Fibonacci generating algorithm

Question

I have written an algorithm that generates a Fibonacci series to the \$Nth\$ number. The code below works fine, but as a beginner I know I must be writing pretty ugly code.

For example I think it is probably not good practice to declare and initialize the variables where I have, but I am confused about what would be the best place.

All comments and advice on code style etc. are welcome.

package algorithms;
import java.util.Scanner;
/**
* This class will generate a list of fibonacci numbers
* @author Richard
*
*/
public class FibonacciGenerator {
/**
 * This method will generate a list of fibonacci numbers
 * @param args
 */
public static void main(String[] args) {
 //declare vars and set initial values
 int number = 0, previousNumber = 0, twoNumbersAgo = 1;
 Scanner input = new Scanner(System.in);
 //prompt user for nth number
 System.out.println("Enter the n'th number for your Fibonacci series: ");
 //set loop counter equal to given nth number
 int n = input.nextInt();
 //loop through as many times as n
 for (int counter=1;counter<=n;counter++){
 //setting each new number in list equal to the sum of the previous two
 number = (previousNumber) + (twoNumbersAgo);
 //print out number
 System.out.print(number+" ");
 //change vars to be next two numbers
 twoNumbersAgo = previousNumber;
 previousNumber = number; 
 }
 //close resources
 input.close();
}
}

Answer 1

Advice 1

One very useful principle is that each method should solve only one problem and not more. In this context, I suggest that you roll a static method that simply computes a desired Fibonacci sequence and returns it as an array/list. Then, another method can call the previous and output the numbers.

Advice 2

Instead of closing a Scanner explicitly, you could use a try with resources. Any Autocloseable can do and will be closed implicitly.

try(Scanner scanner = new Scanner(System.in)) {
 // Use scanner here.
}
// Scanner automatically closed here.

The above will close Scanner when exiting the block, or in the case of exception.

Advice 3

Fibonacci sequence grows exponentially. For that reason you might want to use BigInteger.

Summa summarum

An alternative implementation may look like this:

import java.math.BigInteger;
import java.util.Arrays;
import java.util.Scanner;
public class FibonacciGenerator {
 public static BigInteger[] getFibonacciSequencePrefix(int n) {
 BigInteger[] result = new BigInteger[n];
 BigInteger a = BigInteger.ONE;
 BigInteger b = BigInteger.ONE;
 for (int i = 0; i < n; ++i) {
 result[i] = a;
 BigInteger tmp = a;
 a = b;
 b = b.add(tmp);
 }
 return result;
 }
 public static void main(String[] args) {
 try (Scanner scanner = new Scanner(System.in)) {
 int n = scanner.nextInt();
 System.out.println(Arrays.toString(getFibonacciSequencePrefix(n)));
 }
 }
}

• Martin R: Just note that the original program prints the first N Fibonacci numbers, it does not just print the N'th number. – Judging from the first sentence in the question, that seems to be intentional.
• coderodde: @MartinR Oops, you are right. Edited my answer.
• piepi: I didn’t get the try part. Can you elaborate on it a little?
• coderodde: @piepi Done! If there is more content I could add, let me know!