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)"

Solution

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> {

}

Comments

[ever alian]

java.util.Comparable ? java.lang.comparable right?