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Welcome to Part Six.

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You will learn about methods and interfaces.

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First you will learn the methods.

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After that you will learn the interfaces.

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Lastly you will learn the well known interfaces let's start with the methods methods enhance types

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with additional behavior I will create a couple of types and a few functions.

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After that I will show you how to convert the code to an object oriented style using methods.

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Suppose you want to sell books to keep everything organized.

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I'm going to create a new file.

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First I'm going to declare a new book type OK.

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Now I can create a function that prints a book.

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Lastly I'm going to print the title and price of a given book value it's ready let's use it from the

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main function.

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Let's create a book I'm gonna say Moby Dick and price ten dollars.

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OK let's printed OK let's run the program.

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Nice.

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It works.

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Let's say you decided to sell computer games as well to do that.

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I will create a game type instead of typing the same code over and over again.

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Let's duplicate the book type instead and I will change the book type to the game as you can see.

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You cannot create a function with the same name in the same package.

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I have declared the print book function before. It belongs to the namespace of package main.

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So I need to change the name of the function.

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I also need to change the input parameter to the game type.

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Lastly I'm going to change the variables B to G okay.

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As you can see it's very similar to the book type here.

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I created a game type and a function for printing the game type is ready let's use it from the main

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function.

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I'm going to create a new game

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Let's print it Okay let's run the code.

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It works.

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So far so good.

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As you can see there is a printing function for each type here.

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This is a functional style of programming.

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Now we can talk about the problems of this style of programming and reasons why you might want to use

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methods instead of functions.

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The book type has only two fields title and price.

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You can create a book value like this as you can see the Moby Dick variable contains the data but has

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no behavior.

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I have written only one function for the book type but assume that there will be two functions.

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The first one prints a book the last one discounts the price of a book.

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Both take book values and their primary purpose is to work with books.

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So why don't we combine the data and behavior together I added the functions to the book type the print

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book and discount functions have become the methods of the book type.

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As you can see I no longer need to pass a book value to these methods because Go will automatically

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pass the value of a book to them.

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You will see an example shortly.

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No worries. Now the book type has both data and behavior.

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The book with its data and behavior better represents a real world book product by the way the methods

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of a type are called the method set of that type.

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Here the method set of the book type consists of printing and discounting methods.

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Let me explain how you can attach methods to a type.

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Let's take a look at the print book function.

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We wrote earlier it takes a book value as an argument but the print book function is separate from the

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book to its composite the separate.

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You can run it like this. When you run it,

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the variable Moby Dick is being sent to the printBook function as a copy.

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Now let's attach the function to the book type.

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Now the print book function has become a method.

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Let's talk a little bit about the method declarations syntax.

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The book argument has become something called a receiver.

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It's just a fancy name for an input parameter.

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It's just syntactic sugar.

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If you are coming from other object oriented languages this term may be a familiar to you.

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Here we say receiver because a method automatically receives the value of a type.

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As you know input parameters are variables A receiver is also an input parameter so a receiver is also

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a variable.

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Let's take a look at its parts.

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This is the type of the receiver variable.

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This is the name of the receiver variable.

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Now the book type has a print book method since the Moby Dick is also a book type.

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You can call the print book method on it.

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When you do so, Go

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passes the Moby Dick variable to the print method as a hidden argument before the method executes Go

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copies the value of Moby Dick to the receiver variable B so you can use the variable B to print the

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book.

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Let me give you one more tip about the receiver names.

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We frequently use receiver names so we name them using abbreviations.

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But you can name them however you want here the printBook method belongs to the book type.

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Therefore I used only the first letter of the book type.

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Let me show you how you can attach a method to the book type in the coding editor.

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Let's start by cutting the first parameter from here.

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And paste it to the left in parentheses.

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That's it.

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Ready as you can see the receiver is actually an input parameter written just before the function name.

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We use the method syntax to make Go understand that the printBook function is a method of the book type.

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Now we can use it in the main function but now Go is looking for the print book function in the main

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package but it cannot find it because the print book function now belongs to the book type.

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So instead I'm going to use a value of the book type Moby Dick and I will type a dot.

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Remember the dot operator selects a name from a namespace.

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Here it selects from the namespace of the book type when I pressed the control and space keys together

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my editor lists only the fields and methods of the book type.

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The print book Method is here.

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I can call it like a function.

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The only difference is that I need to run it through a value here.

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I execute it through the Moby Dick variable behind the scenes.

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Go sends the Moby Dick to the print book method as a receiver.

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Let's try it as you can see it works as before.

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Let's create a method for the game type as well.

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Let's start by cutting the first parameter and paste it within parentheses to the left.

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That's it.

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Let's run the methods from the main function.

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Now the print game method belongs to the game type.

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That's why you can see it here.

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Let's try it.

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It also works as before.

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Nice.

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Now let's try it with another game value.

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I'm going to add a new game value like this.

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And I'm going to print it as well.

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Let's try it as you can see when I call the print game method with the Minecraft variable.

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It uses the Minecraft variable but when I run the same method with the Tetris variable this time it

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uses the Tetris variable instead.

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It's because the print game method receives a different game value depending on who is calling it.

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It's like passing the game variable to the print game function.

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The only difference is that gold passes.

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The game value as the receiver.

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Don't forget the receiver is a variable.

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I will show you the first benefit of using methods.

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As I said types have their own namespaces for example since the print book method belongs to the book

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type I can simply say print I mean I don't need to say print book anymore it's because there can be

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methods with the same names among different types.

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Let's go to the game type and do the same thing there.

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I'm going to change the name just to print.

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Now we can go back to the main function and use the new method names there.

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Here I'm going to call the print method instead and another one here let's try it nice.

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It works on the left the book type has a print method you can run the first print method on the left

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only through book values on the right side.

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The game type has a print method as well.

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However this time you can run the second print method only with game values.

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So this means that a method belongs to a single type I can use the same method names because they belong

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to different types.

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Each type has its own namespace.

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If those methods were functions I wouldn't be able to use the same names.

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All right that's all for now.

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See you in the next lecture.