ListView and ArrayAdapter Code Udacity English English (auto-generated)

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Now let's look at the code that you've added in the last coding exercise, and

now let's look at the code that you've added in the last coding exercise and

there's a lot happening in just these three lines of code.

there's a lot happening in just these three lines of code so at a high level

So at a high level, we're creating an ArrayAdapter

we're creating an array adapter that

that contains a list of strings as its data source.

contains a list of strings as its status

Then we're searching through the view hierarchy for

source then we're searching through the

the ListView which was defined in our XML layout.

view hierarchy for the ListView which was defined in our XML layout and then

And then we're associating the array adapter with a list view.

we're associating the array adapter with the ListView for a more detailed

For a more detailed explanation of what's happening,

explanation of what's happening let's

let's take a look at it one line a time.

take a look at it one line at a time

First we create an array adapter object using the new keyword and

first we create an array adapter object using the new keyword and the

the constructor with three inputs.

constructor with three inputs we're

We're storing it in a new variable, the recalling items adapter.

storing it in a new variable that we're calling items adapter where the array

Where the ArrayAdapter is its data type.

adapter is its data type now how did we

Now how do we know to pass in these three inputs.

know to pass in these three inputs well

Well let's take a look at the documentation.

let's take a look at the documentation

So here's the under developer documentation for the array adapter and

so here's the Android developer documentation for the array adapter and

if you search online for ArrayAdapter Android you can also find this page.

if you search online for array adapter Android you can also find this page now

Now let's scroll down to the section called public constructors.

let's scroll down to the section called public constructors and this tells us

And this tells us the different ways we can create a new array adapter.

the different ways we can create a new array adapter each constructor requires

Each constructor requires different inputs.

different inputs so we've been using the

So we've been using the second to last constructor here.

second-to-last constructor here and

And here's the code that we've been using in the numbers activity file.

here's the code that we've been using in the numbers activity file so notice that

So notice that we pass in the context which refers to the numbers activity,

we pass in the context which refers to the numbers activity the layout file and

the layout file and the list of objects.

a list of objects now the reason we need

Another reason we need a context is because we are creating views.

a context is because we're creating

Since the numbers activity is a context

views since the numbers activity is a

we specify the word this to refer to the numbers activity.

context we specified the word this to refer to the numbers activity the second

The second parameter is the list item layout file.

parameter is the list item layout file

So normally we refer to the layout files in the form of R.layout for

so normally we refer to the layout files in the form of our layout for layouts

layouts we've defined, followed by the name of the layout file.

we've defined followed by the name of the layout file so in this case we

So in this case we specify android.R.layout because the Android

specify Android dot r dot layout because the

framework actually predefined an XML layout file called, simple_list_item_1.

android framework actually predefined an XML layout file called simple list item

And we can just use that one that Android predefined for us.

one and we can just use that one that Android predefined for us so the last

So the last printer is a list of objects which will be the data source for

parameter is the list of objects which

the array adapters.

will be the data source for the array

In our case it's words.

adapters in our case it's words now

Now earlier in this lesson we discuss generics

earlier in this lesson we discussed generics when we created a

when we created a string ArrayList.

string ArrayList I remember focus this

Remember focus this is ArrayList and not ArrayAdapter.

is a right list and not arrayadapter

Remember because ArrayList a generic class we could rewrite it

remember because ArrayList is a generic class we can rewrite it to be ArrayList

to be ArrayList and have E inside the angle brackets to indicate that's a generic class.

and have e inside the angle brackets to indicate that it's a generic class now

Now remember that E is type parameter or

remember that E is a type parameter or

placeholder which can be swapped for a specific object type

placeholder which can be swapped for a specific object type such as string when

such as a string when we create an instance of an ArrayList.

we create an instance of an ArrayList

Now creating ONE Generic class allows us to decide

now creating one generic class allows us

the types of data it will store later on, rather than having to create

to decide the types of data I will store later on rather than having to create

many specific classes such as a StringList class, an Integer class,

many specific classes such as a stringless class an integer class a

Boolean class or any other object type list class.

boolean class or any other object type list class so going back to our

So going back to our discussion about ArrayAdapters.

discussion about array adapters here's

Here's the documentation for ArrayAdapters.

the documentation for array adapters now

Now we see here that ArrayAdapter has the angle bracket T

we see here that array adapter has the

syntax which indicates that it's also a generic class.

angle bracket T syntax which indicates that it's also a generic class now T

Now T like E is a type parameter that can be swapped out for

like e is a type parameter that can be

a specific data object type.

swapped out for a specific data object

And specifically T stands for

type and specifically T stands for data

data type because when you say what types of data are in the array and

type because we need to say what types of data are in the array and in our case

in our case specifically, we have an array of data type strings.

specifically we have an array of data type strings now we're creating an array

Now we're creating an ArrayAdapter that specifically takes in strings and

adapter that specifically takes in

will be creating list item views for each of these string elements.

