The findById() method can be used to query using the primary key. The following snippet shows an example:

    @Test
    public void findOne() {
      Optional<Todo> todo = todoRepository.findById(101L);
      assertEquals("Todo Desc 1", todo.get().getDescription());
    }

Optional represents a container object for an object that can be null. Some of the important methods in Optional are listed below:

• isPresent(): Check if Optional contains a non-null value.
• orElse(): Default value if the object contained is null.
• ifPresent(): Code in ifPresent is executed if the object contained is not null.
• get(): To retrieve the contained object.

The existsById() method can be used to check whether an entity with the given ID exists. The following example shows how it can be done:

    @Test
    public void exists() {
      assertFalse(todoRepository.existsById(105L));
      assertTrue(todoRepository.existsById(101L));
    }

The deleteById() method is used to delete an entity with a specific ID. In the following example, we are deleting one of the todos, reducing the available todos from three to two:

    @Test
    public void delete() {
      todoRepository.deleteById(101L);
      assertEquals(2,todoRepository.count());
    }

The deleteAll() method is used to delete all the entities managed by the specific repository. In the specific example here, all the todos from the todo table are deleted:

    @Test
    public void deleteAll() {
      todoRepository.deleteAll();
      assertEquals(0,todoRepository.count());
    }

The save() method can be used to update or insert an entity. The following example shows how the description of a todo can be updated. The following test uses TestEntityManager to flush the data before retrieving it. TestEntityManager is autowired as part of the functionality of @DataJpaTest Annotation:

    @Autowired
    TestEntityManager entityManager;
    @Test
    public void save() {
      Todo todo = todoRepository.findById(101L).get();
      todo.setDescription("Todo Desc Updated");
      todoRepository.save(todo);
      entityManager.flush();
      Todo updatedTodo = todoRepository.findById(101L).get();
      assertEquals("Todo Desc Updated",updatedTodo.getDescription());
     }