Flutter 13 - Generic programming Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody's Brian welcome to the 13th
flutter tutorial we're going to continue
down our dart learning curve here we
have to learn the dart language before
we can work with flutter and we're going
to talk about generics today and we've
actually worked with this before
whenever we made like a list like you
can see this little snippet of code
right here and it's actually as the
arguments in every single program we've
made so far so we can do something like
list you know so I had the e so we have
to define type and we'll say things well
new list and then we can add things in
there now we could have also said string
or bool or whatever we wanted to do so
why do we need generics what's really
the value here well let's say we wanted
to make a function and let's just say
void add and avoid we want to just take
something let's just say a string and we
want to just add it so let's actually
come back here whoops not stringbuffer
geez
all right we just want to add it here so
we're going to say return value plus one
but now let's say we want to take the
same thing and we want to do it with
integers hmm so now we've got to rewrite
this whole darn thing and now this isn't
a string this is an int and you can see
how now it's complaining that it's
already defined and so now we've got to
turn this into add int and then turn
this one and to add string and then we'd
have to know which one to call and you
can see how it gets very confusing very
quickly so yeah we're not messing with
any of that nonsense what we're going to
do is we're going to use generics and
what generics allow you to do is they
allow you to actually take the type and
turn the type into a variable if that
makes any sense whatsoever and we're
gonna actually discuss this but if
you've read through their examples
they're actually very helpful and very
insightful I went through that section
in probably about ten minutes it was
super easy to understand so we're just
going to dive right in then want to say
log now we want to put these little
arrows here that means anything in these
arrows is going to be a type that we're
going to use for generics and we can use
that in our program now and we're just
going to print the value
so what effectively have we done here
we've said hey we're making a method
called log we want to use generics so
we're gonna have a type that type will
change that's what I mean by that's a
variable type now so that type will
change and here's our actual variable
which is going to be of whatever type we
shove in here and it's just going to
print it out print takes an object so it
won't actually freak out on us so now we
can say print link say hello or we can
even say print and some number and they
both work see down here we've got hello
and then whatever number that is giant
long number hmm my laptop meets a weird
noise that kind of scared me a little
bit it was like it's kind of like
Godzilla what was weird anyway so that
is a very super simple example we can
now put anything we want in there so
let's say we want to actually restrict
the type let's go back to our Edition I
wanna say add now let's say he extends
and we want to extend the number class
here now we're not doing inheritance
here that's the confusing bit what we're
doing is we are saying that anything of
the T type has to inherit from the
number class now when I say the T type I
mean this guy right here so anything
that we put into this has to inherit or
extend the number class so let's go
ahead and finish this out we're gonna
say return value plus 1
so what we can say now is print let me
say add just gonna say one and this
should print the number two and there it
is number two so if we take this and we
now try to do something like this you
can see how it's already mad at us
saying an argument string cannot be
assigned to the parameter type num
because it knows it has to extend the
number class and if we try to run it of
course it just explodes on us and says
no can't do it and it even tells you why
type int is not subtype of string or
other where and then it just shows you
so that is one way of limiting that
right there so let's get a little more
complex here let's actually work with a
class and I know I say complex but this
is actually just dead simple to work
with and I'm almost embarrassed making a
tutorial on this because it's so easy to
work with other languages generics get
very complex very quickly but that makes
it very simple I think they kind of
borrow some of the best parts of other
languages but that's just my humble
opinion
so we're just gonna have a dog class the
dogs going to speak and then we're going
to make a new class will call this stray
which is like a stray dog kind of thing
I'm going to import the dog class and
thought this stray is really not going
to do anything I mean we're not gonna
have any properties in it and then we're
going to make a trainer class here and
we're going to import the dog class
because we're gonna be working with that
I'm going to say class trainer and we
want to have our generic type in there
and in here we want to say ok void add
and what we want to have our T value and
whoops I missed a step here it doesn't
make much sense what I'm doing if we
don't have this so what we're going to
do is just make an internal list here
and this is not uncommon for a lot of
programs
alright now this will make a little more
