20 - Generics/001 Generics in Java_en.srt

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Hello, Jim.

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So they are going to answer questions, what are generics in Java and how they work?

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We'll start from understanding of why do we need generics.

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After that, we'll understand how you can parametrized your mascots and classes.

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You're going to understand what Boundy generics in Java are.

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We'll discuss generics with multiple mounts also in this class.

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And we are going to learn what the wild cards are and how to use them.

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Today, we're going to have a lot of practice that will help you to understand generics.

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And at the end of the lesson, I will explain what a type is and what type erasure is.

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Let's start and to make you understand what generics are.

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I suggested starting from examples to make you understand what problem generics intended to solve.

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Imagine the next iteration.

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You have array of strings and you want to have mastered that process is data.

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For the sake of example, let's imagine that you need to have masses that would print all elements of

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array to console in some custom way.

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Here I created massive print array that takes array of strings as Masad argument inside a grade for

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each loop and before printing any new element.

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I also print words aliment and only after that some element is printed.

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Now you realized that you have not only array of strings in your program, you also want to apply the

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same behavior for an array of integers.

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But the existing method will not work with an array of integers since it is specified that it works

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only with the array of strings.

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Let me uncommon this line and you can see a compilation error here.

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What to do in this case, create another method that will work with array of integers, I assure.

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What would you do in case you would need the same method for array of numbers or array of your custom

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type user, for example?

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That is definitely not the best option.

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But what I would recommend to do in this case, it would be perfect to create the method that would

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work with arrays of different types, what they think.

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But how to do this in Java, that is exactly what generic mechanism is created for.

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In other words, we can create parameterized methods and classes that would work with different types.

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And here you already can see the first advantage of using generics you can avoid of code duplication

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and increase cautery usage in your program.

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Let me comment this parametrized method and explain use syntax.

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This method is similar to the one we already have, but before the type of returns value, we have just

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word here.

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I write it in Diamond Operator.

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These characters from both sides are called together as diamond operator whispered Sify, our parameterized

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type in the diamond operator.

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This is also called generic and will explain in simple words what this is about.

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This is a placeholder for some specific type.

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Theoretically, I can write Annunziata here.

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The main thing is that letter that I specified here will be the same as in the Massett parameters and

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Method Masmoudi.

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And after I wrote it here, I can treat this E as a specific type and use it with parameters, for example,

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or anywhere inside the method.

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But here you can see that I have array of type E and while iterating over each element in this area,

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I also use E type for my variable here.

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Now I can call this method for most string and integer array the same as for all other types.

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Let me comment, Prenter, a message that works only with strings and leave my generic method awesome.

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And you can see that I don't have any compilation error anymore here.

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Let's make a definition of generic method now.

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Generic method is a method that introduce its own type parameters.

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I believe now you understand what generics are and why do we need them.

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Also, you know how to parametrized your methods now, but what is a potential issue in this case?

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In this case, for the type inside the method body?

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I can use only Massett that are available for object type, but not any other methods that are specific

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to string or integer or any other type Y because compiler doesn't know what type will be here during

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the runtime.

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I can't call here any string methods because compiler can guarantee that only objects of type streambed

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will be here.

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As we can see on this example, objects of any type might be here and what to do in case I need some

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behavior that is related to some specific hierarchy of objects.

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What if I know that this method wouldn't work with any objects at all?

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But this method has to work with any type that implements comparable interface.

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In this case, I can count on the fact that objects inside my body will have compared to Masset and

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I will be able to use this API in the mess and body.

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Does it make sense?

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To help you understand this, let me open another example here, I opened another file where I prepared

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other examples for you.

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Let's imagine that I have to create a that would be able to find the greatest value out of three variables.

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And I want to implement generic method, but not a bunch of separate methods that works with only strings,

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only integers, only double values.

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How to do that generic with boundaries will help us with this task.

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They're also called bounded type parameters.

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Here you can see massive declaration.

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The methods name is max value.

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The next thing that you can notice here is that we use different letter here now instead of E, we have

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key and he is parameterization of my method here in Diamond Operator.

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I say that this method should work with some type that extends comparable and comparable in turn is

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parameterized with this type.

