#8.6 Kotlin Inheritance with Primary and Secondary Constructors Video Lecture | Kotlin Tutorial for Beginners: Learn Kotlin in easy language - Back-End Programming

hi resisting mm welcome to the next
video of this object-oriented
programming section now in the previous
video we learned the concept of
overriding in case of cortland now in
this video let us check out how do we
use the primary and secondary
constructors when we have inheritance
involved so here inside our project I
have simply modified my code so this is
the derived class of dog and this is the
superclass of animal now notice here we
have the open keyword and inside the
animal class I have the property of
color and inside the dog I have the
property of breed so the dog class
indirectly gets the property variable of
color as well so inside the main class I
am simply setting the color as black and
dog dog breed equal to puzzle so I guess
no doubt till here and now suppose
instead of defining the property
variable here let us use the constructor
to define the property variable let's
say where space breed of the type of
string right and let us now remove it
and now by default inside the
constructor as well you can define the
default value so this is actually known
as the default constructor where you
define the default values for these
variables such as I can define here
black so if you don't pass any value
inside the dog constructor then by
default this variable breed will get the
value of black and this concept of
default parameters we have already seen
in the previous module where we learnt
about the default parameters in case of
methods in Kotlin so as of now let us
remove it so here the compiler is not
happy because we have to pass some value
here so here I will simply cut the bug
from here and paste it inside the
primary constructor of dog right so when
this statement will be executed inside
the primary constructor
this breed variable will get the value
of bug and now the same thing we can
apply for the color variable as well
so here inside the primary constructor
add the header of animal we can simply
define where color of the type of string
and now here as well you can define the
default value let's say black right so
as of now let us remove it and now let
us remove this where color from here we
don't need it so here at the bottom you
will find some error it simply says that
we need to pass some value because the
primary constructor of the animal
contains were in the parameter so here
what I will do is I will simply pass
some value
now that value I have to get it from
this dog constructor so for this I have
to modify my dog constructor let's
modify it let us define the parameter as
color string comma where breed
so now inside the animal I can simply
pass color and now at the top I am
having some error
it simply expects two parameters and now
I have one parameter defined so here as
a first parameter simply pass black
command X and paste it here comma bug
and now so what is happening here is
that when this statement will be
executed it will simply call the primary
constructor of this dog class so here is
our primary constructor so inside this
this color parameter which will get the
value of black and this property
variable of breed will get the value of
bug and now in return I am calling the
superclass constructor that is the
animal class constructor where the value
of color is actually this one right
which is actually black so this
statement will simply pass in the value
of black which will fall here add the
property variable of this class of
animal so here inside the class of dog
let us define our init block and let us
print some value and now let us copy
this init block from here and paste it
inside the animal as well and let us
modify the
animal and here remove the breed right
and now let us run the code and let's
see if everything is working fine so
here we go in the output console we are
getting from animal in it black from dog
in it black and pug so what is exactly
happening is that when this statement is
executed it simply triggers this method
or this constructor which in turn simply
calls the super constructor animal and
simply passed color as a value that
falls here and hence we get the value of
the color inside the animal class and in
the same way here we get the value of
the breed right and here we are printing
the values so in this way when we use
constructors in case of inheritance we
have to compulsorily follow this
guideline always call the superclass
primary constructor if you are having it
right and now at the end just to avoid
confusion always remember the sequence
so here the first parameter is actually
color here so in the derived class as
well the first parameter always make it
color just to avoid any confusion now if
you want multiple constructors in your
class such as if you want to use the
secondary constructor instead of primary
constructor then let's see what changes
do we have to make so here let us remove
the init block and again let us remove
the init block from here and now I will
simply define my secondary constructor
and what I will do is I will simply cut
these parameters from here control X and
paste it right here and let us remove
the declaration here and now the
secondary constructor has its own body
and here as well I will create a
secondary constructor and let us cut
this statement from here and paste it
inside the animal constructor right and
it has its own body and now as
the rule the secondary constructor
cannot contain the where keyword that is
you cannot declare the properties inside
the secondary constructor so what I will
do is I will simply copy it and define
it as a property inside this write like
this and now I will simply remove the
where keyword from here and inside the
constructor I will simply use this dot
color equal to color so as to initialize
our color variable from here right and
now here it shows some error the primary
constructor call expected so here we
need to simply use colon space this
right give a call to the primary
constructor as well this one and now
inside the class doc again we have some
error so what I will do is let us remove
this color from here and now inside this
we need to remove this vert keyword
because we cannot have the properties
inside the secondary constructor so let
us define the where breed of the type of
string equal to let's say a blank string
and now inside the constructor I will
again use this dot breed equal to breed
right and now as per the rules defined
inside the Cortland whenever we are
using the secondary constructor we need
to call the superclass constructor as
well so from this constructor we need to
call this constructor so here what I
will do is I will simply use colon and
then call super and as a parameter I
will simply pass this color so let us
pass this color right so this super
statement will simply call this
constructor defined here right and here
it is showing some error let us do one
thing let us remove it from here and
also from here right
so this rectify is our code so in case
of secondary constructor what do we have
here is this is our class dog that
inherits all the properties from the
animal class right and here we have
defined the secondary constructor so
when this statement at the top is
executed
this this constructor is actually
triggered so color will get the value of
black and breed will get the value of
pub and this statement in return will
simply trigger the superclass
constructor that is defined here right
which in turn will simply initialize the
color variable and again since we have
removed the primary constructor
declaration from here so we do not need
to call the primary constructor from
here so let us remove it and this
rectify is our error and now let us do
one thing let us try to print some
values let's say inside the dog
constructor and let us copy this
statement and paste it inside this
constructor as well and here I will
simply use from animal so as to check if
we are getting the values or not and now
let us run the code so again in the
output we get from animal class we get
the output as black and inside the dog
we are getting the value of black and
bugs
respectively so our code is now working
perfectly fine and each class is getting
its own value with the help of
constructors in Catalan right so in this
video we learned about how do we use the
primary constructors and also the
secondary constructors in case of
Cortland