Rule Definition
Using the implementation class of a collection as the type of your instance ties your code to this specific implementation and makes it difficult to change the implementation class in the future. For example, if you use the java.util.Collection interface, you can change your implementation class from a Set to a List or to a Queue according to your needs and performance requirements.

Remediation
Declare the type as an interface of a more appropriate collection.

Violation Code Sample

private ArrayList ko_02_08_initializer_declaration; // VIOLATION ,ArrayList is an implementation of the List interface.

Fixed Code Sample

private List ok_01_08_field_declaration = new ArrayList(); //Fixed

http://java.sun.com/docs/books/tutorial/collections/interfaces/index.html