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In this video we'll learn what it means to 'override' a method and see how to use the 'super' keyword.
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We just finished talking about
a few methods that are available to
0:00
every object, but
0:04
we got a little confused when the toString
method said that it should be overwritten.
0:05
In Java an override is when a child
class has a method with the exact same
0:10
signature as one in the parent class,
but with a different implementation.
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It's like saying yes,
I do have this method,
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but I'd like to handle it
differently than my parent does.
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Think about all the things you do
differently than your parents,
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that's overriding.
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Let's get more familiar with overrides
by overriding the toString method in
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our animal class.
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Let's close object.java, and
back in our animal class, let's make a new
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toString method with the same signature
as the one from the object class.
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So it should be public return a string,
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be named toString, and
not have any parameters.
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Then let's add a method body and inside,
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let's return what type of animal
this is and what sound it makes.
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Let's type return and
then to get the name of the animal,
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let's use the getClass method,
and then call .getSimpleName
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which returns the class
name without the package.
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Then let's add a + and in quotes,
let's add a ": sound =
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"and then we'll just add our
sound variable, and a semi-colon.
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Then let's run the app, And
now, instead of getting
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the nonsense we got before,
we get something useful, awesome.
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However, we're not done yet,
there's still one thing missing, and
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that's the @Override annotation.
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Let's add a line above our toString
method, and type @Override.
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Annotations like these aren't strictly
required, but you should always do it.
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It lets Java know that we're attempting
to override an already existing method.
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So if we change the name of the method to,
getString,
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it'll let us know that the method does
not override one from its superclass.
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We'll learn more about annotations later.
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But for now,
just know that when you override a method,
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you should add an @Override annotation.
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Okay, let's change that back to toString.
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And then, I want to show you another
way we could have done this.
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Let's copy our return line, And
then delete this whole method.
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And I'll hide the run pane as well.
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Then, let's use Ctrl + O
to open the override menu.
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And let's arrow down to
the toString method and accept it.
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Then let's paste in our return line,
and we're done.
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Intellij has tons of little
shortcuts like this, and
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while I'll do my best to show them off,
that doesn't mean you have to use them.
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There's nothing wrong
with just typing it out.
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Though did you see what it
defaulted to for the return?
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Let's use Cmd or Ctrl + Z to undo and
take another look at this.
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It looks like it's
returning super.toString.
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In Java, the super keyword
refers to the parent class,
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meaning this is calling the toString
method of the object class.
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Okay, let's redo our paste, And
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then let's see why we
need the super keyword.
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Let's say that every time a dog makes
sound, it should also wag its tail.
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To do this, we'll need to override
the makeSound method and our Dog class.
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Let's add a line at the bottom
of the Dog class, and
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use Ctrl + O to override
the makeSound method.
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Then let's a line after the call to super,
and
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type sout, And in quotes,
let say *wags tail*.
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And now, when we call the makeSound
method on a Dog object,
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it'll first call animals make sound
method and then print out wags tail.
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Let's try it up in the main method.
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Let's delete line 7 again, And
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then call dog.makeSound.
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And if we run it, Perfect,
that's just what we were expecting.
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Another place you might see the super
key word is in a constructor.
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Let's see how that works by making
sure that every animal has a sound,
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by requiring it and the constructor.
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Let's start by adding a constructor
to the animal class and
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making sure it takes
on a string perimeter.
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So Animal, and in parenthesis, it'll
take in a string variable called sound.
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Then let's add the brackets,
and we've got our constructor.
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Inside the constructor,
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let's update our sound field to
equal the new sound parameter.
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this.sound = sound, nice, but
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now we're getting an error
in our Dog class.
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Since an animal now requires
a sound in its constructor, we
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need to call through to that constructor
in order to have a valid animal.
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To do that, you just use the super key
word followed directly by parentheses.
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So instead of setting
the sound equal to bark,
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we want to call the constructor
in the animal class by using
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the super key word and
passing in bark for the sound.
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Let's run it one more time and
make sure it still works.
6:00
Awesome, being able to use overrides and
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the super keyword can really help
turbocharge your programming, and
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give you a lot more options for
how to build an app.
6:16
Though giving developers more
options isn't always a good thing.
6:19
More on that in the next video.
6:23
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