A Common Problem

6:34 with Ben Deitch

In this video we'll take a look at the kinds of problems we can run into without generics!

Teacher's Notes
Video Transcript
Downloads

Milk.java

class Milk {
  void drink() {
    System.out.println("You drink the milk.");
  }
}

Oranges.java

class Oranges {
  void juggle() {
    System.out.println("You drop the oranges on the ground.");
  }
}

Box.java

class Box {
  private Object contents;

  void add(Object thing) {
    if (contents == null) {
      contents = thing;
    } else {
      System.out.println("The box is full.");
    }
  }

  Object remove() {
    if (contents == null) {
      System.out.println("The box is empty.");
      return null;
    } else {
      Object thing = contents;
      contents = null;
      return thing;
    }
  }
}

0:00
[MUSIC]
0:09
Hi, I'm Ben, and in this course, we're going to learn about generics.
0:14
But before we get to any formal definitions, I think it'll help if
0:17
we start by looking at an example of the problem generics try to solve.
0:21
Let's start by creating a new project.
0:26
And let's click Next.
0:28
Select the template.
0:31
Hit Next again.
0:32
And let's name our project Generics.
0:35
And we can leave the package as com.teamtreehouse.
0:40
And I'll full-screen my app to make it easier to see.
0:43
Now that we've got an empty project,
0:45
we've got a few things to copy in from the teacher's notes.
0:49
First, let's create a new class, named Milk.
0:55
So right-click on the package, new class, and I'll call it Milk.
1:00
And then let's paste in our Milk class over the existing class Perfect.
1:08
Then let's do the same thing with the Oranges and Box classes.
1:14
So we'll right click, New > Java Class, Oranges, and
1:18
then we'll paste over it with what's in the teacher's notes.
1:27
And then we'll do New > Java Class one more time for the Box class.
1:37
And we'll paste that in.
1:44
Looking back at the Milk and Oranges classes,
1:47
notice that Milk has a function called drink.
1:51
And Oranges has a function called juggle.
1:54
And they each print something out to the console.
1:57
Also, to get classes side-by-side like this, you just right-click on the tab, and
2:02
choose Split Vertically.
2:03
All right, that's enough about Milk and Oranges.
2:06
Let's get on to the Box class.
2:08
As you might have guessed, this class aims to represent a box.
2:12
Though before we dig into the implementation details, let's use Cmd or
2:16
Ctrl+Shift+- to start with just the basics.
2:20
At the top, we have the contents of the box, represented as an object.
2:26
Then we have an add method to add an object to the box, and
2:30
a remove method to take an object out of the box, which returns an object.
2:36
Diving into the add method, we start by making sure the box is empty.
2:43
And if it is, we update the contents variable.
2:47
Otherwise, we print out, The box is full.
2:50
Moving on to the remove function, again, we start by checking if the box is empty.
2:56
And if it is, we tell the user that the box is empty and return null.
3:01
If the box isn't empty, then we pull out the contents of the box
3:06
into a new variable, update the contents to be null, and
3:11
then return that new variable, which is holding the contents.
3:16
So far, so good.
3:17
Let's head over to Main.java and try it out.
3:21
Let's delete the comment, and
3:23
start by creating a new Milk object and a new Oranges object.
3:28
So Milk milk = new Milk,
3:33
and Oranges oranges = new Oranges.
3:41
Then let's leave a space and create a new Box variable to hold our milk.
3:46
So Box, and let's call it boxOfMilk.
3:49
And let's set it equal to a new Box.
3:53
And let's create another Box for our oranges.
3:56
Box boxOfOranges = new Box.
4:02
Awesome, now that we've got our boxes,
4:05
let's use the add function to fill them up.
4:07
Let's add the milk to the boxOfMilk.
4:11
So boxOfMilk.add, and we'll pass in the milk.
4:16
And add the oranges to the boxOfOranges.
4:18
So boxOfOranges.add, and pass in the oranges.
4:25
With our boxes filled, the only thing left to do is take our objects back
4:30
out of the boxes and call the respective functions.
4:33
Drink for the milk, and juggle for the oranges.
4:36
Let's add another space, and start with retrieving the milk.
4:40
Let's type boxOfMilk.remove to get our milk back as an object.
4:47
Remember, the Box class stores its contents as an object.
4:51
So if we want this to be a milk object instead of just an object,
4:55
we'll need to add a cast.
4:58
Let's add the cast at the beginning by putting it in parentheses.
5:05
Then we'll need to add parentheses around the whole thing,
5:11
Before finishing up by calling the drink method, so .drink.
5:16
Then let's do the same thing with the oranges,
5:19
except this time we'll call the juggle method.
5:21
Let's add two left parentheses, and then Oranges for the cast.
5:28
Move over one parenthesis and type boxOfOranges.remove.
5:33
And finally, add the call to juggle.
5:37
To finish up, let's run the code and make sure everything works.
5:43
Perfect, but while this code does work, and there's not necessarily anything wrong
5:48
with it, there are a couple situations that can get us into trouble.
5:52
For example, what happens if we switch the boxes?
5:56
Let's put the oranges in the boxOfMilk, And
6:02
the milk in the boxOfOranges.
6:05
Looks good so far, now let's run the code.
6:10
And of course, we get an error.
6:13
Turns out we can't cast an Oranges object into a Milk object.
6:17
Wouldn't it be great if it would have just stopped us from putting the oranges
6:20
into the boxOfMilk or the milk into the boxOfOranges?
6:24
In the next video, we'll see how to do that by using generics.
6:28
But first, let's go over here and
6:31
click on Indent with 4 spaces to get rid of that warning.

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Generics in Java

A Common Problem

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