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Now that we have completed the Hello World application

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and seen it running, either on the emulator or on a real device,

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let's go ahead and examine this application in a bit more detail

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in order to understand some aspects about Android applications.

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As we have already realized, Android applications

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are all written in the Java programming language.

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Android Studio then compiles the Java code along with the data and resources

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that you supply for your application, and then prepares a package called

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as the .apk package.

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This is what you install on a real device or on an emulator.

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Of course, when you download applications from the Play Store,

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this is automatically done for you.

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So essentially, it involves downloading this .apk package and then installing

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it on your device.

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When you deploy your application from Android Studio, either to the emulator

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or to a real device, it is exactly the same process.

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The .apk package is pushed onto the device and then installed

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on the device.

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What does the .apk package actually contain?

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It contains the compiled Java classes, which are prepared from the Java code

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that you write together with resources and other data files

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that you supply for your Android application.

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We'll examine the remaining two, the resources and data

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files, later on as we go through the course.

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Let's now return to Android Studio and examine the Hello World application

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in a little more detail.

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In Android Studio, you will notice the user interface layout

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that you see corresponding to what you see running on the real device.

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We'll examine this in a bit more detail in one of the later sections.

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Let's switch to the MainActivity.java tab,

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and then, have a look at the source code being displayed here.

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Let's take a closer look at the Java code in MainActivity.java.

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As we examine this code, let me draw your attention

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to two sections of the code.

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First part, you notice that we are extending a class called

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as the Activity class here.

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And the second part, you see that we are overriding

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certain methods inside this class here.

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In particular, let me draw your attention to the fact

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that we are overriding a method called onCreate method here.

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And we have a few lines of code inside the onCreate method.

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The remaining part of the code, we will examine in one of the later exercise.

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Now that you have had a closer look at the Java code, let's now go ahead

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and examine why we subclass the Activity class

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and what exactly is the Activity class that we are subclassing in the code.

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To explain it in a bit more detail, a typical Android application

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is composed of four basic components, activities, services, broadcast

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receivers, and content providers.

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Let me briefly summarize the four kind of components that can

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form part of an Android application.

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Activities are typically dealing with the user interface

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part of your application.

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Services are primarily used for doing background work on your behalf.

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Content providers are dealing with data storage

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and exposing the data within your application to the remaining components

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or to other applications.

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Broadcast receivers are used to deal with any events that need

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to be delivered to your application.

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This is a broad summary of these four components.

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As we go through the course, we will examine some of these in more detail.

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Let's, in particular, examine what an activity is.

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As I have just emphasized, an activity deals with the user interface aspects

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of your Android application.

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Every screen that you see within you application

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is controlled by an activity behind the scenes.

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So how do you create an activity?

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You create an activity by extending the Activity base class.

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And within this class, you will implement the appropriate code

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to create the user interface.

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And the activity also deals with providing the ability for users

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to interact with your applications.

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We'll examine each of these in more detail in some

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of the later examples in this course.

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Typically, when you launch your Android application,

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you click on the icon corresponding to your application.

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Now essentially, what happens is that when you click on the icon

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this results in a call into the Android framework, which

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then goes ahead and creates the appropriate activity that

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displays the user interface for your application.

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Creating the activity involves the Android framework

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calling the onCreate method that you implement within your Activity class.

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Hence, the reason you see the onCreate method in the code

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that we just examined a bit earlier.

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Returning to the code again, you notice that, within the onCreate method,

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we are calling two different methods here again.

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The first one is a call to the super class.

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And you're passing a Bundle there.

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Don't worry about what it is at this moment.

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We'll come to that in a later part.

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The second statement that you see here is setContentView.

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And then, you are supplying something called R.layout.activity_main.

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This parameter supplies the layout of the user

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interface for your application.

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And so when the activity gets created, this particular layout

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will be put on the device's user interface.

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That is the reason why you see the user interface being

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defined in another file called as the layout file.

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We will examine layout files in more detail in the following example.

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Not that we have a basic understanding of an Android application,

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let's now go to the next example where we

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will look at user interface widgets, layouts, and event listeners

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in more detail.