30 - Java Reflection API/002 Annotations_en.srt

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Hello, Jim.

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Today we're going to learn with you what annotations are, how to work with them and how to create new

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ones.

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Before we start this lesson, I must say that it is critically important you watching previous lesson

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about Reflection API.

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If you skip that one, please make sure you watch it, because in this lesson, we are going to use

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Réflexion API to work with annotations.

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Basically, there is not so much as a ways for us to take advantage from annotations without knowing

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the Reflection API.

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In this lesson, we're going to learn everything.

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The related to annotations will start from the definition of annotation, make you understand what annotations

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are on the standard banner.

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We are going to discuss use cases when annotations may be applied and for what purposes.

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After that, we will start reviewing of the source code examples.

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I will explain you what targets annotation is and what retention annotation is.

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We are going to create our own custom annotation and you are going to understand how to create it and

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process it.

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And at the end of the lesson, I will also explain you how to create repeatable annotations.

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Let's stop.

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And the first question that we have to answer sounds like this.

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What is annotation?

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Annotation as a form of metadata provides data about programs.

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It is not part of the program itself.

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Annotations have no direct effect on the operation of the court.

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They annotate the understands this definition.

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The know what is metadata.

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Metadata is a data that provides information about other data or similar data about data in program,

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and we can annotate types, colossus and interfaces, fields, masses, parameters, constructors,

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local variables, packages, modules and others.

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We can even annotate annotation itself.

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Basically, there might be cases when you need to provide more information about your code and use this

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information on different stages on the level of the source code during compilation and at this stage

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of the runtime.

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Late in the lesson we are going to learn this case are some examples I will show you where notation

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can be applied and how long it will be retained.

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You already understood what metadata is and what annotation is, but probably it is still not clear

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for you how this can be helpful and when we need to use annotations in which cases.

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The first one is information for the compiler.

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Annotations can be used by the compiler to detect errors or suppress warnings, for example, such popular

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annotation as override.

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I am sure you used it already is on purpose or it just added it instead of you overwrite annotation

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discarded by the compiler during the compilation.

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And there is a specific purpose of this net information.

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As you remember, annotation tells us that this method should override method from parent type.

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Thus, there are specific requirements to access modify which that matter signature and the list of

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potential exceptions.

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There is a logical place in juju that analyzes this matter information basically for all methods marked

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with overrides, this logic is applied.

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Compile time and deployment time.

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Processing software tools can process annotation information to generate code, XML files and so forth.

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For example, such book in the library and do as number in case you have never worked with.

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No.

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I will tell you about it.

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It generates getters and setters for you and also helps you with generating other things.

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This mechanism also built on top of annotations and reflection API.

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Basically, you can annotate type was information about methods if you need to be generated, for example.

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And when you compile your application and upload classes into Java, those masses that you want it to

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be generated are there on place.

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Is that clear?

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And that's a use case of using annotations is runtime processing.

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Some annotations are available to be excellent at runtime.

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You can set configuration to retain annotations.

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Zero zero runtime I will show you is this an example?

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But let's understand why do we need this?

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You may need to keep annotations during the runtime to build some logic around this meta information.

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You're in the program execution.

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Your homework will be related to this case.

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I believe that we have learned enough basics to start review and examples.

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Anyway, I will explain examples as we go and the true.

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Who believes that understanding of this topic will be easier with source code and the real good examples,

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rather than with slides for the sake of the demo and this lesson, I created a separate package and

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separate them a file.

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You can find them a file in annotations package.

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Let's start from basics.

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Let's discuss annotation format.

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Each annotation is started with at sign, followed by some meaningful name.

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Z Sign character indicates as a compiler that what follows is an annotation.

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The annotation can include elements which can be named and zem values.

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For those elements, elements may contain additional information like in this case, you can see that

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are used to suppress warnings.

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Annotation and element value has value types.

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This is basically for compiler to remove warnings for parameterized, at least in case I would Corman's

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this annotation.

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There will be warnings you as a compilation that I am using parameter types without parameter Ryzens.

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You see this OK and other possible syntax of the similar annotation in case there is only one element

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is declared inside the annotation.

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You can omit element key and just put value like in this example.

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It is also possible to use multiple annotations at once and from Java version eight zero even supports

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of repeatable annotations.

