Java array list returns a double with the highest integer

I have a DrugUseYear class that contains a calculation method for the average drug use per month. The months are in an enumeration class.

An attempt to find the highest value within the resources found.
I understand why my code is wrong, but I'm not sure how to fix it.

public class precipitation {
Private DrugUseYear[] DrugUseYears;

public double calculateHighestMeanAnnualDrugUse () {
Arrays.sort (DrugUseYears);
double maxValue = DrugUseYears[DrugUseYears.length - 1];
return maxValue;