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like i've mentioned perviously, activities are one of the primary building blocks for your applications.

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and an activity is simply a single screen in your application.

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so let's say, an email client, if we have the list of emails, that's one activity,

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when we have the message details for particular massage, that's another activity

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the compose screen, where you write a new massage, that's another activity.

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so it's really important to grasp and learn the fundamentals of activities.

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so i'm gonna create a new android project, and i'm gonna call it LearnActivities

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we're gonna now save it in the default work space, again we're gonna be targeting gingerbread,

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for the name of my application, i'll just call it Activities,

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package : com.leebrimlow.activities

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we're gonna create a main activity, which is the activity that user sees when they first launch your app,

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and what i like to do is i always call that Main.
min SDK version which is targeting gingerbread, and i'm gonna click finish.

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so like i showed you before at this point we actually have a fully working application, the HelloWorld application,

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we have our Main.java which is the file that actually extends the activity class that represents our initial main activity.

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and for the actual content, the visual content, it's referencing a file, called main, which is in the layout folder in my recourses,

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so if i go to resources, layout, there we see that main.xml, and by default it just has HelloWolrd, Main!

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just has some text in side of a textview, so i'm gonna run that just to make sure it's working correctly,

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again we wanna run it as an AndroidApplication, and we'll lunch up the emulator

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and we can see it's running correctly.

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so i quickly wanna point out a couple of things in the android manifest file, so we could see here in the xml view

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inside of our application tag, we have our activity. now one of the things you'll see inside of here

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is this intent-filter. we're gonna be talking about intents in a lot more detail, i'm coming up in another movie,

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but this basically tells the android system by putting in this action

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that this is the activity that should be launched when the application is first launched,

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and it does that by putting in this "android.intent.action.MAIN" so this is basically advertising the android system,

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hey when you lunch me, lunch this activity first,

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and it's this main activity that we have. so again we're gonna get into more details about intents in a little bit.

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but let's say that i wanna add another activity to my application because usually your applications are gonna have multiple activities,

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we basically need to recreate these two files, for different activities.

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so first we need to create another java file that extends Activity. so i'm just gonna right click in my main package,

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and i'm gonna select new->class, and i'm gonna call this one Second, so i'm gonna click browse.

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and here i'm gonna want to extend activity which is in android.app package. click OK.

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and i can just click finish.

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so now it's created another java file called Second, which extends activity.

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but i also need to override, this onCreate method, so that i can actually set which UI i want to put into this activity,

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so i can just come inside of here, and give myself some space, start writing oncreate and hit ctrl+space.

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and now we can this first option, will create the override method for me.

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now i haven't actually created a new xml file, so i don't anything to put here yet, so that's the next step.

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i need to create an xml file, similar to main.xml which is gonna hold the UI for the Second activity.

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so this third button here in the toolbar opens up a wizard to help create a new android xml file. so i'm gonna click that.

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and here i need to give it a name so i'm gonna call this second.xml.

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now the naming conventions here are really strict so you can't have a capital letter here, you can't have different symbols,

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and it has to be lower case, so i'm gonna put second.xml. and now it's asking us what type of resource would you like to create.

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well i wanna create a layout resource, very much like main.xml so i'm gonna choose layout.

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and now it's asking me what do i want the root element for that xml file to be.

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essentially what is gonna be my root layout container, so there are a bunch of different layout options that you have in android,

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and we're gonna get to that also in a future movie. we're gonna leave the default as linear layout which is pretty much the simplest most standard layout option

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as we add things, they laid out in a linear fashion one after another.

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so we're gonna click finish. and now it's created a new xml file and here we are in the graphical layout view.

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so from this point i can actually just start adding various UI controls to this. so let's say i want very much like the main.xml

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i just want this one to have a textview, so i'm gonna drag that out here, we can see there it is.

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and in here i can actually go into my properties, (let me actually just make this stuck to the side there).

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now i can set all the properties for this textview. so if i scroll down,

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i can see one of the properties here is actually the text property or what's gonna be inside of this textfield, what is the text gonna say,

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so i'm just gonna change that to "This is the second activity".

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and that's all what we're gonna do at this point, so it's gonna be very simple when we get to this activity,

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it's just gonna display this massage. if we wanna go and look at what the xml looks like,

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of course we can go back and look at that, and you'll notice that things are not wrapped nicely,

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well one thing we can do is to right click, goto Source, and then cleanup document, and then just click OK.

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and it's actually going to make things be laid out a little bit better. but again very much like main.xml we have a linear layout with a textfield

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OK so now that we have the second activity, but we need a way to actually be able to lunch it, because remember when we first come into the app

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we're coming to this main activity, so what we're gonna do here is to first i'm gonna delete this textview.

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and now we're gonna put a button in here, so i'm just gonna drag this button into the activity.

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if we wanna look at the properties here, we can. the ADT tools will actually automatically give any controls that you drag out here an ID,

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that you can actually come in and customize this id which is how you reference the control throw out your project,

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but for right now we're gonna leave it at the default.

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so now i have this button, now i need to actually write the code so that when the user taps this button it's gonna launch the second activity. 

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so i'm gonna go to my main.java file. and under where i set the content view,

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the first thing i need to do is to get a reference to that button control that i put in the xml.

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so to do that i'm gonna create a button property, and i'm just gonna call it b, to keep it simple.

