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
In this video we'll learn how to use type casting (or casts) to move an Object up (or down) it's family tree. We'll also see how to use the 'instanceof' operator to determine if an Object is of a specific type.
This video doesn't have any notes.
Related Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign upRelated Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign up
We just saw how we can group
together different objects
0:00
by using an array with
a common base class.
0:03
Now let's see how to use a cast so
we can call makeSound on our dog object.
0:06
First, since we got rid of our dog
variable, let's make a new one.
0:11
Let's add a new line after our
object array and type dog.
0:16
And we'll name it dog, and set it
equal to the first item in our array.
0:20
So list at position 0, which gives
us an incompatible types error.
0:25
This is where the cast comes in.
0:33
A cast, or type casting,
is when you tell Java that an object
0:36
is a more specific
descendant of that object.
0:40
Essentially moving the object
up its family tree.
0:44
For example, if we were storing
a car as a vehicle object and
0:48
needed to turn it back to a car object,
we would use a cast.
0:53
To use a cast, you just write the name
of the class between parentheses.
0:57
So right after the equal sign,
1:02
let's add a cast to change
list 0 to a dog object.
1:05
So parenthesis and
inside them we write the class, Dog.
1:10
Nice, it doesn't look
like we have any errors.
1:15
And if we run it, We get the right output.
1:19
But before we move on,
1:26
let's see what happens if we
try to merge these two lines.
1:27
Let's hide the run panel,
and then delete line seven,
1:31
and change dog to list at position zero.
1:39
Then let's add the cast,
and it doesn't work.
1:45
It turns out that the call to
makeSound happens before the cast.
1:50
So to make sure that
the cast happens first,
1:56
we just need to use some parentheses.
2:02
Okay, we've got an array of objects and
2:08
seen how we can use casts to move
an object up its family tree.
2:11
But this only works because we know
exactly where our dog is in the array.
2:15
If we accidentally tried to cast
our dog food object to a dog,
2:21
we'd crash the program.
2:25
And just to make sure, Yep,
we get a class cast exception.
2:27
Cmd or Ctrl+Z to undo, and
we're back to working code.
2:38
So instead of just picking
out the dog object,
2:43
let's loop through our list array and
only call makeSound if the object
2:46
actually has a makeSound method,
meaning the object is an animal.
2:51
Let's delete line 7 again, and let's use
a for each loop to loop through our array.
2:56
For object, we'll call it object, and,
3:04
which is represented by a colon,
3:09
our list array, and add the brackets.
3:13
And inside the for loop, we need to
determine if this object is an animal.
3:17
Luckily, there's a keyword for this.
3:22
Let's write an if statement.
3:24
And for the condition,
let's check that object, instance of.
3:28
Animal.
3:36
And notice that instanceof is
all one word and all lowercase.
3:37
Using the instanceof operator
allows us to check if one object
3:44
is an instance of another.
3:49
So inside the if statement,
3:51
after we add our brackets,
object is guaranteed to be an animal.
3:54
However, even though it's guaranteed to be
an animal, we'll still need to cast it.
4:00
Let's type object., and choose makeSound
4:06
to have IntelliJ automatically
add the cast and call the method.
4:10
Awesome, now let's run the app.
4:16
And perfect,
we're still getting the same output.
4:21
Using casts and the instance of operator
4:25
gives us a lot more freedom
with how we handle our objects.
4:28
Why don't you get a little more practice,
and
4:31
in the next video, we'll take
a deeper look at the object class.
4:34
You need to sign up for Treehouse in order to download course files.
Sign upYou need to sign up for Treehouse in order to set up Workspace
Sign up