Generic Issue in LocalDate

Question

I have the following code. I was expecting Arrays.asList(LocalDate.of(2014, 1, 1), LocalDate.of(2015, 2, 2)) to convert into the list of LocalDate. But I am getting compile time error in max(dates). What might be the cause of this error?

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class C {

    public static <T extends Comparable<T>> T max(List<T> d) {
        if (d.size() == 0) {
            return null;
        }

        T maxValue = d.get(0);
        for (T temp : d) {
            if (maxValue.compareTo(temp) < 0) {
                maxValue = temp;
            }
        }

        return maxValue;
    }

    public static void main(String[] args) {
        List<LocalDate> dates = new ArrayList<>();
        dates.add(LocalDate.of(2015, 9, 15));
        dates.add(LocalDate.of(2010, 9, 1));

        max(dates);     //This line generates compile error.
        max(Arrays.asList(LocalDate.of(2014, 1, 1), LocalDate.of(2015, 2, 2)));
    }
}

Compile Time Error: The method max(List) in the type C is not applicable for the arguments (List)

Answer 1

LocalDate implements Comparable<ChronoLocalDate> and not Comparable<LocalDate>

Therefore change:

public static <T extends Comparable<T>> T max(List<T> d) { ... }

to:

public static <T extends Comparable<? super T>> T max(List<T> d) { ... }