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and in the way which we can get a reference to UI components in our xml is to use the findViewById method,

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and this will actually go out if we give it an Id, and return us a reference to that control, so in order to actually get the id,

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we're gonna wanna go to R.id there you can see our button control with that name that was given by default,

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so this will now return a reference and save it in this b property that we created,

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if we hit ctrl+1 here, we can see it's giving us an error, it will give us a quick fix which is to add cast to this button,

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so we're basically casting it to a button control, so we now have our reference to that button stored in this b property.

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so the next thing we need to is to set up the onClick listener, so that when ever the user clicks that button,

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we can respond to it and actually cause something to happen in our application so the way which we can do that is to say,

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b.setOnClickListener we can see it's the first choice here, so i'm gonna select that and now inside of here what we're gonna do

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is to create a new onClickListener so we're gonna say new, and i'll just start typing it,

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and then hit ctrl+space. and we can see it's gonna create an anonymous inner event handler in which we can response to this event,

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so when i click this we can see it's automatically created that stop for me, again if i hit ctrl+1

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i'll be able to get some quick fixes here and i just wanna import the onClickListener class,

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and i'll come down here, and put a semicolon. so now inside of this onClick method,

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this is where i can actually do something when the user has clicked that button, so all i'm gonna do is to lunch this other activity

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so the way in which we start activities, is to actually call the startActivity method,

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so to this method we pass in an instance of the intent class, and the intent class which we're gonna cover a lot more in a future movie,

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but this actually just broadcast, hey i want to activate this particular component or this activity.

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so we're just gonna do an inline here, i'm gonna create a new intent.

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now there's a bunch of different constructer overrides here that i can pass in, the one we're gonna use specifically to lunch activities is this one,

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in which we pass in a context and then we pass in which class which represents the activity do we want to call?

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so now when it comes to context, this is something that you'll have to use quite a bit as you doing android development.

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so any time we request to the system to launch something, we send in a context so that, that component knows who is requesting it essentially.

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so now there's a few different ways to get the context, but because we're in an anonymous inner method here,

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the easiest way to get the content is to just say Main.this

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which is referencing our main file here, and this basically tells the system that hey the main activity is requesting this action.

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so the next thing we send in is the class of which activity we want to launch, well that is Second

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and then we give it the class property, and then we can close that.

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so really quick just to recap, we've gotten a reference to our button which we define in our xml here,

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we've set up the onClickListener, we just did it as an anonymous inner type event handler here,

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there are other ways to it as well, which we'll cover when we get to talking more about button controls,

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when the user clicks, and you'll notice in android it says click event though the user typically will be doing it with their finger, it's the same thing.

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we're just gonna start a new activity we're passing in this intent object, which basically is saying who is requesting this,

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well it's this activity right here which is Main. and then which activity we'll want to launch.

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so let's go ahead and launch that. and we'll save these resources.

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and let's go to the emulator.

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so we can see it's launched now and when i click on the button, now you can see what happens when i launched this is we actually get an error,

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and it says Sorry the application Activities has stopped unexpectedly.

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so now this is gonna happen to you often during your development, so how do you respond to this, and how do you figure out what's wrong?

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well let's go back to Eclipse, now what we wanna do is to go to a deferent perspective, so right now we're in the java perspective.

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if we click this and then go to other. we can see that there's one specific to android called DDMS.

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and this contains a bunch of different tools in which you can monitor how your application are actually running on the emulator or actually on a real device.

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one of the most useful areas is LogCat so as your applications are running in the emulator or a device,

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there are constantly sending out massages about things, and we can already see that we have a big one in red, which is generally not a good thing.

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so if we scroll up here we can see what happen, FATAL EXCEPTION : main ActivityNotFoundException.

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so it couldn't find that activity that we created, so why was that?

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it's because we haven't actually added it to the manifest file. so let's go back to the manifest file,

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remember any activity that exists in your application, it needs to be defined here in the manifest file.

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so all i'm gonna do is to create a new activity here.

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and i'm just gonna start typing name and then hit ctrl+space give me the code hinting,

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now for name we actually need to reference the name of it, so of we hit period that basically gets us to the end of our default package here,

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and i'm just gonna type in Second and that's all we need to do actually inside of here because we don't need to define any intents.

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that's all we need to do with this point just to define this activity.

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OK so let's go ahead and relaunch this and go back to our emulator.

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and let this load-up.

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OK so i'm gonna click this button now, and now you can see it's actually gone to the second activity but where is the textview which says hey this is the second activity?

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well if we go back to our java file. you'll notice the difference between this and the main one is that we haven't set the contentView

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which basically tells the activity which xml file to use for it's visual content.

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so what we're gonna do is to call setContentView and then give the id of the layout file that we wan to use.

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so to get that we say R. and then go to layout and we wanna use the second.xml file here.

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and with that we should be done. so let's launch it one last time.

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there it is, we click the button and now it's actually showing us the second activity with the visual content from that second xml file.

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so again that's what it takes to add a new activity to your application. again you're gonna be creating a java file,

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if your gonna be using xml for the UI, you're gonna need to create that.

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and then a really important part is that you need to define it in your manifest file.

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so we're gonna be getting a lot more into activities and how they work with intents, but that's a basic overview of creating activities. 

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