Variable cannot be resolved in an If-Statement

I'm trying to do the below tutorial question.

// Create a method called greatestCommonFactor
// It should return the greatest common factor
// between two numbers.  
//
// Examples of greatestCommonFactor:
//   greatestCommonFactor(6, 4)   // returns 2
//   greatestCommonFactor(7, 9)   // returns 1
//   greatestCommonFactor(20, 30) // returns 10
//
// Hint: start a counter from 1 and try to divide both
// numbers by the counter. If the remainder of both divisions
// is 0, then the counter is a common factor. Continue incrementing
// the counter to find the greatest common factor. Use a while loop
// to increment the counter.

And my code is below

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Ex4_GreatestCommonFactor {

    // This is the main method that is executed as
    // soon as the program starts.  
    public static void main(String[] args) {
        // Call the greatestCommonFactor method a few times and print the results
    }

    public static int greatestCommonFactor(int a, int b){
        int i = 1 ;
        while (i >= 1 ){
            i++;
        }
        if (a%i == 0 && b%i == 0){
            ArrayList factor = new ArrayList<Integer>();
            factor.add(i);  
        }
        else if (a%i <= 1 || b%i <= 1){
            Collections.sort(factor);
            List<Integer> topnum = factor.subList(factor.size() - 1, factor.size());

        }
        return topnum;
    }
}

So I have 2 questions.

1) In my elseif statement, I get an error where factor cannot be resolved to a variable. How do I "carry over" the factor ArrayList from the previous If statement into this elseif statement?

2) I also get a similar error where topnum cannot be resolved. Is this also a placement error for this line of code in my method, or am I making a completely different mistake?