Implementing class Fibonacci with a recursive method in Java

Question

Here's a part of my code that is relevant to the problem solving

public long Recursiv(){
    Fibonacci fibo = new Fibonacci(this.getNumar());
    long fibnum;

    if(this.getNumar() == 0) return 0;
    if(this.getNumar() <= 2) return 1;
    else
        fibnum = (fibo.Recursiv()-1) + (fibo.Recursiv()-2);
        return fibnum;

}

It is part of the Fibonacci class that I've created which has a constructor, a getter and setter methods.

My goal is to implement and print even fibonacci numbers in the console e.g. 2, 8, 34...etc.

In the main method, whenever I apply the method on the object I receive a java.lang.StackOverflowError message with the issue that the line which makes the assignment is the problem.

What should I do?

Run in the debugger, name methods starting with lowercase letters and post complete code examples. — Mad Physicist
– Mad Physicist, Commented Dec 7, 2015 at 19:16

Codor · Accepted Answer · 2015-12-07 19:34:51Z

2

The calculation of the Fibonacci sequence can be done by implementing the method as follows.

public long Recursiv(int n)
{
    if (0 == n)
        return 0;
    else if (1 == n)
        return 1;
    else
        return this.Recursiv(n - 1) + this.Recursiv(n - 2)
}

However this would require changing of the method's signature.

Edit:

Apparently I've misunderstood the question a bit; the desired argument is given in the constructor and the method should use instances with smaller arguments for evaluation. The method could also be implemented as follows.

public long Recursiv()
{
    if(this.getNumar() == 0)
        return 0;
    if(this.getNumar() <= 2)
        return 1;
    else
        return new Fibonacci(this.getNumar() - 2).Recursiv()
             + new Fibonacci(this.getNumar() - 1).Recursiv();
}

edited Dec 7, 2015 at 19:34

answered Dec 7, 2015 at 19:29

Codor

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3 Comments

Geosphere Over a year ago

True, I was attempting to do it without the input, I was wondering if there is another way.

Geosphere Over a year ago

No worries, that's interesting so basically at the return it is creating new instances of the Fibonnacci class, am I right?

Codor Over a year ago

Yes, this apparently works, but it is a rather complicated way of doing the evaluation.

Stephen P · Accepted Answer · 2015-12-07 19:29:00Z

2

You need a method that calculates the nth fibonacci number and returns the result — so you have to tell it which number in the fibonacci sequence you want.
That requires a parameter.

public long fib(int n) {
    ...
}

Now a fibonacci number is the sum of the two previous fibonacci numbers; that is

fib(6) = fib(5) + fib(4)
// or more generally
fib(n) = fib(n-1) + fib(n-2)

so (without writing the whole thing for you) your method needs to accept and pass the necessary parameters:

public long fib(int n) {
    if (n == 0) return 0;
    //  etc. -- handle the defined/degenerate cases
    return fib(n-1) + fib(n-2)
}

answered Dec 7, 2015 at 19:29

Stephen P

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