Testing multiple interface implementations with same tests - JUnit4

Question

I want to run the same JUnit tests for different interface implementations. I found a nice solution with the @Parameter option:

public class InterfaceTest{

  MyInterface interface;

  public InterfaceTest(MyInterface interface) {
    this.interface = interface;
  }

  @Parameters
  public static Collection<Object[]> getParameters()
  {
    return Arrays.asList(new Object[][] {
      { new GoodInterfaceImpl() },
      { new AnotherInterfaceImpl() }
    });
  }
}

This test would be run twice, first with the GoodInterfaceImpl then with the AnotherInterfaceImpl class. But the problem is I need for most of the testcases a new object. A simplified example:

@Test
public void isEmptyTest(){
   assertTrue(interface.isEmpty());
}

@Test
public void insertTest(){
   interface.insert(new Object());
   assertFalse(interface.isEmpty());
}

If the isEmptyTest is run after the insertTest it fails.

Is there an option to run automatically each testcase with a new instance of an implementation?

BTW: Implementing a clear() or reset()-method for the interface is not really an options since I would not need it in productive code.

Answer 1

Create a factory interface and implementations, possibly only in your test hierarchy if you don't need such a thing in production, and make getParameters() return a list of factories.

Then you can invoke the factory in a @Before annotated method to get a new instance of your actual class under test for each test method run.