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The @Override annotation in Java instructs the compiler to verify proper inheritance. Specifically, it verifies proper overriding of methods. In this video, we'll see this annotation in action.
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The first annotation we're going
to look at is called override.
0:00
Now, you can't get too far in Java
without running into this one.
0:04
The override annotation is processed and
discarded when your code is compiled.
0:08
Essentially what it does,
is send a message to the Java compiler,
0:13
asking hey, when you compile this
code can you tell me whether or
0:16
not this method I've annotated
properly overrides a method.
0:20
If the answer is no,
then the compiler will produce an error.
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It is good practice to use this annotation
0:29
because it allows you to use the compiler
to verify proper inheritance.
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One of the most common examples
of using the override annotation
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would be a toString() method.
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Let's experiment with writing with
one of these, and see how we can
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us the override annotation to ensure that
we are properly overriding a method.
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In this workspace I have a simple
Java project with a few packages.
0:51
You can find a link to the project in the
teacher notes if you'd like to download it
0:54
and import it to your favorite IDE,
such as Intel JRE Eclipse.
0:58
For this course,
I'll stick with workspaces.
1:02
To demonstrate the use of
the override annotation,
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we'll make a simple class
in the override package.
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How about we call it, cheese?
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We'll create a new file.
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Name it Cheese.Java.
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And we'll set up the structure of
our class here, public class Cheese.
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Let's not forget our package.
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Brilliant, now we won't go into too
much detail crafting the perfect cheese.
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There are many fine cheeses out there, so
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we don't need to go reinventing the wheel,
get it?
1:45
Wheel, like a cheese wheel?
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Oh, nevermind.
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Anyway, one of the most common tasks
we wanna perform with an object
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is to display a string
summary of the object.
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You've likely learned that this is
accomplished via the toString() method.
2:00
Let's write a simple one that
returns a hard coded string.
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We'll start with out signature,
public string toString().
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And we will return a simple string
that says, 'String cheese'.
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Pretty straightforward at this point.
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Now let's say a beginning programmer has
accidentally added a string parameter to
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the toString() method.
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String something.
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You and I know that this isn't right,
but the Java compiler is none the wiser.
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We would definitely see some puzzling
behavior if we tried to use the toString()
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method by creating a new cheese object,
then displaying it in the console.
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Let me switch to my main class in
that same package and do just that.
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So, in the main class,
we will create a new Cheese object,
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calling it default constructor.
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And then, with a simple System.out.println
statement, display myCheese.
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Let's compile and run the application.
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I'll change directories to src,
standing for source.
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To do this, I'll use the cd command,
which stands for change directory.
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Then, I'll compile our code with a javac
command being sure to list the full path
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to our main class.
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Looks like it complies successfully.
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So I'll run our application
with the java command,
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using the dot notation that
includes the full package name.
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The output we're getting is
the cheese object's hash code,
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which means our toString() method
wasn't actually called, but
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rather the Java object's
toString() method.
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This is a logical error
that occurs at run time.
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If we ever have a chance to convert
runtime errors into compile time errors,
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we'll take it.
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And, I'd rather fix a compiler
before releasing my software,
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than upset a user with
a pesky runtime error.
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By simply adding the override
annotation to our toString() method,
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the Java compiler will perform
a check on inheritance.
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Notice that our method does not constitute
proper overriding and generate an error.
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So, we'll add the annotation,
go back to my
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Cheese class, add the override annotation,
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go back down to our console,
recompile, and voila.
4:39
We have now detected our
error during compile time.
4:46
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