Question

now I have been at this for a while and there is an error I am having. Now the program I am making is an address book, and I am using an insertion sort to sort an arraylist of objects which I call books(address entries). Now I soon discovered that my sorter is not sorting properly so I made a simple program to test the sorter and again it does not work. I was wondering if you guys could have a look at it and help me out.

Here is my Sorter:

import java.util.ArrayList;
public class Sorts {

    /**
     * Sorts and array of integer from low to high
     * pre: none
     * post: Integers has been sorted from low to high
     */
    public static void insertionSort(ArrayList<String> test) {
        Comparable temp;
        int previousIndex;
        ArrayList<String> objectSort = test;

        for (int i = 1; i < objectSort.size(); i++) {
            temp = objectSort.get(i);
            previousIndex = i - 1;

            while ((objectSort.get(previousIndex).compareTo((String) temp)) == 1 && (previousIndex > 0)) {
                objectSort.set(previousIndex + 1, objectSort.get(previousIndex));
                previousIndex -= 1; //decrease index to compare current item with next previous item
            }
            if (objectSort.get(previousIndex).compareTo((String) temp) == 1) {
                /* shift item in first element up into next element */
                objectSort.set(previousIndex + 1, objectSort.get(previousIndex));
                /* place current item at index 0 (first element */
                objectSort.set(previousIndex, (String) temp);
            } else {
                /* place current item at index ahead of previous item */
                objectSort.set(previousIndex + 1, (String) temp);
            }

        }
    }
}

My simple program to test it is:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args){
        ArrayList<String> test = new ArrayList<String>();

        test.add("Roxy");
        test.add("Proxy");
        test.add("Moxy");
        test.add("Samuel Adams");

        Sorts.insertionSort(test);

        System.out.println(test);

    }

}

To sum it up, I am having troubles with my ArrayList sorter. The problem is it wont sort correctly and I do not know why. Thank you so much in advance. If you have any questions feel free to ask. 🙂

Answer 1

First problem: you’re expecting compareTo to always return 1 for “greater than”. It just returns a value greater than 0 which may be a different positive integer. So both your == 1 comparisons should be > 0.

There may be other problems, but that’s the first one I’d look at.