Towers of Hanoi; recursive method means method never completes?

Question

public class han {

    public static void main(String[] args) {
        hanoi(5, "A", "B", "C");   
    }

    private static void hanoi (int n, String from, String other, String to) {
        if (n==0){
            return;
        }
        if (n>0){
            hanoi(n-1, from, to, other); //comment
            System.out.printf("Move one disk from %s to %s \n ", from, to);
            hanoi(n-1, other, from, to); //comment
        }
    }

}

I'm learning recursion and the classic example is Towers of Hanoi. I'm kind of stumped however, with the commented lines in the above code snippet.

How does this program ever reach the 2nd commented code if the method restarts at the 1st commented code? Something like the below to illustrate:

1: some junk
2: goto 1
3: more junk

How can it ever reach 3 if 2 restarts the function? Same for the Tower of Hanoi snippet: how does it ever reach the 2nd commented code?

Answer 1

It does not restart. You can imagine it as a tree which is growing new branches and leaves.

After your first call to hanoi() it calls itself again and again until it reaches the base case:

if (n==0){
    return;
}

This means that when the call at the first commented line reaches the base case in all of its branches it returns to that point and continues execution.

If there is no base case (some conditional statement which makes your function return eventually) your function will be stuck in an infinite loop.

I think that the Tower of Hanoi is albeit a classic it is rather hard to understand you should start with an easier problem like calculating elements of the fibonacci sequence or a practical algorithm: the depth first search

If you have trouble understanding recursion I can suggest The Little Schemer which is an excellent book exactly about this topic. I can hardly imagine a better and more thorough explanation than that.