Error Message: The method sort(List<T>) in the type Collections is not applicable for the arguments (ArrayList<Date>)

Question

Keep getting error message, but don't know why. Can't get code to sort the list using Collections.sort()

Here is what I have. 3 java files.

The Interface file.

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

The Class file.

public class Date implements Comparable<Date>{

private int year;
private int month;
private int day;

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

public int getYear(){
   return this.year;
}

public int getMonth(){
   return this.month;
}

public int getDay(){
   return this.day;
}
public String toString(){
   return month + "/" + day + "/" + year;
}


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

}

The Client class

import java.util.*;

public class DateTest{
   public static void main(String[] args){

  ArrayList<Date> dates = new ArrayList<Date>();
  dates.add(new Date(4, 13, 1743)); //Jefferson
  dates.add(new Date(2, 22, 1732)); //Washington
  dates.add(new Date(3, 16, 1751)); //Madison
  dates.add(new Date(10, 30, 1735)); //Adams
  dates.add(new Date(4, 28, 1758)); //Monroe     


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

}




}

The error message I get is "The method sort(List) in the type Collections is not applicable for the arguments (ArrayList)"

Answer 1

Because Collections.sort expects java.lang.Comparable and not your Comparable interface, change your Date class to implement the java.lang.Comparable.

public class Date implements java.lang.Comparable<Date>{
 ..
}

If you still want to define your own Comparable for some reasons and you still want to use Collections.sort then the your Comparable has to be java.util.Comparable

interface Comparable<T> extends java.lang.Comparable<T> {

}