Test filtering

Diffblue Cover allows users to filter generated tests based on a set of tags associated with each test, in both the IDE plugins and the Web-UI.


Reflection/No reflection

Diffblue Cover may generate tests that use reflection as a way to access parts of the code that are protected. Below are some of the reasons why reflection might be used:

  • Inaccessible private fields or methods
  • Overly complex code
  • A lack of getters and setters

Mocking/No mocking

If Diffblue Cover does not have access to dependent code for a method at analysis time, then it will mock the behaviour of that dependency. This may also include parts of the JDK library which have yet to be modelled by Diffblue Cover.

The following classes will always be mocked:


Assertions/No assertions

There are some scenarios where Diffblue Cover is unable to produce a valid assertion for the test.

  • A void method
  • Methods that do not exhibit side effects
  • Tests where only assertNotNull is used as an assertion

If the test uses thrown.expect in order to check that an exception has been thrown, then this is counted as a valid assertion.


If a method is not fully covered by test(s) generated by Diffblue Cover, then the method may be analysed again in a subsequent "phase". The next phase will use different options in order to try to cover the method fully.

Test star ranking

The star ranking system has been developed to tag tests based on their readabilty, the complexity of the method under test and the quality of the inputs.

1 - 2 stars (★ - ★★)

Generated tests that have been ranked with 1 or 2 stars will include one or more of the traits (as defined above) which may hamper readability:

  • No assertion
  • Mocking
  • Reflection

3 - 5 stars (★★★ - ★★★★★)

Generated tests that have been ranked with 3 to 5 stars are considered to have a good level of readability and only make use of mocking and reflection where necessary. Higher levels of ranking are awarded if the method under test is sufficiently complex and/or the input(s) for that test are of high enough quality (e.g. the input values use human readable strings).

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