单元测试方法 void 参数

主要使用 mockito ArgumentCaptor

    @Test
    public void test_User_Follow_User() {
        //预设 User
        User user = new User();
        user.setId(userId);
        ...

        //模拟调用 findById 方法时返回值
        when(userDao.findById(userId)).thenReturn(Optional.of(user));

        ArgumentCaptor<FollowUser> followUserCaptor = ArgumentCaptor.forClass(FollowUser.class);
        verify(followUserDao, times(1)).save(followUserCaptor.capture());

        Assert.assertSame(user, followUserCaptor.getValue().getUser());
    }