How to fix my merge sort?

Question

When I compile my merge sort, it compiles fines but there is no output.

This needs to be done recursively. Also, I do run my printList() function in my main, so it isn't that I am forgetting to print the list. I've been staring at this for the past 30 minutes.

public void printList(){
    for(int i = 0; i <  fullNames.size(); i++){
        System.out.println(fullNames.get(i));
    }
}

public void sort(){
    fullNames = mergeSort(fullNames);
}

public ArrayList<String> extract(ArrayList<String> ex, int low, int high){
    ArrayList<String> answer = new ArrayList<String>();
    for(int i = low; i <= high; i++){
        answer.add(ex.get(i));
    }

    return answer;
}

public ArrayList<String> merge(ArrayList<String> first, ArrayList<String> second){
    int f = 0; 
    int s = 0;
    ArrayList<String> answer = new ArrayList<String>();
    while(first.size() > 0 || second.size() > 0){
        if(s != second.size() && f != first.size()){
            if(first.get(f).compareTo(second.get(s)) > 0){
                answer.add(second.get(s));
                s++;
            }

            else if(first.get(f).compareTo(second.get(s)) < 0){
                answer.add(first.get(f));
                f++;
            }
        }

        else if(f == first.size()){
            while(s != second.size()){
                answer.add(second.get(s));
                s++;
            }
        }

        else{
            while(f != first.size()){
                answer.add(first.get(f));
                f++;
            }
        }
    }

    return answer;
}
public ArrayList<String> mergeSort(ArrayList<String> names){
    int k = names.size()-1;

    if(k > 1){
        ArrayList<String> first = extract(names, 0, k / 2);
        ArrayList<String> second = extract(names, (k / 2) + 1, k);

        mergeSort(first);
        mergeSort(second);

        return merge(first, second);
    }

    else{
        return names;
    }
}

Answer 1

The while condition in your merge will never be false because the size of first and second is never adjusted within the loop:

while(first.size() > 0 || second.size() > 0){
        if(s != second.size() && f != first.size()){
            if(first.get(f).compareTo(second.get(s)) > 0){
                answer.add(second.get(s));
                s++;
            }

            else if(first.get(f).compareTo(second.get(s)) < 0){
                answer.add(first.get(f));
                f++;
            }
        }

        else if(f == first.size()){
            while(s != second.size()){
                answer.add(second.get(s));
                s++;
            }
        }

        else{
            while(f != first.size()){
                answer.add(first.get(f));
                f++;
            }
        }
    }