pit mutation - if ( x !=null ) return null else throw new RuntimeException

Question

I have a method that returns a custom object

public MyObject getTheObject(){
  ...
  return muObject;
}

its unit test checks that the object returned by getTheObject() method is not null

@Test
public void testGetTheObject(){
  ...
  assertNotNull(actualObject);
}

and test passes.

When running mutation test with Pitest it shows one SURVIVED mutation which says something like this:

mutated returned of Object for value for ..../getTheObject to ( if ( x!= null ) null else throw new RuntimeException )

The question is what should our unit test look like to get rid of this issue, and KILL that mutation

Answer 1

I cannot reproduce this problem (i.e. same mutation is correctly KILLED). I don't think your question includes enough context (other "pitest didn't work" SO questions seem to revolve around use of Spring and/or Mockito both of which also amend bytecode/make additional classes)

https://stackoverflow.com/help/minimal-reproducible-example

• Gradle 5.2.1
• JUnit 5
• Pitest 1.4.5

    public class SO55104467 {
      public static class MyObject {}
      private final MyObject muObject = new MyObject();
      public MyObject getTheObject(){
        return muObject;
      }
    }

    class SO55104467Test {
      private SO55104467 sut = new SO55104467();
      @Test
      public void testGetTheObject(){
        SO55104467.MyObject actualObject = sut.getTheObject();
        assertNotNull(actualObject);
      }
    }

Answer 2

It is "return value mutator" of PIT (when return type is Object) and it is going to replace non-null return values with null and throw a java.lang.RuntimeException if the unmutated method would return null.

i.e mutated code:

public MyObject getTheObject(){
  ...
  return null;
}

Above mutator is expecting that your Junit should fail but for Runtime exception reason. assertNotNull(getTheObject()) is just to check whether return object is null or not and there is no way to get Runtime exception just by checking object value.

To kill this mutator you can call any method of that object (e.g getter()). Now if you call any method over null object (which is return by above muted code) then you will get null pointer exception (i.e Runtime exception) and your Junit will fail for muted code.

Ans:

assertThat(getTheObject().getterOfAnyField(),is(expectedValue));

Or

assertNotNull(getTheObject().getterOfAnyField());