Error: Bound mismatch, Collections.sort, interface, compareTo Method

I want to compare a new class called CalendarDate. I made an interface Comparable, and I make the public class CalendarDate implementing Comparable. I also wrote a compareTo(CalendarDate Other) method in the class.

However, when I try Collections.sort(birthday), it says Bound mismatch. (It has details: Bound mismatch: The generic method sort(List) of type Collections is not applicable for the arguments (ArrayList). The inferred type CalendarDate is not a valid substitute for the bounded parameter >)

what is wrong with my code? Thanks.

P.S. There is a RED line under the word sort in CalendarDateTest.java.

Comparable.java

package Chapter10;

public interface Comparable<T> {
    public int compareTo(T other);
}

CalendarDate.java

package Chapter10;

public class CalendarDate implements Comparable<CalendarDate> {
    private int month;
    private int day;

    public CalendarDate(int month, int day) {
        this.month = month;
        this.day = day;
    }

    public int compareTo(CalendarDate other) {
        if (month != other.month) {
            return month - other.month;
        }
        else {
            return day - other.day;
        }
    }

    public String toString() {
        return month + "/" + day;
    }
}

CalendarDateTest.java

package Chapter10;

import java.util.*;

public class CalendarDateTest {
    public static void main (String[] args) {
        ArrayList<CalendarDate> birthday = new ArrayList<CalendarDate>();
        birthday.add(new CalendarDate(1, 23));
        birthday.add(new CalendarDate(5, 18));
        birthday.add(new CalendarDate(12, 17));
        birthday.add(new CalendarDate(2, 29));
        birthday.add(new CalendarDate(8, 6));

        System.out.println("birthdays = " + birthday);
        Collections.sort(birthday);
        System.out.println("birthdays = " + birthday);
    }
}