Golang Tutorial #22 - Interfaces

Key Takeaways

[Music] hello everyone and welcome back to the golang tutorial so in this video we're going to be talking about interfaces now interfaces are very important and very powerful inside of golang and in fact inside of any real typed language if you're coming from a more dynamic language like javascript or python you've probably never heard of an interface and in fact you don't really need to use one in those languages i mean i suppose you could but there's not really a real good purpose for it whereas in a language like golang it makes a lot of sense now what is an interface one interface is really a way of looking at a set of related objects or types so let's have a look at this example here to really explain this we have a circle and we have a rectangle and they both have this similar behavior in that we can call area on them i can call a rect area i can call a circle area and they both have a different implementation but they both have an area so in theory you know the rectangle in the circle could be looked at in the lens of they both have an area they're both of shape they both have some kind of area i don't need to know anything more about them other than that they have this area and while i should be able to use that area on them that's kind of what i'm getting at with the interface the interface lets you look at these types as if they're the same it kind of hides the details underneath and says we just care about the fact that these both have area or they both have a perimeter or they both have something right that's when interface lets us do it lets us define some kind of behavior that is similar between objects or similar between types so that we can use these types kind of one in the same with their related behavior now a good example of this is if we wanted to make a slice for example of c1 and r1 so maybe i want to do this i want to say shapes colon equals and i want to make a slice we want it to be some type i don't know what the type is so i'm just going to write type for now and then we want c1 r1 now of course we don't know what to put here and this is going to be an error right we need to put some kind of type when we define this slice but what type do we put here we have no idea right now what i want to do though is i want to look through shapes and i want to call area on both of the shapes i want to be able to do something like shape 0 dot area and shapes 1 dot area and in fact if we look we assume that this is valid it's not but assume it's valid then really this should work right because both the circle and the rectangle have this area method so i should be able to do shape0.area and shapes1.area and that means that maybe if i had a huge slice of shapes i could do a for loop and loop through them and call dot area on all of the shapes and sum up the area right or something like that but right now we can't do that and this is where an interface comes in it lets us do exactly the behavior i'm trying to illustrate to you so i'm just going to define an interface and then we'll talk about it more in depth so i'm going to define type shape interface like that and inside of here i'm going to say area float 64. oh not inside the bracket put it there all right so what i've done now is i've defined a shape interface and all this says all the whole point of this interface is really just to say that anything any type any struct that has this area method that returns float 64 is of type shape it is it implements the interface shape so we say that when some kind of struct so one of these types that we've defined has the behavior defined or has the fields defined inside of this interface that it implements the interface and when it implements the interface we can use the interface as kind of like an upper level type of this struct now i know this is a lot of big words it's confusing we'll see how this makes sense in a second but really just think of the fact that since both circle and rectangle have an area method here they implement this this interface right because all that's said here is if it has area that returns float64 it is a type shape that's what this interface says now if i go ahead in here and i add um i don't know let's just call pow i'm just making up a random one here and we say pow float64 well now both circle and rectangle do not implement the shape interface because they don't have a method called pow that returns float64 that's the basics that's all you really need to know of course if we had a method that took some parameters then i would have to define them in here i would have to say something like x int and now we would need to have not only a method called pow that returned float64 but a method called pow that took a value x that was an integer and returned float64 so that is kind of how the interface works now once we've kind of understand how you implement an interface and really how you define it which i think i've just explained pretty much how do we use it well we use it just like any other type in fact i can actually just plug it right in here and save and this is totally fine what does it say your shapes declared and not used yeah so that's a common error that's not to do with what i've done but see this is how this works since the circle and the rectangle both implement the shape interface i can use the type shape and plug them both inside of this slice now the important thing to understand though is that as soon as i do this as soon as i use an interface as a type all of the other behavior that i know is true about circle and rectangle i cannot access through this interface so i can only use the dot area method on anything inside of shapes because that's all that this knows that i know right if you think about it the only thing similar between the rectangle and the circle is the fact that they have area so since that's defined inside of the shape i can only use dot area when i'm looking at shapes on c1 i can use anything i can call the radius on r1 i can look at the width and the height but on this shapes list right here this shape's um slice i can only use dot area so let me show you what i mean if i do for underscore shape colon equals range shapes oops like that let's go shapes then what i can do is go something like fmt.print