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In other words, some type T should use it extends comparable interface or implement comparable interface.

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This is called upper bound with parameterization because we set boundary from the top.

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Doesn't make sense.

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Also, one more common here.

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It is possible to make multiple sounds like it is specified here in command line.

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I will leave it here, command it so that you could learn this when you will download the source code

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that I am showing right now.

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So you can say that we should extend both comparable interface and compare the interface.

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That's just for the sake of example.

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I believe you understand that instead of comparator, you can specify any other type here and only in

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case our custom type T will implement both of these interfaces.

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Only in this case we could work with this method and methods, arguments of type T, you even can see

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that I have a compilation error because neither string or integer nor verbal implements both of these

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interfaces simultaneously.

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Let me erase this to proceed with our demo potentially that you can also use time that is specified

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for parameterization as a returned type so you can get the type that was used as parameters here.

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The main thing and all of this is that this type should be the same everywhere across the MassArt in

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case the first argument is string.

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It is not possible that other arguments wouldn't be of different type or return type would be different.

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And now Compiler is aware that this type will be compatible with comparable interface.

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That means that all variables that are passed as arguments to this method have compared to MassArt.

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That's why all logic that is written in this method is rely on the fact that we can use this method

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and there are no any compilation errors.

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That's great.

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And here you can see in my method that I can invoke max value method with different types of arguments.

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No matter is as this integer double or string type, let me run this program from the console output.

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We can conclude that our method works as expected.

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Now you know how the Parameterize method was bound.

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Hobbs's makes things clearer.

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Let's go to the next example and this example.

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I'm going to show you how you can parameterize your class here.

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You can see Diamont operator right next to my class name.

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That means I parametrized my class, for example.

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I can declare property of type key and Masset parameters like in this set method also can be of my type,

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e.g. I can initialize my field with the values that will be passed as massas argument here.

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The method will return me a value of type that we used to parametrized my class.

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And here's example.

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I declare the variable of type generic.

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Mazrui was a name integer in box and used Diament operator to parameterize this type by integer.

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After that I created the object and used Diament operator after Anjelah version seven.

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There is no need to duplicate parameterization type in Diament operator here, but you can if you wish.

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For example, here I left integer type and then my second variable was a name string box that I parametrized

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by string.

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I left blank my second Diament operator.

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In most cases type is specified only when you declare a variable.

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And here you can see that my integer box can work only with integers and there is a compilation error

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in case I would try to set any string.

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Object is a similar story with my string box.

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That is because these two variables are parametrized with different types and in the first case set

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Masset except on the integers.

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And in the second case, set method accepts only strings.

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When I use get MassArt and receive integer and string type accordingly.

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And here you can see that I print values to console and used special format specifiers for that some.

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Numbers and string accordingly, let me run this program, you can see that everything works as expected

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without any errors.

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Let's create a definition of generic type.

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Now, when you saw the example and generic type is a generic class or interface that is parameterized

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over types.

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And now let's continue our lesson.

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Even if you would not use parameterization and generics for your own custom classes, you would use

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generics a lot with classes from Java collections framework.

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It is super useful because it wasn't possible to create collections for all possible types.

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That's why all types from Java collections framework are parametrized and use generics.

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Let me open source code of arriviste.

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You can see that it is parameterized with each type and if I open at Masset, you can see that it adds

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elements of each type.

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And get maced in return elements of each type, does it make sense sometimes even parametrized with

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two types, for example, on implementations specify separate type for key and value.

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Let me open hash map or squad here.

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You can see that I can specify type for key and a separate type for Lélia just to placeholders and input

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MassArt.

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You can see that I put key of type K and value of type we and type of return value is the same as type

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of the value we, because according to this interface we receive the last value that is associated with

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a given key.

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But the details of the interfaces of our collections framework are covered in the separate lessons.

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Now you know how to parameterize your class and you know how types from Java collections framework are

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parametrized.

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We have one more important and practical thing to talk about.

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I want to talk with you about the wild cards with upper and lower bounds.

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Let me open another demo file that I prepared before this lesson.

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To make you understand this example, let me show you all classes that will take part in our demo.

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I declared all classes in one file to keep everything grouped for the demo and to not switch tabs and

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engines that we have.

