How to make this sorting Algorithm work

Hi, I'm pretty new to java and I'm trying to work on this sorting algorithm question.

here's the main bit:

Code:

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public static void chooseSongs(ArrayList<Song> l, int k)  {
                int []tmpArray = new int[a.length];
                mergeSort(a,tmpArray,0,a.length - 1);
        }

        private static void mergeSort(int[]a, int[]tmpArray, int left, int right) {
                if( left < right) {
                        int centre = (left + right) /2;
                        mergeSort(a,tmpArray, left, centre);
                        mergeSort(a.tmpArray, centre + 1, right);
                        merge(a, tmpArray, left, centre + 1, right);
                }
        }

        private static void merge(int[]a, int[]tmpArray, int leftPos, int rightPos, int rightEnd){
                int leftEnd = rightPos - 1;
                int tmpPos = leftPos;
                int leftBeg = leftPos;

                while(leftPos <= leftEnd && rightPos <= rightEnd){
                        if(a[leftPos] < a[rightPos]){
                                tmpArray[tmpPos++] = a[leftPos++];
                        }else{
                                tmpArray[tmpPos++] = a[rightPos++];
                        }
                }

                while(leftPos <= leftEnd){
                        tmpArray[tmpPos++] = a[leftPos++];
                }

                while(rightPos <= rightEnd){
                        tmpArray[tmpPos++] =a[rightPos++];
                }

                for(tmpPos = leftBeg; tmpPos <= rightEnd; tmpPos++){
                        a[tmpPos] = tmpArray{tmpPos};
                }
        }

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ummm it looks long and tedious but it's basically just the merge sort algorithm I've been given.

The code is using arraylist but to make it to work I'll have to change it toarrays. Because I'm a newbie and my java skill is poor I don't really know what to do. Any help would be great.

ps: I've attached the code for the whole thing if the above looks confusing (this application is suppose to sort a list of songname, artist and time into order)

Try debugging it by adding println() statements at key points and watch how control flows. use a short list of records to sort.