strings and we'll be creating list item views for each of these string elements

So back to this comparison table which demonstrates why it's better to create

so back to this comparison table which demonstrates why it's better to create

one generic class versus creating a class for every type of list possible.

one generic class versus creating a class for every type of list possible

Here, we see that for

here we see that for array adapter we

ArrayAdapter we can replace the T with any other object data type.

can replace the T with any other object

In our case, we're going to replace the T with the string data type.

data type in our case we're gonna replace the T with the string data type

All right, well, there was a lot happening in that one line of code.

all right well there was a lot happening in that one line of code so let's move

So let's move on to the second line.

on to the second line so here we're

So here we're looking in the view hierarchy for

looking in the view hierarchy for a view

a view with the ID name list and the findViewById method returns a view and

with the ID name list and the fine view by ID method returns a view and then we

then we cast it into a list view data type.

cast it into a list view data type the

The casting will work if we found a text view or a image view.

Kassie won't work if we found a textview or an image view in this case the

In this case the casting works because we actually did find a list view

casting works because we actually did find a ListView in the

layout so we store this ListView into a

So we store this list view into a variable called ListView.

variable called ListView and we can call

And we can call methods on it and

methods on it and interact with it while

interact with it while the app is running.

the app is running now on to the last

Now on to the last line of code. And here it says listView.setAdapter and

line of code and here it says ListView

it takes in itemsAdapter as its parameter.

dot set adapter and it takes in items adapter as its parameter so we're

So we're calling the setAdapter method on the ListView object we just defined

calling the set adapter method on the ListView object we just define in a

in the previous line.

previous line now if we go to the

Now if we go to the ListView documentation and search setAdapter.

ListView documentation and search set

We see that this method takes one input parameter.

adapter we see that this method takes one input parameter it requires an

They require an object of type ListAdapter but

object of type list adapter but hmm in

in our code we only have an ArrayAdapter, not a list adapter.

our code we only have an array adapter not a list adapter so let's click on

So let's click on list adapter to find out more information.

list adapter to find out more

Right, so now we're in the list adapter documentation and if we scroll down

information all right so now we're in the list adapter documentation and if we

a bit to the public methods we see that it has to abstract methods.

scroll down a bit to the public methods we see that it has two abstract methods

If we look at the top of the doc it says that this adapter is an interface and

if we look at the top of the dock it says that this adapter is an interface

remember we learn about interfaces in the last lesson and

and remember we learned about interfaces

how on click listener is all interface.

in the last lesson and how onclicklistener is an interface but we

But we need to provide an implementation for the methods, all that means that we

need to provide an implementation for the methods well that means that we need

need to create a class that implements the ListAdapter interface,

to create a class that implements the

so that we can properly set an adapter on the ListView.

list adapter interface so that we can properly set an adapter on the ListView

Well luckily the Andrew team has already done this for us, and

well luckily the Android team has already done this for us and we're free

we're free to create our own class that implements the ListAdapter interface,

to create our own class that implements the list adapter interface but a ray

but ArrayAdapter fits our needs.

adapter fits our needs so let's take a

So let's take a look back at the ArrayAdapter documentation page and

look back at the rain adapter documentation page and we can look at

we can look at the class hierarchy diagram.

the class hierarchy diagram so array

So ArrayAdapter extends from BaseAdapter.

adapter extends from base adapter so

So let's check that out.

let's check that out so it says here

So it says here that BaseAdapter is an abstract class.

that base adapter is an abstract class

And look, it implements the ListAdapter interface.

and look it implements the list adapter interface so this means that base

So this means that BaseAdapter can be used whenever ListAdapter is required.

adapter can be used whenever list adapter is required and a ray adapter

An ArrayAdapter can be used wherever a ListAdapter is required.

can be used wherever list adapter is

All right, so I know there was a lot of links to follow through and

required all right so I know there was a lot of links to follow through and it

it might have been a maze.

might have been a maze so thanks for

So thanks for trying to keep up and

trying to keep up and I'm gonna help

I'm going to help summarize all the information that we just talked about.

summarize all the information that we just talked about now in this graphic

Now in this graphic that shows the differences between the concrete class,

that shows the differences between the

abstract class, interfaces we're going to discuss

concrete class abstract class and interfaces we're gonna discuss how

how adapters applying to each of these three categories.

adapters applying to each of these three

So the list view set adapter method requires a list adapter as its input.

categories so the ListView set adapter method requires a list adapter as its input so

So the list adapter is an interface.

the list adapter is an interface and

And this means that no states are implemented and

this means that no states are

all methods are abstract.

implemented and all methods are abstract

It also turns out that the Android team has created a base adapter class,

it also turns out that the Android team has created a base adapter class which

which is an abstract class that provides an implementation

is an abstract class that provides an

of some of the list adapter methods.

implementation of some of the list

But it also leaves some other methods as abstract.

