Java - Radix Sort Incompatible types?

Question

For my class we have to write the Java code for the Radix Sort algorithm. I understand the algorithm, and I thought I had the solution, but I'm getting an incompatible types error in my code and I don't understand why.

    import java.util.*;

    public class RadixSort {

        public static void radixSort(int[] list) {

        // will be passed to getKey to decide what to divide the number by before %10
        int div = 1;

        // repeat once for the max number of digits of the numbers
        for (int i = 0; i < 3; i++) {
            ArrayList bucket[] = new ArrayList[19];

            // distribute the elements from the list to the buckets
            for (int j = 0; j < list.length-1; j++) {
                int key = getKey(list[j], div);

                // add 9 so that if key is negative, it will now be 0 or positive so an out of bounds exception isn't thrown
                // so bucket[0] means the key was -9, and bucket[18] means the key was 9
                if (bucket[key+9] == null)
                    bucket[key+9] = new ArrayList();
                bucket[key+9].add(list[j]);
            }

            // move the elements from the buckets back to list
            int z = 0;
            for (int x = 0; x < 19; x++) {
                if (bucket[x] != null) {
                    for (int y: bucket[x]) {  // incompatible types???
                        list[z] = y;
                        z++;
                    }
                }
            }
            // multiply div by 10 for the next run-through
            div = div*10;
        }
    }

    public static int getKey(int i, int j) {
        return (i/j) % 10;
    }

    // test method
    public static void main(String[] args) {
        int[] list = {922, 243, 409, 885, 96, 21, -342, 119, 540, 12, -732, 8, -3, 2};
        radixSort(list);
        for (int i = 0; i < list.length; i++)
            System.out.print(list[i] + " ");
    }    
}

I marked the line where I'm getting incompatible types. I don't want just how to fix it, but why the types are incompatible. I genuinely have no idea and I want to understand why. Thanks for any help.

Answer 1

You declared bucket as a List of Object items, and an Object item can't be cast to an int value. So it throw a compilation error there. To fix the error, you can use Generics as @Elliott mentioned, or rewrite the code there as follow:

for (Object y: bucket[x]) {
   list[z] = (Integer) y;
   z++;
}

Answer 2

In order to use Generics you will need to type your ArrayList, that is

ArrayList<Integer>