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Some parent class and child class extends parent and grandchild.

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Class extends child.

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Is it clear?

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Cool.

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Let's move on.

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Now imagine that I have at least parametrized by type child and I have mastered that can work with any

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list objects that are parametrized by time parent.

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It is called process parent elements.

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How do you think MacColl process span's elements MassArt and POS collection of child elements there?

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Logically, it seems to be OK since each child class should have all methods that parent class has and

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compilers should not be worried about this.

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But if I ask you this question, you already suspected something, don't you?

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You are right.

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If you are suspicious, this wouldn't work.

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You can see compilation error here.

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Why?

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Because list of child elements is not a list of parent elements.

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That said, what what to do in this case.

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Usually in this case, engineers use wild cards.

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Let me show you the next method we have process elements, method, the text list of elements that is

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parametrized by any type that extends Berent.

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This question mark is called wildcard and can be treated as any type.

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This is called upper bound at wild card.

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In this case, I can pass list that contains elements of type child to this method.

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And here you can see that there are no errors.

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Does it make sense?

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Now I want you to put all the attention to the next example.

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When you use cards, that means you can get elements of type parent from this collection.

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I believe that is clear that you will not be able to extract element of type child from this list because

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there is no guarantee what type would be here instead of the question mark during the runtime.

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It can be child, but it also can be parent type.

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And that's why custom foreign object to child type would cause exception.

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And most likely, parent doesn't have all methods and behavior that is described in the child type.

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Is it clear?

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OK, we understood that we can get elements of type parent in case we use upper bound at wild card.

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But can we add elements to this content?

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Unfortunately we can't.

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And here the examples you can see that I can't add neither parent nor child nor grandchild objects I

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cannot only know value.

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You might be wondering why why I can't add object of parent type to collection of elements, parametrized

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by parent type.

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But let's look at this from another side.

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This is a very smart mechanism created in Java to save engineers from potential errors.

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The main issue is that because we don't know what type would be here during the runtime, that's why

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interactions with such collections are limited.

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Imagine that during the runtime I will pass list of elements of type child and I will add element of

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time parent.

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And after I modified this collection in this method, I will get the element and I would think that

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all elements in this container are of child type and I will call some child specific method.

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What will happen?

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I will get error during the runtime because indeed I will get here object of parent time and this object

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will have child specific method.

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That is why Compiler wants to eliminate cases like this and just doesn't allow me to add any elements

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in this method.

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Does it make sense and leave now you understood how upper bound at wild cards work.

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Now let's take a look at another case.

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What to do in case I have to create.

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That will be able to add new elements to the collection in this case, we have to use lower bound at

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wild cards here you can see that I created two more lists.

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One of them is parametrized by parent type and another one is parametrized by child type.

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And you can see that I call process elements to Masset and parse the first and the second list object

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there.

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But I can't pass list of elements that are parametrized by grandchild type.

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Let's investigate process elements to MassArt.

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This method has lower bound and wild card.

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You can see super keyword here.

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What does that mean?

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That means that this method can take as an argument any object of type liste parametrized by any type

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that is parent or in other words, super type with respect to child type.

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That's why I can't pass Barrentine or super type of child.

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This could be any super type, including parent and even object is clear.

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If yes, then let's proceed.

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In this case I can get on the elements of type object and I believe you already understood why.

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Because there is no guarantee what super type of child type will be here during the runtime.

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But what types I can put into this collection, I can put any time that this child with respect to child

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type here, you can see that I can add object of type child and grandchild.

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And also I always can add now, for example, I can't add any super type of type child, including parent

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type and object y if this sounds completely not logical, then consider the next case.

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Imagine that we have passed here container full of objects of type child and in case compiler let me

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add the parent object.

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And after this collection was modified, I retrieve object of child type from here and called child

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specific method and program fails because that object doesn't have child specific behavior and case

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I passed here at least parametrized by any other type that is super with respect to child type.

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There is no harm in this at all to add child or grandchild object in this container because each of

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the specific types for sure has behavior of its parent type.

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Does it make sense?

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We can make a conclusion in case you want to create a method that will work with collections, parametrized

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by different types, and you want to use this method to modify the collection and to add elements there

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you have to use lower bound at wild cards.

