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A Common Problem

Name: A Common Problem
Uploaded: 2019-01-02T12:26:20Z
Duration: 394 s
Description: In this video we'll take a look at the kinds of problems we can run into without generics!

6:34 with Ben Deitch

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

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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;
    }
  }
}

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[MUSIC] 0:00

Hi, I'm Ben, and in this course, we're going to learn about generics. 0:09

But before we get to any formal definitions, I think it'll help if 0:14

we start by looking at an example of the problem generics try to solve. 0:17

Let's start by creating a new project. 0:21

And let's click Next. 0:26

Select the template. 0:28

Hit Next again. 0:31

And let's name our project Generics. 0:32

And we can leave the package as com.teamtreehouse. 0:35

And I'll full-screen my app to make it easier to see. 0:40

Now that we've got an empty project, 0:43

we've got a few things to copy in from the teacher's notes. 0:45

First, let's create a new class, named Milk. 0:49

So right-click on the package, new class, and I'll call it Milk. 0:55

And then let's paste in our Milk class over the existing class Perfect. 1:00

Then let's do the same thing with the Oranges and Box classes. 1:08

So we'll right click, New > Java Class, Oranges, and 1:14

then we'll paste over it with what's in the teacher's notes. 1:18

And then we'll do New > Java Class one more time for the Box class. 1:27

And we'll paste that in. 1:37

Looking back at the Milk and Oranges classes, 1:44

notice that Milk has a function called drink. 1:47

And Oranges has a function called juggle. 1:51

And they each print something out to the console. 1:54

Also, to get classes side-by-side like this, you just right-click on the tab, and 1:57

choose Split Vertically. 2:02

All right, that's enough about Milk and Oranges. 2:03

Let's get on to the Box class. 2:06

As you might have guessed, this class aims to represent a box. 2:08

Though before we dig into the implementation details, let's use Cmd or 2:12

Ctrl+Shift+- to start with just the basics. 2:16

At the top, we have the contents of the box, represented as an object. 2:20

Then we have an add method to add an object to the box, and 2:26

a remove method to take an object out of the box, which returns an object. 2:30

Diving into the add method, we start by making sure the box is empty. 2:36

And if it is, we update the contents variable. 2:43

Otherwise, we print out, The box is full. 2:47

Moving on to the remove function, again, we start by checking if the box is empty. 2:50

And if it is, we tell the user that the box is empty and return null. 2:56

If the box isn't empty, then we pull out the contents of the box 3:01

into a new variable, update the contents to be null, and 3:06

then return that new variable, which is holding the contents. 3:11

So far, so good. 3:16

Let's head over to Main.java and try it out. 3:17

Let's delete the comment, and 3:21

start by creating a new Milk object and a new Oranges object. 3:23

So Milk milk = new Milk, 3:28

and Oranges oranges = new Oranges. 3:33

Then let's leave a space and create a new Box variable to hold our milk. 3:41

So Box, and let's call it boxOfMilk. 3:46

And let's set it equal to a new Box. 3:49

And let's create another Box for our oranges. 3:53

Box boxOfOranges = new Box. 3:56

Awesome, now that we've got our boxes, 4:02

let's use the add function to fill them up. 4:05

Let's add the milk to the boxOfMilk. 4:07

So boxOfMilk.add, and we'll pass in the milk. 4:11

And add the oranges to the boxOfOranges. 4:16

So boxOfOranges.add, and pass in the oranges. 4:18

With our boxes filled, the only thing left to do is take our objects back 4:25

out of the boxes and call the respective functions. 4:30

Drink for the milk, and juggle for the oranges. 4:33

Let's add another space, and start with retrieving the milk. 4:36

Let's type boxOfMilk.remove to get our milk back as an object. 4:40

Remember, the Box class stores its contents as an object. 4:47

So if we want this to be a milk object instead of just an object, 4:51

we'll need to add a cast. 4:55

Let's add the cast at the beginning by putting it in parentheses. 4:58

Then we'll need to add parentheses around the whole thing, 5:05

Before finishing up by calling the drink method, so .drink. 5:11

Then let's do the same thing with the oranges, 5:16

except this time we'll call the juggle method. 5:19

Let's add two left parentheses, and then Oranges for the cast. 5:21

Move over one parenthesis and type boxOfOranges.remove. 5:28

And finally, add the call to juggle. 5:33

To finish up, let's run the code and make sure everything works. 5:37

Perfect, but while this code does work, and there's not necessarily anything wrong 5:43

with it, there are a couple situations that can get us into trouble. 5:48

For example, what happens if we switch the boxes? 5:52

Let's put the oranges in the boxOfMilk, And 5:56

the milk in the boxOfOranges. 6:02

Looks good so far, now let's run the code. 6:05

And of course, we get an error. 6:10

Turns out we can't cast an Oranges object into a Milk object. 6:13

Wouldn't it be great if it would have just stopped us from putting the oranges 6:17

into the boxOfMilk or the milk into the boxOfOranges? 6:20

In the next video, we'll see how to do that by using generics. 6:24

But first, let's go over here and 6:28

click on Indent with 4 spaces to get rid of that warning. 6:31

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Generics in Java

A Common Problem

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