Java Programming Tutorial - 58 - Abstract and Concrete Classes Video Lecture

alright guys welcome to your 58th jump
tutorial and in this tutorial I'm going
to be talking about the two types of
classes and in case you didn't already
know we already talked about the two
types of classes I just didn't tell you
the technical definitions of them now
you guys already know what abstract
class is from the less toriel might as
well put that back there
so an abstract class is a class that you
can't create objects from so it's only
useful for polymorphism and inheritance
and stuff like that in these other
classes that aren't abstract even though
they don't have a name
people call these classes concrete and
the concrete class is a class that's
specific enough where you can make an
object's or excuse me you can make an
object from it so even though it doesn't
say concrete that's what people call it
just because they have to call it they
just don't want to be like not abstract
so again food you wouldn't want to
create something because it's too broad
a topic but tuna and pie those are
pretty specific
aka concrete so you can create objects
from concrete classes but not abstract
so again let me demonstrate this by
saying all right whenever we try to
create a food object it's too broad and
abstract so it gets an error but if we
go ahead and create we can again we can
have this food reference but if we
create it from the class like new tuna
since this isn't abstract we don't get
um well if I got rid of that then we
wouldn't get an error right here because
tuna has no restrictions we can create
objects from this now let's get rid of
this and move on to our next topic and
so you know what an abstract class is
and it pretty much is useless unless you
extend it or its inherited from
something and as you can see tuna
inherits food and Popeye inherits fruit
if nothing inherited food it will be
useless because you can't create objects
from it so that's what an abstract class
is but just to make things more
confusing I want to tell you that
it's can be abstract - and what exactly
is an abstract method well an abstract
method is a method that must be
overridden so for example we created
this method in food and since tuna and
Popeye inherited food we created a
method eat in tuna and a method eat in
pot pie in both of these overwrote food
I mean this wasn't necessary that we
created these methods but we did it
anyways just for fun so again an
abstract method is a method that must be
over red in the sense it must be
overwritten which means you need to
create a body for it then this body in
the main food class right here this is
unnecessary whenever you're creating an
abstract method so let's go ahead and
create an abstract method right here
let's go ahead and get it rid of
everything in our food class how would
we create an abstract method pretty much
the only thing you need to do is add the
head in a semicolon so do public will
make it public abstract since it's the
abstract method void eat since it
doesn't return anything and it's named
eat and now you can see this is a method
but it doesn't have a body
and if we're saying alright i never
created a method without a body
well since tuna needs to override it
pretty much it needs to create a body
for it and popeye needs to create a body
for it as well then that is why um in
your declaration right here you don't
need a body right here so again an
abstract method in your superclass it
doesn't need a body
so another thing i want to point out
anytime you have an abstract method and
a superclass that entire class needs to
be labeled abstract - right there so you
can't have like an abstract method and
you can't have this class be not
abstract or else you're going to get an
error like that so ever
someone's knocking on my door I think
hold on oh I'll let them keep knocking
I'll get it after this tutorial and
another thing I want to point out is
when we declared this public remember I
said I can't change the scope when we
override it so we need to make these
public as well so make that public and
if you see that little error right here
um multiple markers can't reduce the
visibility that is why anytime you see
an error with the word visibility you
probably need to change public to
private or private to public or add one
or the other so again like I was saying
anytime you have an abstract method you
need to make the class abstract too but
here's the thing you can have an
abstract class but have abstract or non
abstract methods inside it so again in
this is a we're getting here because of
something else here because anytime you
have a non abstract method you need a
body but anyways I just wanted to point
that out anytime you have an abstract
method you needs to be inside an
abstract class but anytime you have a
non abstract method it can be inside an
abstract class well you just heard it I
don't need to repeat myself so the last
thing I need to point out is let's go
ahead and make this method abstract
again just like this anytime you have an
abstract method you need to implement it
in one or excuse me
in any of the subclasses that extends
the superclass now I know that sounds
confusing but listen to this all right
this tune the class and this potpie
class both inherit from this main food
class in this food class has an abstract
method of eat in it that means this tune
class and this potpie subclass they both
need to use the eat method or or in
other words they both need to implement
or override this eat method if you don't
have this eat method then you're going
to get an error in your class just like
this type pop I must implement
the inherited abstract method food so
again any time you have an abstract
method in a superclass then the
subclasses need to override that method
and that's like making a promise this
food says all right you want to inherit
from me okay I'll let you but I'm
telling you I have this abstract method
and if you inherit from me you're going
to need to override it and Poppi's like
all right I promise I'll override it
since you have an abstract method I
promise if you let me inherit from you I
will override it for you so if like the
classes could talk to each other that's
kind of the conversation they would be
having and for saying all right if I
need to override this method anyways
what's the point of even having it in
here well it's pretty much for
consistency in some polymorphism and I
know that doesn't make a lot of sense
but in the next tutorial I'm going to be
building a program to demonstrate
everything I went over in the last two
tutorials
these last two tutorials were pretty
much the rules and concepts that you
have to follow and I couldn't just skip
over it because it's incredibly
important for what we're going to be
learning in that's next up with
tutorials and this is pretty much a core
of polymorphism these rules so in like
five tutorials ago when I called a class
or excuse me it was an interface and I
implemented it that's what implemented
means implement means pretty much you
need to override the abstract methods so
that's enough for this story I probably
should go see who's knocking on my door
but thank you guys for watching um don't
forget to subscribe
I got like phlegm bone up in my throat
but anyways thank you guys for watching
don't forget to subscribe and I'll see
you next tutorial