
        else
            return i+recur_head(i-1);
    }

Tail Recursion Code with Java.

    /***
     * sum from 1 to n. tail recursion
     * @param i
     * @param total
     * @return
     */
    public int recur_tail(int i, int total){
        if (i == 1)
            return 1+total;
        return recur_tail(i-1, total +i);
        
    }

See the class with

 javap -c

public class Recur {
  public Recur();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public int recur_head(int);
    Code:
       0: iload_1
       1: iconst_1
       2: if_icmpne     7
       5: iconst_1
       6: ireturn
       7: iload_1
       8: aload_0
       9: iload_1
      10: iconst_1
      11: isub
      12: invokevirtual #2                  // Method recur_head:(I)I
      15: iadd
      16: ireturn

  public int recur_tail(int, int);
    Code:
       0: iload_1
       1: iconst_1
       2: if_icmpne     9
       5: iload_2
       6: iconst_1
       7: iadd
       8: ireturn
       9: aload_0
      10: iload_1
      11: iconst_1
      12: isub
      13: iload_2
      14: iload_1
      15: iadd
      16: invokevirtual #3                  // Method recur_tail:(II)I
      19: ireturn
}

for Java Developer

Java programmers tend to avoid recursion through the use of loops.

int factorial(int n) {
   int result = 1;
   for (int t=n; t > 1; t--)
       result *= t;
   return result;
 }

Reference

http://www.drdobbs.com/jvm/tail-call-optimization-and-java/240167044