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As you can see in this example, a little bit later in our lesson, one will comp the creation of our

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custom annotations.

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I will explain you how to create repeatable annotations.

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Let's open the source code of any annotation.

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For example, let's look at suppress warnings annotation.

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As you can see, there is no end in logic and code here.

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There is no any algorithm or data processing or anything else.

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Some students ask me then how annotations work, how overwrite annotation works, and all this basically

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annotations are nothing more than just an information and what you can do.

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Learn this information.

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This is up to you, and this logic is programmed in a separate place.

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OK, that was annotations basics.

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Now, you know, at least on the high level, what annotations are us?

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Know known annotations deeper and to learn them better?

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Let's create our own annotation.

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If you are already familiar with G-Unit will implement our own kind of test annotation if you are not

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familiar with your units yet.

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Just a few words.

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G-Unit is a framework for unit testing.

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It provides a lot of different features and the Mournes impossibility to run specific methods separately

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to execute separate test cases.

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The purpose of that massive is to reproduce the state of the system as a specific moment of time, and

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the Cert is expected to be here.

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All examples will be stored under this package, so you will easily found them in attachments to this

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lesson in case you want to run examples on your computer.

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Let's start.

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I create test annotations.

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First, I declare annotation in a separate file, and you can see in this example that the use interface

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here was at sign.

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The second important step is to add matter annotations to specify the scope and the target of our custom

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annotation.

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I'm talking about annotations, zero retention and target.

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Let's run.

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What are they?

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So what do we need to target annotation for?

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This annotation indicates a context in which an annotation type is applicable.

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In simple words, where you can use the sanitation target, annotation can take array of any values

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of element type enum.

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So even in case you forgot where you can use annotations.

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Just open element type in them and you would find enum for each possible placement of your annotation.

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Even despite the fact is, a on annotation may take array of elements in our specific example, we would

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use only one in a message because in our specific business context, we don't want to allow our client

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use it in other places is an exhortation is retention.

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This information indicates how long annotations was annotated.

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Time are to be retained.

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That is exactly what we have discussed at the beginning of our lesson.

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When I explained that annotation may be used during Zabeel time by the compiler and discarded from the

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class on the glass level but discarded during the anthem, one class is uploaded in.

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And during runtime, this annotation can take any value of retention policy in them, even in case you

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forgot what's retention levels for annotations are, you always can open the source code of this in

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them and explore this.

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As I said, there's three possible options here.

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They are source Closs, and one time in our case, we use runtime retention policy because we need this

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annotation to be retained.

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That's a runtime.

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Now, at this time, to add custom elements, imagine that you want to specify expected exception in

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order we can test negative scenarios, for example, in case we expect exception to be thrown.

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But there was no any exception.

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Drone zones This particular test should be considered as field and the stanzas declare an element in

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annotation.

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We have to declare mass was the name of the element itself.

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Return type will be the type of the element.

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For example, in this specific case, we would like to specify loss of the expected exception.

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Thus, returns type for this message will be close when we declare this matter.

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We should remember important things, since this massive can throw an exception.

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There is a restriction on the return types.

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Basically, you can return on your primitives string class in them, annotations and arrays of these

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types.

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Also, this must has default value.

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For example, in this particular case, default exception, to be certain is of non plus.

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This is our custom clause that extends Roble and that basically is empty.

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This is needed because I can't have default no values.

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But in case I wouldn't specify, a default value client will be forced to parse the value for this element.

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But what clients should Boston in case there is no expected exception?

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That's why I have default non clause here.

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Similar example of default value in our other element.

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Each test can be named and default name is empty string.

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But the client of this code can pass any custom name.

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One of the restriction was regards the default value.

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It can't be null.

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Is that clear?

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We are done with annotation creation.

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Let's now understand how to use it.

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As you can notice, there is no logic defined in the annotation.

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That means that I should have some place where logic for annotation process is described in this particular

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example.

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It can be some tests around the class similar to the G unit, where we have separate runners that can

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execute my tests.

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I open tests around the class.

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It has run tests massive that takes class.

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It contains tests to run as mass argument.

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I use this reference to a class object to get the array of all masses.

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After that, I feel the masses to keep in-stream only masses that have test annotation.

