合并排序nullpointer异常

Question

这是我的合并排序算法的合并方法

private void merge(T[] data, int first, int last) {
temp = new Comparable[data.length] ;
        int mid  = (first+last) /2 ;
        int it1 = 0 ;
        int it2 = first ;
        int it3 = mid + 1 ;
        System.out.println("mid = " + mid + " length = " + data.length);//for testing purposes

        while(it2 <= mid && it3 < data.length)
        {
            System.out.println("it2 = " + it2 +" it3 = "+ it3);//for testing purposes
            if(data[it2].compareTo(data[it3]) < 0)
                temp[it1++] = data[it2++] ;
            else
                temp[it1++] = data[it3++] ; 
        }

        while(it2 <= mid)
            temp[it1++] = data[it2++] ; 

        while(it3 < data.length)
            temp[it1++] = data[it3++];  

        for (int i = 0; i < data.length; i++) 
            data[i] = (T) temp[i] ;

但是在调用此合并方法时，我一直收到java.lang.Integer.compareTo（未知源）错误。我不确定我是如何在data[it2].compareTo(data[it3])行收到此错误的，因为while循环确保it2和it3都不会超出范围。

由于