adapter methods but it also leaves some other methods as abstract now this is

Now this is where the ArrayAdapter comes in.

where the array adapter comes in the

The ArrayAdapter is a concrete class that has methods implemented.

array adapter is a concrete class that has methods implemented so no methods

So no methods are left abstract, and

are left abstract and we can create an

we can create an object instance of the ArrayAdapter and use it in our app.

object instance of the array adapter and use it in our app so since we verified

So since we verified that ArrayAdapter is ListAdapter,

that a right adapter is Alyssa daftar we

we can pass in the itemsAdapter variable as the input

can pass in the items adapter variable

To the list view dot set adapter method call.

as the input to the ListView dot a set adapter method call hey awesome work and

Hey, awesome work. And now you know how powerful these three little lines of code are.

now you know how powerful these three little lines of code are I know as a lot

I know it's a lot of information to take in at once but

of formations taken at once but it's

it's definitely worth it to take the time to understand all of it

definitely worth it to take the time to understand all of it because the adapter

because the adapter is a really common pattern in Android.

is a really common pattern in android so

So, that adapter knows about the data source whether it's an array or a list.

the adapter knows about the data source whether it's an array or a list and it

And it knows how to present each of these items as a view.

knows how to present each of these items

Meanwhile the ListView handles showing these views on screen,

as a view meanwhile the ListView handles showing these views on screen detecting

detecting the user's touch gestures and

the user's touch gestures and

maintaining state on where the user is within the entire list.

maintaining state on where the user is within the entire list and here are the

And here are the three lines of code that summarize

three lines of code that summarize this

this interaction between the ListView and the ArrayAdapter.

interaction between the ListView and the

We'll dive more into that later on.

array adapter we'll dive more into that

For now you can think of it as a separation between the user interface

later on for now you can think of it as a separation between the user interface

and the data model.

and the data model it's not exactly a

It's not exactly a clean separation

clean separation because the right

because the array adapter deals with the views but

adapter deals with the views but

primarily the ListView is in charge of the details of the user interface.

primarily the ListView is in charge of the details of the user interface while

While the adapter is in charge of the data.

the adapter is in charge of the data now

Now because it's separated, you can swap out one part for another.

because it's separated you can swap out one part for another for example you can

For example, you can hook up the same array adapter to a grid view.

hook up the same array adapter to a grid

The array adapter stays exactly the same and

view the array adapter stays exactly the

instead of showing list item rows, the grid view now asks for grid item views.

same and instead of showing this item rows the grid view now asks for grid

So this becomes position 0, position 1. And so on and so on.

item views so this becomes position 0 position 1 and so on and so on and the

And the adapter provides each of these views still.

adapter each of these views still so have you

So have you seen the drop down menu in Android?

seen the drop-down menu in Android well

Well, in Android speak these are called spinners.

an Android speak these are called the

And guess what?

spinners and guess what to populate each

To populate each item in the drop down, we attach an adapter to it.

item in the drop-down we attach an

So each item in the menu comes from an item in the data source.

adapter to it so each item in the menu comes from an item in the data source so

So as you can tell you run into the adapter pattern as you build

as you can tell you'll run into the

adapter pattern as you build Android

So it's critical that you understand it now.

apps so it's critical that you understand it now in the near term we're

In the near term we're going to see the pattern come back again in lesson five.

gonna see the pattern come back again in

For this next task, try to modify some of the list view adapter code and

lesson 5 for this next task try to modify some of the ListView adapter code

see what happens.

and see what happens

Then try to use the grid view instead of the list view.

then try to use the grid view instead of

So grid view also recycles views and

the ListView so grid view also recycles

needs an adapter to know what information to display.

views and needs an adapter to know

So this is all for experimentation purposes.

information to display so this is all for experimentation purposes and at the

And at the end of the task I'll ask you to go back and

end of the task I'll ask you to go back

reverse your code to using list views.

and revert your code to using list views

So here are the specifics of the practice.

so here are the specifics of the

Instead of using a list view,

practice instead of using a ListView try

try using a grid view to display the words in the numbers activity.

using a grid view to display the words in the numbers activity you can use the

You can use the same adapter that we created earlier.

same adapter that we created earlier and

And if you do need some help,

if you do need some help you can refer

you can refer to the article that I've linked below that discusses grid view.

to the article that I've linked below that discusses grid view but I want to

But I want to let you know before you go too far down the rabbit hole and

let you know before you go too far down

start copying and pasting a lot of code from the article.

the rabbit hole and start copying and pasting a lot of code from that article

You should be aware that this task of changing from the ListView to a GridView

you should be aware that this task of changing from the list view to a grid

can be done in the eWalk app by simply changing a couple lines of codes.

view can be done in the Miwok app by simply changing a couple line of codes

So after you've tried out this task go back and

so after you've tried out this task go

change your code to using a list view again.

back and change your code to using a

All right, best of luck and have fun experimenting.

ListView again alright best of luck and have fun experimenting

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