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The purpose of the Java Collections API is to provide a well defined hierarchy of interfaces in order to hide implementation details.
Implementing classes must be used to instantiate new collections, but the result of an instantiation should ideally be stored in a variable whose type is a Java Collection interface.
This rule raises an issue when an implementation class:

 is returned from a public method.
 is accepted as an argument to a public method.
 is exposed as a public member.

Noncompliant Code Example

public class Employees {
  private HashSet<Employee> employees = new HashSet<Employee>();  // Noncompliant - "employees" should have type "Set" rather than "HashSet"

  public HashSet<Employee> getEmployees() {                       // Noncompliant
    return employees;
  }
}

Compliant Solution

public class Employees {
  private Set<Employee> employees = new HashSet<Employee>();      // Compliant

  public Set<Employee> getEmployees() {                           // Compliant
    return employees;
  }
}