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But in case you wanted to use this method only to read elements from the container to process that data,

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then you have to use upper bound at wild cards because upper bound wild cards allow us to read from

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the container specific type and use its methods to get all necessary data.

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You already learned how to use bounded wild cards, but should we use wild cards without bounds?

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I would say that this is super rare case.

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That's why.

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Let me show you this on example.

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Let me open source code of absurd collection.

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You can see that absolute collection is parametrized by any type.

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But let's look at contains all.

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Macit, can you see wild card here.

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That means that contains some method can work with any collection that contains any types inside.

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Then what is the difference between each type and wild card?

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The point is that each type will be erased and substituted with a specific type everywhere with the

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type that we will specify, whereas wild cards would always mean any type, but not the exactly one

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type that we used to parameterize instance of our object.

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Does it make things clearer?

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I will leave linked to this example in attachments to this Larssen so that you could investigate it.

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But pay attention that this specific example you won't be able to run because I described various scenarios

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that causes compilation error.

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But still you can play around with compilation properties to understand wildcards together with upper

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and lower bounds.

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Let me open the next demo file to show you the next example and this example.

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I want you to understand the generic exist only for our compiler.

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There are no generics during the runtime because of the type erasure type.

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Erasure can be explained as a process of enforcing type constraints only at compile time and discarding

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the element type information at runtime.

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That means that during the runtime it can be that objects of another type will appear in collection.

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Let me show this example and give my comments as we go and create at least Werrimull and list is parametrized

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by integer type.

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Everything looks good so far, but now I created a list without specifying any generic type.

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That means that during the runtime, the EIB that is declared in the source code of list will be erased

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and.

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Aware where it was used, it will be substituted with object type, for example, at Mass, it will

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work with any object.

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But in case I use bouncin generics, like, for example, in our damos was a generic for our method

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during the compilation.

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This type will be erased and substituted with the first bound class, in this case comparable.

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Does it make sense?

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You can see that our compiler tells us that there is a one here.

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It is not an error, but still works of our attention in the opinion of compiler.

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If I am a mouse over this warning, I see explanation.

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Message that tells me is at least is a type.

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What is a raw type.

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A raw type is the name of a generic class or interface.

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Without any type arguments.

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We can still work with such types.

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But Compiler wants to warn us that something bad may happen.

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Now I Telx at least reference is pointing out to the same object as integers reference.

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And I put string object in that list.

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And again, I see he has a warning about time safety, but not an error.

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This is very unlikely that you will be able to reproduce this mistake.

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But I want to prove you that generics exist only to help us during the compilation time, but not during

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the runtime.

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And as you can see, I successfully edit my string object to the list, but when in other parts of the

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program or somewhere else will use the reference of our parametrized type integers and when will call

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that method, we will get object of type integer because this variable has parameterized type.

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But what would happen during the runtime?

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What do we think?

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Lets me around the program and show you this.

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And we see runtime exception, glossiest exception.

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Java tells us that in line 15 it can't costarring object to integer.

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I believe you got the trick.

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That's it.

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Now you know about even more than some senior software engineers, believe me.

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Let's recap what we have learned today.

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Today, we learned a lot of different things.

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Now you know what generics are and what potential problems they help us to solve.

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Youso how to parameterize your method in Java, we learned what Boundy generics are together.

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We created parameterized Closs.

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You saw what wild cards are.

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And we practice a bit with upper bound at wild cards and lower bound with wild cards.

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And at the end of the lesson, we talked about hyperalgesia during the runtime.

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And now let's look at your homework.

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I believe you already implemented our custom list from the other homework I left link to that homework

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here just in case.

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Also, I count on the fact that you implemented Iterator and listed the return for this container.

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Here is a link to the homework about iterators.

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Now, I want you to make my list Interface Parametrized, and I want you to use generics for the fourth

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E class implementation myside that I want you to make here in your classes that implements Iterator

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and least iterator interfaces.

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Also parametrized.

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This will be a great practice for you to work with generics that it home.

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This task will be like a piece of cake for you.

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That's all what I have for you for today.

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Thanks a lot for your attention.

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See you in the next lesson.