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Queueing

9:10 with Travis Alstrand

Let's explore the `java.util.Queue` interface and learn how to poll for the next item to be processed.

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Alright, we're now ready to get familiarized with the final data 0:01

structure interface I wanted to make sure you picked up from this course. 0:04

And that is the queue interface. 0:09

Queues are used when you want to store elements prior to processing. 0:11

They provide convenient methods 0:15

for grabbing the next element and removing it from the list. 0:16

This is called polling. 0:20

By default, queues work in FIFO, first in, first out. 0:22

I put stuff in the queue that I want something else to process when it's ready. 0:26

And that program does so by calling the pull method on the queue. 0:30

If there is an element, it gets removed and returned. 0:34

If not, it returns a null. 0:37

This is used a lot in loops. 0:39

While there's a next element in the queue to process, process it. 0:41

Let's allow our users to choose a song to add to a queue 0:45

and add a way for our karaoke jockey to poll for the next song. 0:49

Okay, before we 0:54

get cooking, I noticed something a little off with what I did last time. 0:55

I forgot to add the heading when we were prompting for songs. 0:59

So, you know, before this prompt for index happens, all of a sudden 1:02

there's this blast of songs without any message there. 1:05

So let's add one there. 1:08

System.out.printf 1:10

Available songs for %s 1:16

%n and 1:21

artist. 1:24

That should look much better. See? 1:25

We all make mistakes. 1:27

It's part of the learning process, so I'm totally cool with it. 1:28

Let's go break some more stuff. 1:31

Okay, so let's go explore that queue interface. 1:33

Let's pop open the documentation, 1:36

and let's take a look at some of its implementing classes. 1:38

So we have array deque. 1:41

Here's that linked list. 1:43

Let's jump into this array deque. 1:45

I think I remember them talking about use this if you're not sure. 1:47

So this is a 1:51

resizable array implementation of the deck interface. 1:52

Array decks have no capacity restrictions. 1:56

They can grow as they need, not thread safe. 1:59

Sounds good. Let's use that. 2:01

So this has an add method where you add the element 2:03

and it will return true if that worked. 2:06

It also has poll so it retrieves and removes the head of the queue 2:11

represented by the deque. In other words, the first element of this deque. 2:15

So a deque, as opposed to a cue 2:24

that we were talking about, is a double-ended cue. 2:26

Right? 2:29

The name deque is short for a double-ended cue. 2:29

Cool. 2:32

So let's go use it. 2:33

So first things first, 2:36

let's import it so I don't forget like last time. 2:37

Karaoke machine, here we go. 2:40

So import both the implementation and the interface. 2:42

And let's add a member variable for this. 2:59

We'll say private queue 3:04

called msong queue. 3:08

And here we'll initialize it. 3:13

Array deque, double ended queue, 3:19

and we want songs in it. 3:21

Cool. 3:25

Alright, so now that we have it, let's take care of that to do 3:27

we left here in the choose method. 3:29

So to do add song to the queue. 3:31

So now we have the queue and we know that we can add 3:34

the song. 3:36

Great! 3:42

Now let's show the count in the prompt action, 3:43

so people know how long they'll be waiting, right? 3:46

So there are songs available and %d 3:49

in the queue. 3:51

So let's add that extra msongqueue.size. 3:55

Cool. 4:05

I think we can now call this ticket here. 4:05

As a singer, I should be able to choose a song and place it in the queue 4:08

so I have the opportunity to sing. 4:11

I think we can call that done-zo. 4:13

You know, while we're in here, why don't we look at this KJ one? 4:16

As a karaoke jockey, or KJ, I should be able to call out the next song 4:19

in the queue to sing and the video should be available 4:24

so that karaoke occurs. Let's do that while we're in here. 4:26

Okay so let's add a method down here. 4:30

Let's make it public so anybody can call this one. 4:34

It doesn't need to return anything. 4:38

Call it playNext, and it also doesn't need to take anything. 4:40

So let's go ahead and use that queue. 4:45

So the mSongQueue, 4:50

and we saw that there's this thing called poll, right? 4:51

So what that will do is it's going to take the next one that's available, 4:54

if there is one, and remove it from the queue as well as return it. 4:57

But if it doesn't exist, it's going to be null. 5:01

So just in case somebody says hey, let's play the next one, and they didn't 5:04

look at the queue count, we should tell them how 5:07

this is supposed to work, right? 5:09

Good error messages are always helpful. 5:10

So if song equals null, we'll say, sorry, there are no songs in the queue. 5:13

Use choose from the menu to add some. 5:24

And so else, if there is a song, let's go ahead 5:31

and show them the song. 5:34

We'll give them some space. 5:38

Open %s to hear %s by %s. 5:40

We'll do some new lines after that too. 5:45

So we're saying, open the video URL 5:49

to hear the song title from the song artist. 5:51

Or at least in the style of, I guess. 5:54

Okay. 5:56

Finally, 6:01

let's add that to the menu so that KJ can do his job here. 6:01

So let's copy and paste this add line. 6:04

Then we'll say play. 6:09

Play next song in the queue. 6:19

And let's go ahead and we'll add that to our switch down here. 6:22

So if somebody does enter play, 6:27

we'll just call that method playNext, and let's remember to break. 6:28

Alright, so let's go 6:35

ahead and kick this off. 6:36

Well, it looks I misspelled an import. 6:45

I bet you've all been yelling at me through your screens, huh? 6:48

It happens to all of us. Let me fix that. Aye, 6:51

yi, yi. Hmm. 6:59

Object cannot be converted to song. 7:09

Line 122. 7:12

That's where we're polling. 7:15

That looks right. 7:17

Maybe I declared it wrong. 7:18

Oh, here we go. 7:21

I forgot to put the generic type. 7:22

So we saw there it said incompatible types. 7:24

When I declared it, I forgot to say it was of type song. 7:27

So that is the message that you'll get sometimes if you use these interfaces. 7:31

Glad this error happened, actually. 7:35

If you don't put that type 7:37

definition there, what happens is it returns an object. 7:38

So here we go. Let's try that again. 7:41

There we go. 7:45

We got this thing working. Okay. 7:45

So we'll add. 7:48

This time I want to add a song by Level 42 7:49

called Lessons in Love. 7:52

And I'll just do example.com again. 7:58

Okay, so there we go. 8:02

Lessons in Love was added. Great song. 8:03

There's one songs available and zero in the queue. 8:06

So I'm going to go ahead and choose. 8:09

I'll choose Level 42. 8:11

I want to choose lessons in love 8:13

and now we'll see that there are one song - 8:15

there are one songs available. There we go with that bug again 8:18

and one in the queue. So let's go ahead and play that. 8:22

So it says open this to hear yada yada yada. 8:27

So if this were a real link, I could highlight this 8:30

and right click and select go to and it would open it up for me. 8:32

Cool and down here we can see that it's been removed from the queue. 8:37

All right, so let's go close that ticket. 8:40

So we are doing good. 8:44

We're all done except for storing. 8:46

Let's figure out a way to keep this information around. 8:47

Nice. 8:52

Now we know about the queue interface, and that wraps up all the Java Collection 8:53

Framework interfaces that I wanted you to get a chance to play with. 8:57

Our karaoke machine is just about ready for its first party. 9:01

All we need now is a way to load it up with some songs. 9:04

Let's go knock out that last ticket. 9:07

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