not printf i want it to print a line these autocompletions always mess me up all right line and then let's go shape dot area this is valid and i can do this but i could not for example call shape dot radius right and the reason i can't do that is because of course radius is not defined in the type shape so i can only use things that are defined in the interface when i'm accessing the interface which of course makes sense so let's have a look at this now i'm just going to run this down here so go run tutorial.go and we get the area printing out so we have 63 point whatever and 35 and that is the basics of the interface if you implement the interface then you can use the objects inside of here inside of the type interface and use all of the behavior that's defined in the interface it's just a way of looking at objects that implement this type of behavior you look at them in this lens not in the circle lens which would give us access to the radius as well right and that is the basics behind this i don't know what else can we show you let's look at a function so another great thing with interfaces is that you can use them as any kind of type it doesn't just have to be a slice you can use them now as parameter types right as return types you can use them as a variable type whatever you want you can use them in a map right that's why they become so flexible because now you can use all these different types under this interface right so now let's do func get area and let's inside of here actually say we're going to take s which is a shape and then we're going to return a float 64 and all we'll do is return the s dot area so this is very similar to what we've done down here all we're saying is that now we're going to have this function called get area instead of printing shape dot get area let's just print the get area of shape so now i just pass that shape here which could be a circle it could be a rectangle in fact it could be anything that just has an area method on it and then i return s dot area boom we can print that out and this works totally fine again one of the great things with an interface is that we can do that now i just want to show you what happens if i make these methods actually accept the pointer so of course i showed you in the previous videos that pointers and what is it pointers and values are different and we need to understand the difference between them so i need to kind of go over that here if we add this method so it accepts a pointer not just the actual instance or not just the kind of like a copy like we've said before then if i save this we're going to have to change some things so here look what we're getting cannot use c1 type circle as type shape in array or slice literal circle does not implement shape area method has pointer receiver so what this is saying is i cannot use these so c1 and r1 which again is just the it's not the pointer it's kind of like the value right i cannot use that inside of here because how am i going to call area on not a pointer of circle right how am i going to do that well i can't so what i need to do when that's the case is i need to make sure that when i create a slice or i use these as a shape i'm actually passing the pointer so now we're totally fine this works we can do that but that's because again we had the asterisks here so if we remove the asterisks and in fact i can do this now if i just remove it from circle and we save here let's see what we're getting save is it not giving me any error oh yeah i guess i can pass the pointer well now i can not pass the pointer for c1 and this still works but if i try to not pass the pointer for r1 and we save we should get the red squiggly yeah we get the red squiggly popping up so it's good practice to always pass the pointer when you're making kind of slices and stuff like this just so that if you need access to the pointer you have it and it doesn't hurt to pass the pointer because you can still access the methods even if the pointer is there but if you have a method that has the pointer on it then you need to make sure you pass the pointer otherwise it won't implement the interface correctly so i hope that kind of makes sense i understand that's probably a little bit confusing and you need to do some more practice to really understand that but just when you see examples or you see error messages that are telling you like oh it has reference it doesn't have pointers stuff like that mess around with the ampersand and the asterisks because that's probably your error and one of the most common errors i run into all the time writing go code is the fact that i'm mismatching pointers and types or pointers and references and values and all that stuff so that is kind of it for interfaces the last thing i will say is that objects can implement different interfaces so you don't only have to have one interface they can actually implement three four five as many interfaces as you want and in fact i can actually go type shape two interface and then here go area float 64. and now technically both my circle and my rectangle implement the shape interface and the shape 2 interface right and now shape 2 is just a different name to look at my objects and in fact it's going to be the same view but i could have made this a different method right so if i made this now perim so maybe that's perimeter then if i wanted these two objects to implement this interface and this interface i would need to make sure that i add perimeter to a circle and two rect so add methods for a perimeter and then they would implement both of those so i hope that makes sense again that has been interfaces that's really all i can show you there's not too much more to go through but i would encourage you guys to kind of look them up practice with them a little bit because they are very important and they're really useful to understand that this is kind of the way we view these structs and that when we're viewing them in this way we can only access or perform any of the behavior that's defined in this kind of view that's the way i like to think about it again hopefully that makes sense let me know in the comments down below with that being said i hope you guys enjoyed if you did make sure to leave a like subscribe and i will see you in another youtube video