any() vs any(Class.class) Mockito - junit

Question

I am not able to understand why below two tests are not giving the same result.
#Service
public class SomeManager{
private final SomeDependency someDependency;
#Autowired
public SomeManager(SomeDependency someDependency){
this.someDependency = someDependency;
}
public List<returnType> methodToTest(Arg arg){
List<JsonObject> jo = someDependency.search(arg);
return jo.stream().map(returnType::parse).collect(Collectors.toList());
}
}
Test with any(). This Test pass.
#RunWith(SpringJUnit4ClassRunner.class)
public class TestMethodToTest(){
#Test
public void TestMethod(){
SomeDependency someDependency = mock(SomeDependency.class);
List<JsonObject> expected := \some valid list of JsonObject\
// Here I used any() method.
when(someDependency.search(any())).thenReturn(expected);
SomeManager someManager = new SomeManager(someDependency);
List<returnType> actual = someManager.methodToTest(any(Arg.class));
assertArrayEquals(acutal.toArray(), expected.stream().map(returnType::parse).toArray());
}
}
But since search(Arg arg) method of SomeDependency takes parameter of class Arg so I changed above test like this:
#RunWith(SpringJUnit4ClassRunner.class)
public class TestMethodToTest(){
#Test
public void TestMethod(){
SomeDependency someDependency = mock(SomeDependency.class);
List<JsonObject> expected := \some valid list of JsonObject\
// Here I used any(Arg.class) method.
when(someDependency.search(any(Arg.class))).thenReturn(expected);
SomeManager someManager = new SomeManager(someDependency);
List<returnType> actual = someManager.methodToTest(any(Arg.class));
assertArrayEquals(acutal.toArray(), expected.stream().map(returnType::parse).toArray());
}
}
This second test fails with output java.lang.AssertionError: array lengths differed, expected.length=1 actual.length=0.What's the possible reason behind this?
Note: The value expected.length=1 in output depends on what value is provided by the user as valid list of json objects in the test.

Answer 1

The difference stems from the fact that any matches null, while anyClass does not match null. See ArgumentMatchers javadoc:
any() Matches anything, including nulls and varargs.
any​(Class<T> type) Matches any object of given type, excluding nulls.
You are passing null to your method under test here:
List<returnType> actual = someManager.methodToTest(any(Arg.class));
any() returns null which you pass to method under test.
Note that using argument matchers this way is illegal - you should only call them inside calls to when and verify. You should pass a real instance of Arg to method under test.
See Mockito javadoc
Matcher methods like any(), eq() do not return matchers. Internally, they record a matcher on a stack and return a dummy value (usually null). This implementation is due to static type safety imposed by the java compiler. The consequence is that you cannot use any(), eq() methods outside of verified/stubbed method.