Collections.sort() error

Question

I am trying to sort a list of type A named BinOrder in class B according to Class A\'s int r.

However i am receiving this error for the line Collections.sort(BinOrder);<

Answer 1

To be able to use the single-argument version of Collection.sort() on an ArrayList of A, A should implement the Comparable interface:

public class A implements Comparable<A> {
  ...
  @Override
  int compareTo(A rhs) {
    ...
  }
}

Answer 2

Here's the signature of Collections.sort :

public static <T extends Comparable<? super T>> void sort(List<T> list)

A must implement Comparable for this method.

You try to pass BinOrder to this method, when BinOrder is of type ArrayList<A>, but since A does not implement Comparable<A>, it doesn't fit the signature of the method.

Either change A to implement Comparable, or use the sort method that accepts a Comparator :

public static <T> void sort(List<T> list, Comparator<? super T> c)