Recursively pass counter variable in Java

Question

I have a function which calls itself recursively:

public int foo(int num, int counter)
{
    if (num > 0)
    {
        counter++;
        num--;
        foo(num, counter);
    }

    return counter;
}

From main method I call function with:

System.out.println(bst.foo(3, 0));

I expect such behavior:

public int foo(int num, int counter)
{
    // counter = 0
    // num = 3
    if (num > 0)
    {
        counter++; // counter = 1
        num--; // num = 2
        if (num > 0)
        {
            counter++; // counter = 2
            num--; // num = 1
            if (num > 0)
            {
                counter++; // counter = 3
                num--; // num = 0
                if (num > 0)
                {
                    // don't execute as num = 0
                }
            }
        }
    }

    return counter; // return 3
}

But function always returns 1 and I have no idea why.

Answer 1

You're passing the value of counter, not the variable itself. Your recursive call can't modify the value of counter in the outer invocation. Java is pass-by-value.

One possible solution is to do

 counter = foo(num, counter);

in the recursive call. Or, alternately, just return foo(num, counter), since you don't do anything with counter afterwards except return it.