How do I sort an ArrayList of strings in alpabetical order using a compareTo method?

Question

I'm an amateur programmer and I've created a program that contains an arraylist that holds different phone book contacts and I'm trying to sort the arraylist lexicographically using a compareTo method. I don't know how to properly call the method in the main method so that it sorts properly and I'm not allowed to use collections sort. Can anyone help me?

Here's some of my code:

public class Phonebook implements Comparable<Phonebook> {
        private String first, last;
        public Contact(String first, String last) {
        this.first = first;
        this.last= last;
}
        public String getFirst() {return first;}
        public String getLast() {return last;}

        public String toString() {
            return first + " " + last;
    }
        public int compareTo(Phonebook another) {
        int a = last.compareTo(another.last);
        int b = first.compareTo(another.first);
        if (a== 0 && b== 0)
            return 0;
        if (a == 0 && b!= 0)
            return b;
        return a;
    }
    }

public class PhonebookList implements Iterable<Phonebook>{

    ArrayList<Phonebook>phonebook;

    public PhonebookList() {
    }
    public PhonebookList(Contact[]contacts) {
        phonebook=new ArrayList<>(Arrays.asList(phonebooks));
    }

import java.util.*;
public static void main(String[] args) {
        // TODO Auto-generated method stub

        PhonebookList list= new PhonebookList();

        Phonebook ph1= new Phonebook ("Brandon","Johnson");
        Phonebook ph2 = new Phonebook ("Samantha","Joseph");

        list.add(ph1);
        list.add(ph2);
}

Are you required to implement a sorting algorithm as a part of this assignment then? — Martin'sRun, Oct 03 '19 at 04:27
@Martin'sRun not really, my professor just told us that the names stored in the arraylist should be stored in sorted order by the compareTo() method — user10649856, Oct 03 '19 at 04:44
Comparable and the compareTo method defines the ordering, whats smaller and whats bigger, but sorting has to be done separately if you are to use an ArrayList. Since collections.sort is not allowed I assume other alternatives like list.sort or stream.sorted aren't allowed as well? — Martin'sRun, Oct 03 '19 at 04:55
Looks like the assignment is intended to use your own sorting with comparable. — Martin'sRun, Oct 03 '19 at 05:48

score 0 · Answer 1 · answered Oct 03 '19 at 09:24

The compare to method returns 1 if the item is "larger", 0 if they are equal or -1 if the item is "smaller".

This should work:

int compareTo (PhoneBook o) {
    for(int i = 0; i < first.size(); i++) {
        if (i >= o.first.size())
            return 1;

        if ((int)first.charAt(i) > (int)o.first.charAt(i))
            return -1;

        else if ((int)first.charAt(i) < (int)o.first.charAt(i))
            return 1;
    }

    for(int i = 0; i < last.size(); i++) {
        if (i >= o.last.size())
            return 1;

        if ((int)last.charAt(i) > (int)o.last.charAt(i))
            return -1;

        else if ((int)last.charAt(i) < (int)o.last.charAt(i))
            return 1;
    }

    return 0;
}

This just checks if the ASCII value is larger. (All letters have to be lower- or uppercase)

How do I sort an ArrayList of strings in alpabetical order using a compareTo method?

1 Answers1