How to mock new instance of class in JUnit 5

Question

I am trying to mock the new instance of CourseLoader in the method I'm testing so when I call loadAll() it returns my mock.

I have setup a mock() in the unit test but it seems to be ignoring what I'm doing, is some of the setup wrong?

Method I'm testing

public ArrayList<Module> getAssignedModulesWithDetails() {
        CourseLoader courseLoader = new CourseLoader();
        ArrayList<Module> assignedModules = new ArrayList<>();
        
        for (Course course : courseLoader.loadAll()) {
            ArrayList<Module> courseModules = course.getModules();
            for (Module module : courseModules) {
                if (module.getInstructor() != null && module.getInstructor().equals(this.getId())) {
                    assignedModules.add(module);
                }
            }
        }

        return assignedModules;
    }

Unit test

@Test
    public void testYouCanGetAssignedModulesWithDetails() {
        CourseLoader courseLoaderMock = mock(CourseLoader.class);
        ArrayList<Course> mockedCourses = new ArrayList<>();

        ArrayList<Module> mockedModules = new ArrayList<>();
        Module module1 = new Module(this.module1, "Module1", true, 4, false, this.id);
        Module module2 = new Module(this.module2, "Module2", true, 4, false, this.id);
        Module module3 = new Module(this.module3, "Module3", true, 4, false, this.id);
        mockedModules.add(module1);
        mockedModules.add(module2);
        mockedModules.add(module3);
        Course mockedCourse = new Course(UUID.randomUUID(), "Mocked Course", true, mockedModules);

        mockedCourses.add(mockedCourse);
        when(courseLoaderMock.loadAll()).thenReturn(mockedCourses);

        ArrayList<Module> assignedModulesWithDetails = this.instructor.getAssignedModulesWithDetails();

        assertEquals(0, assignedModulesWithDetails.size());
    }