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In this video, we're going to continue from the

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previous video where we learn how we can use

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assert.fail to explicitly fail a test and explore

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where we actually might want to use that mechanism.

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So let's jump in. Now you might be thinking where is

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that going to be useful?

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Why would I want to fail a test?

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Well, you might want to do that for example,

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if you're testing some exception handling.

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So you can imagine having a try

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catch,

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some kind of ValidationError e.

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And if we supply bad input, to the system and

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the test that we're invoking the test method of.

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Imagine this comment here is replaced with an

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actual method call to the class that you're

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testing and you specifically give it bad input. If you

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give it bad input and you expect a ValidationError

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to occur, then you expect that you immediately

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then going to end up in the catch block.

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Which means that if you end up on this line here,

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then you didn't end up in the catch block.

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Which means that the class you're

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testing didn't throw a ValidationError.

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At that point, you might want to fail the test

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and say, something like the validation didn't kick in

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or some more descriptive nice error message than that,

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and obviously ignore the fact that this is red here.

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I've just typed in a random error

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that our fictional API might throw.

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Actually in fact as well, thinking about it

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wouldn't be an error because narrows really like

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a stop the world cup type thing.

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Instead it would be an exception.

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So maybe there's a checked exception which is

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thrown out from this API that you've written.

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So we're just supposing that here. And at

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this point you're probably thinking, but you've just

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told me a few seconds ago, that we can put expected

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equals and then an exception class that we're expecting

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to be thrown and that will make the test pass.

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And that's true. You can do that.

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And then you're probably thinking, well, in which

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case, why would I want to even have

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the try catch in the first place?

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Why would even want to have this catch block?

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And the reason is, because in the catch block you

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might want to do some kind of extra assertions.

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So for example, if you set things on this validation

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exception, if you set different parameters or you

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expect the ValidationException to carry some kind of

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state with it, then maybe you want to do some

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assertions which check, what's been set on it

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and what the state is that it's carrying.

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So that's one example where you could use that and get

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to know how to write better tests with JUnit.

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As you get more experience with it, you'll

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find other opportunities to use this as well.

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But that's just one example which

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we wanted to give you. So let's move on.