Java recursive functions explained // Using recursion to sum an array of numbers

1public class Recursive {
2  public Recursive() {
3    /* We create the array of numbers we want to sum,
4       It can be any subclass of java.lang.Number */
5    Double[] test = {100d, 0.05d, 88d, 99d, 0.05d, 88d, 99d, 0.05d, 88d, 99d};
6    /* We call our function */
7    System.out.println(sum(test));
8  }
9  /* This is the initial function, it calculates the starting fields and results *automatically*/
10  public Number sum(Number[] numbers){
11    return sum(numbers[numbers.length - 1], numbers, numbers.length - 2);
12  }
13  /* This is our RECURSIVE FUNCTION */
14  public Number sum(Number initialValue, Number[] numbers, int location){
15    /* If we've reached the end of the array we return the final RESULT */
16    if(location < 0){
17      return initialValue;
18    }
19    /* Or else we are recursive */
20    else{
21      /*First we cast the java.lang.Number to its subclass so we can do the *sum */
22      if(numbers instanceof BigInteger[]){
23        return sum( ((BigInteger)initialValue).add((BigInteger)numbers[location]), numbers, (location - 1) );
24      } else if(numbers instanceof BigDecimal[]){
25        return sum( ((BigDecimal)initialValue).add((BigDecimal)numbers[location]), numbers, (location - 1) );
26      } else if(numbers instanceof Byte[]){
27        return sum( ((Byte)initialValue) + (Byte)numbers[location], numbers, (location - 1) );
28      } else if(numbers instanceof Double[]){
29        return sum( ((Double)initialValue) + ((Double)numbers[location]), numbers, (location - 1) );
30      } else if(numbers instanceof Integer[]){
31        return sum( ((Integer)initialValue)+ ((Integer)numbers[location]), numbers, (location - 1) );
32      } else if(numbers instanceof Long[]){
33        return sum( ((Long)initialValue)+ ((Long)numbers[location]), numbers, (location - 1) );
34      } else if(numbers instanceof Short[]){
35        return sum( ((Short)initialValue)+ ((Short)numbers[location]), numbers, (location - 1) );
36      } else{
37        return sum( ((Float)initialValue)+ ((Float)numbers[location]), numbers, (location - 1) );
38      }
39    }
40  }
41  public static void main(String[] args) {
42    new Recursive(); 
43  }
44}