Find Common Elements In Collection?

I need to create an algorithm that finds the common element(s) in all arrays that has a signature of public Comparable[] findCommonElements(Object[] collection) that has an efficiency of at most O(knlogn), uses a query array, and accepts as input a collection of arrays. I am aware my time would be better spent learning how to use array lists and hash sets, but I am supposed to use concepts already covered, and these have not been.

I feel like this code should work, but it is returning null for the array of common elements. Which means it obviously is not working correctly. I am also likely going to need implementing the sort algorithm, but I wanted to get the part of finding the common elements set first.

public class CommonElements2<T extends Comparable<T>>
{
Comparable[] tempArr;
Comparable[] common;
Comparable[] queryArray;
/*
sort algorithm goes here
*/
public Comparable[] findCommonElements(Object[] collections)

[code]....