WEBVTT 00:00.150 --> 00:03.120 Hello again! In this video, we are going to look at error handling. 00:04.590 --> 00:07.080 We are not talking here about bugs in your code. 00:07.440 --> 00:12.150 These are things that can go wrong at run time, when your program interacts with the real world. 00:12.750 --> 00:17.550 So, for example, if you try to read data from a file and the file does not exist. If you are 00:17.550 --> 00:23.580 trying to send data over a network and the connection is down. If you receive data which is incorrect, 00:23.610 --> 00:25.200 or in the wrong format. And so on. 00:25.740 --> 00:31.770 So these are situations which arise at run time, when the environment does not behave in the way that 00:31.770 --> 00:33.150 your program is expecting. 00:35.970 --> 00:41.700 If we are just writing toy programs, we can print out a message and say goodbye, but if we want to 00:41.700 --> 00:45.900 watch professional quality programs, we need to do a lot better than that. 00:50.170 --> 00:53.260 When an error occurs, we need to communicate with the user. 00:53.650 --> 00:57.250 We need to say what the problem is, and what actions the user can take. 00:57.880 --> 00:59.500 Normally there are three possibilities. 01:00.070 --> 01:00.880 They can try again. 01:01.540 --> 01:05.590 They can do something to try and resolve the problem, or they can just ignore it and carry on. 01:06.920 --> 01:10.670 We may also need to inform other concerned parties. 01:11.150 --> 01:16.370 For example, if our company's connection to the internet is down, we may need to notify the network 01:16.370 --> 01:17.120 management team. 01:18.980 --> 01:21.810 We should avoid giving information which is not helpful to the user. 01:21.830 --> 01:27.770 So things like error numbers, or Java stack traces. which may be helpful to the developer, but they are 01:27.770 --> 01:31.010 completely unhelpful to the person stuck in front of the computer. 01:31.460 --> 01:35.660 If the situation really is beyond help, then we can exit the program. 01:35.870 --> 01:37.490 But that really should be your last resort. 01:40.310 --> 01:41.630 Where should we handle errors? 01:43.100 --> 01:49.220 The obvious answer is to handle the error where it occurs. And quite often that is going to be good enough. 01:49.790 --> 01:54.800 For example, if you are asking for user input and they type in something which is in the wrong format, 01:55.160 --> 01:56.270 we can ask them to try again. 01:57.290 --> 01:59.930 But there are situations where it can be useful to handle 01:59.930 --> 02:02.840 errors in a different part of the code where it occurs. 02:05.260 --> 02:09.700 As an example, if we are downloading a file, so we have some networking code, at a low level, 02:09.700 --> 02:11.860 which is doing the actual fetching of the data. 02:12.340 --> 02:17.230 And suppose that when something goes wrong with the networking code, we want to display a dialogue box. 02:18.730 --> 02:23.800 If we decide to handle the error where it occurs, that means that every time there could be an error, 02:24.130 --> 02:27.160 we need to write some code to display the dialogue box. 02:27.700 --> 02:32.200 So this means we are going to have a lot of GUI code mixed in with our networking code, which is going 02:32.200 --> 02:33.940 to make the code more complicated. 02:34.720 --> 02:38.890 If we are trying to follow the logic of the networking, that is going to be difficult, because there is 02:38.890 --> 02:42.970 all this user interface code that keeps jumping up and distracting us. 02:44.980 --> 02:49.920 And if we adopt this approach for every feature that our program has, then we are going to have error handling 02:50.260 --> 02:51.440 spread all over the program. 02:51.790 --> 02:57.610 There is probably going to be a lot of duplicated code. And also, it is going to be hard to work out where 02:57.610 --> 03:00.580 the code that produces a particular dialog box comes from. 03:01.090 --> 03:03.910 So this is going to be code which is going to be hard to maintain. 03:07.460 --> 03:13.430 So in this situation, a better approach would be to have a specialized part of the code, which just deals with 03:13.430 --> 03:14.090 error handling. 03:14.510 --> 03:19.400 And then when an error occurs, we can send it off to this part of the code. And that will display the 03:19.400 --> 03:21.230 dialogue box, or whatever we need to do. 03:21.920 --> 03:26.990 So that means we need to be able to transfer the information about the error, from the place where the 03:27.230 --> 03:29.450 occurs, to the code which handles it. 03:30.050 --> 03:33.260 And we are going to be talking about how to do that in the rest of this section. 03:34.250 --> 03:35.750 Okay, so that is it for this video. 03:36.170 --> 03:37.040 I will see you next time. 03:37.040 --> 03:39.370 But until then, keep coding!