Spy

Spy is a variation of a mock/stub, but instead of only setting expectations, spy records the calls made to the collaborator. The following example explains this concept:

class ResourceAdapter{  
  void print(String userId, String document, Object settings) {
    if(securityService.canAccess("lanPrinter1", userId)) {
      printer.print(document, settings);
    }
  }
}

To test the print behavior of the ResourceAdapter class, we need to know whether the printer.print() method gets invoked when a user has permissions. Here, the printer collaborator doesn't do anything; it is just used to verify the ResourceAdapter behavior.

Now, consider the following code:

class SpyPrinter implements Printer{
  private int noOfTimescalled = 0;
   @Override
  public void print(Object document, Object settings) {
    noOfTimescalled++;
  }
  public int getInvocationCount() {
    return noOfTimescalled;
  }
}

SpyPrinter implements the Printer.print() call, increments a noOfTimescalled counter...