Heads up! To view this whole video, sign in with your Courses account or enroll in your free 7-day trial. Sign In Enroll
Preview
Start a free Courses trial
to watch this video
When overriding Object.Equals we should also override Object.GetHashCode. Here's why and how.
In the previous video,
0:00
we got this compiler warning when we
overrode object.equals in the Point class.
0:01
TreehouseDefense.Point overrides
Object.Equals but
0:06
it does not override Object.GetHashCode.
0:11
The hash code of an object is an integer
that is unique to that object.
0:15
Different objects will
have different hash codes.
0:19
Every object created is
given a unique identifier.
0:22
So by default,
this ID is used as an object's hash code.
0:24
This ID, if you will,
is actually just its memory address.
0:29
Being able to identify an object
by its hash code is very handy and
0:33
the GetHashCode method is used
by many C# collection types.
0:38
An array is just one type of collection.
0:42
There are other types of collection
classes that rely on being able to
0:45
identify an object using just an integer.
0:48
The most common collection that uses
the GetHashCode method is .NET framework's
0:51
dictionary class.
0:55
A hash code is only an attempt to create
a unique identifier for an object.
0:57
It is possible for two different
objects to have the same hash code.
1:02
Just like my name is Jeremy but
there are also other people named Jeremy.
1:06
In fact it's a rather common name.
1:11
If someone called out my
name in a large crowd,
1:13
there's a good chance that more
than one person would respond.
1:16
How could they know which person
is the Jeremy they're looking for?
1:19
Well, they'd have to rely
on more than just our name.
1:23
They'd probably need to
get a look at each of us
1:27
in order to say which person they wanted.
1:30
This is why the methods Equals and
GetHashCode go hand in hand.
1:32
If two objects map to the same hash code,
1:37
the only way to tell them apart
is to check if they're equal.
1:40
Had we only overridden
the GetHashCode method,
1:44
we'd get a similar compiler warning
asking us to override Equals too.
1:47
Let's get a better idea of how
the GetHashCode method works
1:52
by overriding it in Point.
1:55
So we'll type public
2:00
override int GetHashCode.
2:04
In order to implement GetHashCode,
we need to make a couple decisions.
2:10
First if two objects are equal as
we've defined equality to be up
2:14
here should their hash
codes also be equal?
2:18
We almost always want this to be the case.
2:21
And this is what users of
the GetHashCode method will expect.
2:24
Here we see that we determine if the Point
objects are equal by comparing the X and
2:28
Y coordinates with each other.
2:34
We should also use X and
Y when creating the new hash code.
2:35
But here's the problem that we always
run into when creating hash codes.
2:39
We can only return a single
integer as the hash code.
2:44
Here we have two integers, X and Y.
2:48
We can't return them both.
2:50
Let's say we just returned
X as the hash code.
2:52
That means that all points with the same
X value would have the same hash code.
2:55
That isn't very unique.
3:00
Our goal is to create a hash
code that is unique as possible.
3:02
The best chance we have of doing
this is by combining X and
3:06
Y, somehow, into a single integer.
3:09
There are lots of ways we could do this.
3:13
We could just add them together.
3:15
If X was five and Y was two we'd
end up with a hash code of seven.
3:17
There are lots of ways to add two
different integers together to get seven.
3:24
All of those points would
also have the same hash code.
3:27
We could reduce those combinations
by half just by multiplying X or
3:31
Y by a prime number before
adding them together.
3:36
By doing this we're ensuring
that X equals 5, and Y equals 2
3:39
gives us a different hash code
than X equals 2 and Y equals 5.
3:44
So here we'll say return x
3:49
Times a relatively small prime number 31,
and then add Y.
3:56
There are lots of ways to
create hash codes, and
4:04
they are all compromises of some sort.
4:07
For what we're doing this
is probably good enough.
4:10
I should mention that when computing hash
codes it's best to compute them from
4:13
the hash codes of the fields
that we're combining together.
4:16
This is because their hash codes
are already fairly unique.
4:20
So we should write this as
X.GetHashCode * 31 + Y.GetHashCode.
4:24
In the case of integers
the result is the same since
4:31
the hash code of an integer
is just its value.
4:37
So the hash code of five would be five.
4:44
If X and Y were of any other type it
4:47
would be important to call
their GetHashCode methods.
4:49
This is just a good practice to always use
GetHashCode when computing a hash code.
4:52
We'll learn more about how hash
codes are used in other courses
4:58
where we use more advanced
.NET collection types.
5:01
You need to sign up for Treehouse in order to download course files.
Sign up