Question

I have an array of a custom type that I want to sort by one of its String attributes. For some reason, the following code is producing wrong results. Could you point out where I might have made a mistake?

class PatientLNComparator implements Comparator<Patient>{
        @Override
        public int compare(Patient p1, Patient p2) {
            String p1_LN = (p1 == null) ? null : p1.last_name;
            String p2_LN = (p2 == null) ? null : p2.last_name;

            if(p2_LN == null)
                    return -1;
            else if(p1_LN == null)
                    return +1;
            else if(p1_LN.equals(p2_LN))
                    return 0;
            else if(p1_LN.compareTo(p2_LN) > 0)
                    return -1;
            else
                    return +1;
        }
}

Answer 1

One problem to start with – your comparator is inconsistent if you give it two patients with null names, or two null patient references. In particular:

Patient p1 = null;
Patient p2 = null;

int x = comparator.compare(p1, p2);
int y = comparator.compare(p2, p1);

The signs of x and y ought to be different – but they’ll both be -1.

After that, it depends on how you want to compare the names. I would usually use

return p1_LN.compareTo(p2_LN);

if you want to sort in ascending order. Note that to sort in descending order you shouldn’t just return -p1_LN.compareTo(p2_LN), as if the comparison returns the Integer.MIN_VALUE, the negation won’t work. Instead you’d want to return p2_LN.compareTo(p1_LN);.

Note that if you’re using this scheme, you don’t need to call p1_LN.equals(p2_LN) either – that will be handled by the compareTo call.