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After that, I filter all static masses to keep only non static masses in the stream.

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And now I need to call test my asset on the object.

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I create new instance using test class.

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After that, I need to speak logic for annotations that have expected exception and those who don't.

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You remember that annotations results?

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Bits of fired by client exception still has exception was not by default.

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I use this information during implementation of my logic because in case I expect any exceptions specified

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by client other than default, I need to check that the expected exception has been thrown come back

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losses of exceptions thrown.

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Here is the logic for that.

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By the way, in a reflection API in case exception has been thrown on the invoked received in the invocation

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target exception that serves as a wrapper for a regional exception.

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That's why I need to get course massive here to get the reference to the exception that caused invocation

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target exception in case I don't expect exception to this rule.

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Massad just should be executed successfully, and I just invoke mass differently.

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This is not the full replica of test annotation from G-Unit, but we have a goal in this lesson not

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to run G-Unit, but to understand how to work with annotations and process zap.

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Now let me show you class with tests.

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I open the glass, such as code test samples.

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This class contains.

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Is different mascots that are marked with test annotation.

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Each test has name for the sake of the current demo.

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I have two test cases here mascots that is executed without exception and massive that throws exception

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of expected type.

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Nothing complex here.

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And now let's write a code that will use my test runner to run these test cases.

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I open another clause at the so-called test annotation demo.

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I read instance of test runner.

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I could run test method and the person's glass that contains my test cases that so let's run this program

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and console.

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You can see that test without exception has been passed because we received exception that we expect

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and test without exception also has been passed because it was executed without any exception.

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I recommend you to look through the source code that I left in attachments to the lesson.

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Run it on your local computer to understand better how it works.

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And the last thing for the desert I'd like to share with you is repeatable annotations.

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To be honest, in my opinion, this is a very rare case because in case you need to pass multiple values,

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you always can define additional elements in the original annotation or declare array of values for

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some element.

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Anyway, it was hard for me to come up with real life example in this case, but I just want to demonstrate

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use a feature that was added in Java version eight.

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Let me open our demo example.

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Can they see that I have Orser annotation here duplicated?

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Normally it is not accepted, and to be able to use the same annotation twice for the same element,

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you need to perform additional actions.

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The first thing to do is to add annotation, repeatable annotation definition, like I did for Orser

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annotation.

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The second thing to do is to create container for repeatable annotations for compatibility reasons.

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Repeating annotations are stored in a container annotation.

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In order for the compiler to do this, two declarations are required in your code.

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That's why another annotation that we need to create is here.

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I call it Auster's container annotation must have element value that returns array with objects of repeatable

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annotation type that we want to repeat.

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In this case, value element will contain array of Orser annotations.

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Is that clear?

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Basically, that's all what I wanted to share with you.

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Let's review what we have learned in this lesson.

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In this lesson, we have learned what annotation is also explained to you, why we might need an addition

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and when we can use it.

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Now, you know where annotations may be applied.

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You know why we need targeted annotation and you know what element type in them is.

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Also, you learned what retention annotation is and what retention policy in them is.

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In the lesson.

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We created our custom annotation, and at the end of the lesson, I explained to you how to create repeatable

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annotations.

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Basically, that's a summary of all what we have learned today.

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I have a homework for you in order you can practice in creation of annotations.

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You also can find a link to the whole in attachment to the lesson.

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Let's understand what you need to do in this task.

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You need to create to do annotation that will allow us to track the progress of note.

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Implement that message.

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To do annotation has different elements such as author, priority and stages.

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Some of them have default values and also Element doesn't have annotation can be applied only to masses

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and should be available under on that.

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We'll see to create separate classes at the school to do analyzer in that class, we should have massive

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three chance string value that is generated to do report pay attention that in order your homework would

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be considered to be done.

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You need to follow specific partners for each method analysis.

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One is described here.

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You should specify method name or search priority on stages.

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Is it clear any way you will be able to find my solution also in attachments does assessment, but try

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to solve this task without any help and by yourself first and only in case solution is not on this for

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you.

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Use my solution as a reference and as always, in case you have any questions, feel free to add your

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questions in comments on this video.

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Thanks a lot you for your attention.

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Have a great day and see in the next lesson.