unchecked call to compare(T,T) as a member of the raw type Comparator

Question

We are migrating our existing code from Java 1.5 to Java 8, and while compiling with -Xlint, I came across warnings like "unchecked call to compare(T,T) as a member of the raw type Comparator".

I roughly understand that I need to somehow tell which datatype I'd be passing in to Comparator, but I'm using generics so I can pass in different datatypes, so how can I eliminate this warning.

Here's a snippet of my code where I'm seeing this warning:

private static void  mergeSort(Object[] source, int[] src,int[] dest,
                                        int low, int high, int off,  Comparator c) {
        int length = high - low;

        for (int i=low; i<high; i++)
                for (int j=i; j>low && c.compare(source[dest[j-1]], source[dest[j]])>0; j--) {
                    int t =  dest[j];
                    dest[j] = dest[j-1];
                    dest[j-1] = t;
                }
            return;
}

And, this is being called as:

public void setSort( Comparator<T> comparator) {
List<T> currentList = getCurrentList();

        int[] dst = new int[currentList.size()];
        for (int i=0; i< currentList.size(); i++)
            dst[i]=i;
        int [] src = dst.clone();
        mergeSort (currentList.toArray(), src, dst,0, dst.length, 0,  comparator );
}

Since I'm already saying Comparator<T> comparator in setSort() method, why am I still getting the below error?

warning: [unchecked] unchecked call to compare(T,T) as a member of the
raw type Comparator  [multiant]               for (int j=i; j>low &&
c.compare(source[dest[j-1]], source[dest[j]])>0; j--) {

Answer 1

As far as I can tell, you have to change the definition of mergeSort:

public static <T> void mergeSort(Object[] source, int[] src, int[] dest,
        int low, int high, int off, Comparator<T> c) {

And than you have to do a cast:

 @SuppressWarnings("unchecked")
 T[] sourceC = (T[]) source;

You know that the array is of type T but you can't declare a generic array. But that's ok, since T is going to be erased to Object at runtime either way.

And the for loop slightly changes to use that:

for (int j = i; j > low && c.compare(sourceC[dest[j - 1]], sourceC[dest[j]]) > 0; j--)