Java fibonacci stack overflow error

Question

public static void main(String[] args) {
    one();
    System.out.println();
}

public static void one() {
  Scanner sc = new Scanner(System.in);
  System.out.println("Enter a number:");
  int i = sc.nextInt();

  for (int j=0; j<i; j++) {
      System.out.println(fibonacci(j));
  }
}

public static int fibonacci(int num){
    if (num == 1) {
        return 0;
    }
    else if (num == 2 || num == 3){
        return 1;
    }
    else {
        return fibonacci(num-1) + fibonacci(num-2);
    }
}

When I run this will get Exception in thread "main" java.lang.StackOverflowError but if I change one() to

public static void one() {
  Scanner sc = new Scanner(System.in);
  System.out.println("Enter a number:");
  int i = sc.nextInt();
  System.out.println(fibonacci(j));
}

I wont get error, even I just enter 1, could I ask why?

Answer 1

You don't handle the case of num == 0 , so when you call fibonacci(0) the recursion never ends, leading to StackOverflowError when the stack is full.

You can solve it by changing the range of your loop

 for (int j=1; j<i; j++) {
     System.out.println(fibonacci(j));
 }

or by changing the stopping condition of your recursive method:

public static int fibonacci(int num) {
    if (num <= 1) {
        return 0;
    }
    else if (num == 2 || num == 3){
        return 1;
    }
    else {
        return fibonacci(num-1) + fibonacci(num-2);
    }
}

That said, it would be much more efficient to store the intermediate results of fibonacci(i) , and re-use them when calculating fibonacci(n) for n > i (instead of making unnecessary expansive recursive calls).

Answer 2

In the for-Statement you start with 0. This is the first Input to fibonacci-Funktion. Try:

public static void one() {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number:");
 int i = sc.nextInt();

 for (int j=1; j<i; j++) {
     System.out.println(fibonacci(j));
 }

Answer 3

I think you missed to handle the zero. Just try the following loop,

   for (int j=1; j<=i; j++) {
    System.out.println(fibonacci(j));
    }