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code-style IllegalType (Checkstyle)

Avoid using implementation types (i.e., HashSet); use the interface (i.e, Set) instead

import java.util.*;
public class Bar {

 // should be "private List list"
 private ArrayList list = new ArrayList();

 // should be "public Set getFoo()"
 public HashSet getFoo() {
  return new HashSet();
 }
}

Rationale: Helps reduce coupling on concrete classes. In addition abstract classes should be thought of a convenience base class implementations of interfaces and as such are not types themsleves.