Below we have a Student class with Private Class Variable(id and name), Service and Private Methods.

Student.java

public class Student {
    private Integer id;
    private String name;

    @Autowired
    private StudentService studentService;

    private String getStudentDetails(){
      return "id: " + getId() + "; name: " + getName();
    }  
}

Mocking a Private Field
we cannot access the private field id to assign a value for testing, because there isn’t a public setter method for it.
We can then use ReflectionTestUtils.setField method to assign a value to the private member id

StudentTest.java

@Test
public void setValueWithNoSetter() {
    Student student = new Student();
    ReflectionTestUtils.setField(student, "id", 101); 
    assertTrue(student.getId().equals(101));
}

Mocking a Private Method
In a similar way we can invoke private method – getStudentDetails() as below in student class
StudentTest.java

@Test
public void whenNonPublicMethod_thenReflectionTestUtilsInvokeMethod() {
    Student student= new Student ();
    ReflectionTestUtils.setField(student, "id", 101);
    ReflectionTestUtils.setField(student, "name", "Mugil");
 
    assertTrue(ReflectionTestUtils.invokeMethod(student, "employeeToString").equals
                 ("id: 101; name: Mugil"));
}

Mocking a Private Dependencies
StudentTest.java

@Test
public void whenNonPublicMethod_thenReflectionTestUtilsInvokeMethod() 
{
   Student student = new Student();
   StudentService studService = mock(StudentService.class);
   when(studService.getStudentStatus(student.getId())).thenReturn("Active");

   ReflectionTestUtils.setField(student, "studentService", studService);

.
.
}