Collection VS ArrayList.

[Suresh Rajadurai]

Hi Folks,

I compiled the following 2 java programs, the first one gives compilation error, the 2nd one compiles fine.
The first program, the compiler does not recognize the command "Collections.sort(s)". Could anyone please throw some light on it please.

Program 1:

import java.util.*;

class test
   {
	public static void main(String arg[])
	   {
		Collection s = new ArrayList();
		s.add("B");
		s.add("C");
		s.add("A");
		Collections.sort(s);
		System.out.println(s);
	   }
   }

Program 2:

import java.util.*;

class test
   {
	public static void main(String arg[])
	   {
		ArrayList s = new ArrayList();
		s.add("B");
		s.add("C");
		s.add("A");
		Collections.sort(s);
		System.out.println(s);
	   }
   }

Thanks in advance.

Suresh

[Dawn Charangat]

Hello Suresh..,

The sort() method in Collections utility class takes in only objects of the type "List". Just since
the List interface extends from Collection interface doesn't mean you can directly cast it into the method.
May be this could help solve your confusion.

Dawn

[Paul Beckett]

The reason the sort method is applied to a List and not just any Collection is because not all Collection implementations are ordered.

Lists are ordered, that iteration over the elements is guaranteed to return them in a consistent order.

Sets are unordered, that is iteration over the elements is not guaranteed to return them in a consistent order.

[Suresh Rajadurai]

Hi Paul , Dawn,

Thank you so much for the beautiful explanation. Much appreciated.


Regards

Suresh.