sense what we're doing then we're going
to flat arrow this into your animals and
and we're just going to add the value
there
so we're going to be able to add remove
to that internal list and then we're
just going to say boy and tree and if
you have no idea what I'm doing here I
encourage you to watch my previous
tutorials we're just going to do that
and then we're going to say for its I
equals zero is less than three because
why not and then going to increment so
now we're going to say value dot speak
this looks weird let's talk about this
okay so what we're doing here is this is
our generic class the dog is just a
normal class the stray just extends
whoops we actually didn't even do that
extends the dog class and then the
trainer itself is just a generic class
now when I say generic I mean it's using
this and you can see internally we're
using this list and we're actually using
the same type and we're just going to
add remove values to that list and then
we're going to work with that list now
you notice how instantly it's saying the
method speak isn't defined for class
object what does that mean well it means
that we actually have never defined this
we haven't said what it extends so it's
going to automatically extend the object
class which is the base class for pretty
much everything in dart the base class
object does not have a method called
speak which is why it's saying tada
object doesn't have that method now
let's try to work with this and just see
what happens here and let's say
and whoops I don't want dog I want stray
now we are going to make you guessed it
a trainer and we're gonna say trying to
think of a good name it's why I'm like
sitting here for a minute going mm-hmm
let's say it's Trey and let's call him
why not let's call Bob and then let's do
this oops
Bob dad we're gonna add Fido into this
so Bob's gonna train Fido basically is
what's going on here so we are adding
Fido we're going to Train all the dogs
in the list and then we're gonna remove
Fido because Fido has been trained and
let's see what happens if we run this
and it says woof woof woof so now back
up wait a minute what's going on here
notice how we've done a couple things we
haven't added the generic tire we
haven't added the type in here the dart
was smart enough to know that because
it's here we're doing it also in the
trainer class it's still saying method
speak is it defined this is a perfect
example of what's called polymorphic
programming which means that your
program changes based off the input if
we were to put a integer in here it's
going to die because integer doesn't
have a speak in here let's actually try
that let's say
yeah let's say one and one that's
probably not gonna work let's try it
yeah see boom class and has no instance
method of speak so the same code did
work but didn't work if you kind of
follow what I mean it works for what we
programmed it for but it didn't work for
integers so what we need to do now is go
back in and modify this a bit and you
guessed it we are going to say extends
the dog class now suddenly it clears up
and it knows that hey dog and anything
that inherits from or extends from dog
actually has that so now this should
just work fine well for foaf there we go
so just some little caveat says you
should be a little careful with generic
programming it's a bit of an advanced
topic a lot of questions that I get when
I talk about generic programming in
other languages is why I mean when do
you actually use this you'll see a lot
of generics in core libraries things
like lists for example so if you're
making a program that takes plugins for
example you would have a base plug-in
that all other plugins would extend or
inherit from and in your generic
programming you would just like we did
with the trainer here you would extend
that base plug-in that would be a really
good example another thing would be like
if you're making like a music player you
would have a generic class called song
or file or you know whatever and
everything would extend upon that and
then you would have your your base
methods in there so another question I
always get is how do you have more than
one like there's tea what if you wanted
like a couple you just simply separate
this sim
similarly simply separate them by commas
like that we have E and H in there now
we could use a and age but then you have
to in our file here you see how they're
suddenly exploding we don't have to say
int and string and they would have to
match up
oops try help if I spelled string
correctly here hmm yeah really really
screwed that one up here sorry guys and
then you would have to you know
obviously work with the extra types as
needed
but I'm gonna get rid of those because I
don't want it to be part of the example
here just remember that generics are a
bit of an advanced topic and really it's
a powerful tool the problem with a
powerful tool is that when you hit
yourself in the foot with it it's going
to hurt a lot worse and you'll get some
weird errors like this value speak like
if we did an extend dog if we just let
it extend object automatically and we
throw the wrong value tight or the wrong
variable type in there suddenly the
program starts crashing we're trying to
figure out why so just be mindful of
that as always I thank you for watching
I hope you found this educational
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can and that's it thank you for watching