Create test for service method that saves an object and returns long value

I am trying to test a service method. The method takes in information, uses this information to create an object. Than it saves this object. Than it returns the id of that object.

When I am trying to test this, my test keeps failing, stating that the returned object is null. I am wondering how I can test this. At this
point I am trying to mock the generated object with an getId() that returns 1L. Than I let the save method return the mocked object. Also this doesn't seem to work.

I could really use some help how I should properly make a test for this method.

Example method:

public long addNotification(ObjectWithInformation objectWithInformation){
       NewObject newObject = // retrieve information based on objectWithInformation
       repository.save(newObject);
       repository.flush();
       return newObject.getId();

}

How I am currently trying to test (test compiles, getId() still returns nullvalue):

@Test
void addNotification(){
     NewObject newObjectmock = mock(NewObject.class);
     Mockito.when(newObjectmock.getId()).thenReturn(1L);
     when(repository.save(Mockito.any(NewObject.class))).thenReturn(newObjectmock);

     service.addNotification(ObjectWithInformation objectWithInformation);

     //verify methods, which I cannot reach due to getId() returning null in the service method.

}

You cannot perform tests on an object generated inside a function, nor can you mock it in advance. This can only be achieved with functions that accept object as parameters and return those objects.

You're mock is correct, but junit cannot train the newobject inside the function to return an id. So you have to train your repository to return a mock object with the id (this is what you did correct). Your function has a between step, you pass the trained method and return a null value cause the newObject holds no id value. You will have to return the id immediatly in your function.

Also use saveAndFlush instead of save() and flush(). Both will work but the first is cleaner and works better for the test.

this will make it work:

when(repository.saveAndFlush(Mockito.any(NewObject.class))).thenReturn(newObjectmock);

public long addNotification(ObjectWithInformation objectWithInformation){
       NewObject newObject = // retrieve information based on objectWithInformation
       return repository.saveAndFlush(newObject).getId();
}