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Object Equality
3:39In this video we'll explore what it means for two Objects to be equal
When talking about objects,
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there's typically two ways
to think about equality.
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There's strict equality, which makes
sure they're literally the same object.
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And then there's looks like equality,
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where as long as the two things have the
same properties, then they are the same.
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Here's an example.
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Let's say we've got a playing card
class with two fields, one for
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the number on the card and
one for the card suit.
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Then let's create two new cards and
make them both a two of clubs.
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By default, Java uses strict equality,
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meaning these two cards are not equal and
would have different hash codes.
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But in some situations,
you might want these to be equal.
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I mean, after all,
they are exactly the same card.
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In this case, rather than basing equality
on them literally being the same object,
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you'd want to base equality on
the properties of that object instead,
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which can be accomplished by overriding
the equals and hash code methods.
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Let's see how to do this
with our animal class.
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But first,
let's start in the main method and
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see what we get when we compare two
dogs that are exactly the same.
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Let's delete the findFood line,
And then rename this to dog1.
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Then let's add a dog2 on the next line.
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Dog dog2 = new Dog.
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And let's finish up by printing
out whether they equal each other.
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So S out, dog1.equals(dog2).
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Then let's run it, And
it looks like they're not equal.
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Awesome, now we need to change our
animal class so that equality will be
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based only on the properties of the class,
which, for us, is just the sound property.
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To do this,
we'll need to override the equals and
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hash code methods to be
based on the sound property.
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However, this typically
isn't done by hand.
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So let's use the shortcut.
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Add some space at the bottom of the class,
and then hit Command+N for
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Mac, or Alt+Insert for
Windows to bring up the generate dialog.
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Then pick equals and hashCode,
and just keep hitting Enter.
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Nice, let's run the code again.
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And now, since they have the same
sound property, they're the same dog.
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So I guess that means if we
ever want different dogs again,
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they'll need to make different sounds.
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How about instead of all dogs saying bark,
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we update the dog constructor
to take in a sound.
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In the dog class, let's add in
a sound parameter to the constructor.
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String sound, and
let's update the call to super
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to just pass along our sound parameter.
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Then let's fix the errors
in our main method
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by making dog1 say bark and dog2 say woof.
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And if we run it again,
We're back to having different dogs.
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There's a ton you can do with objects and
inheritance.
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In fact, I wouldn't be surprised if you
start thinking of the world as a bunch of
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objects, with all sorts of
different family trees.
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And for that, I apologize, but
that's just the life of a developer.
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Until next time.
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