1. Android 3: Exploring Apps and the Development Environment
Kirk Scott

3. Introduction
This unit surveys some of the code aspects of an Android app
It also surveys how you can find these aspects of an app through the Eclipse interface
The unit is not intended as a comprehensive treatment of these topics

4. It is intended to answer the questions that might spring to the mind of a programmer who is seeing app development for the first time
It is also motivated by this simple goal:
How could you change MyFirstApp, the “Hello World” app so that it displayed a different message?

5. The outline for this unit is shown on the following overheads
As you will see, this outline corresponds to parts of the project as they would appear in the Project Explorer in Eclipse

6. Outline 3.1 The Project Explorer
3.2 /src/com.example.myfirstapp/ Main_Activity.java
3.3 /gen/com.example.myfirstapp/ BuildConfig.java
3.4 /gen/com.example.myfirstapp/R.java
3.5 Android 4.3 and Android Private Libraries

7. 3.6 /bin/res/AndroidManifest.xml
3.7 /bin/res/MyFirstApp.apk
3.8 /res/layout/activity_main.xml
3.9 /res/menu/main.xml
3.10 /res/values/strings.xml
3.11 Grand Finale

8. 3.1 The Project Explorer

9. The screenshot on the following overhead shows the hello world app as it was shown in the previous overheads
The key point of interest at this moment is the Project Explorer on the left hand side
As given here, the MyFirstApp folder has been expanded to show the items immediately underneath it

11. No matter where’ve you’ve gotten to in Eclipse, you can always restore the Project Explorer
Take these options in the menu:
Window, Show View, Project Explorer

12. Expanded View of the Project in the Explorer
The screenshot on the following overhead shows the subfolders in the Project Explorer expanded to show the items in the outline of this unit
The following sections of this unit look at items of interest in these expanded folders one after the other from top to bottom

14. Once things are expanded, not everything is visible in the screenshot at the bottom
If you are following along on your own machine, depending on the setup, you may have to scroll down to see some of it

15. In the screenshot, Android 4.3 hasn’t been expanded
It will be dealt with separately
Future versions of these overheads may continue to show Android 4.3 even if the version number has changed, assuming that nothing else about it that’s being explained here has changed

16. 3.2 /src/com.example.myfirstapp/Main_Activity.java

17. The screenshot on the overhead following the next one shows what you see when you double click on Main_Activity.java in the Project Explorer
This is essentially the Java source code for the app
Notice that unlike a Java application, there is no main() method

18. The app code is slightly reminiscent of applet code, if you are familiar with that
The app class extends the Activity class
Instead of a main() method it has (overrides) an onCreate() method
Most of the details mean nothing at this point, but note that in the code, reference is made to a class name R

20. 3.3 /gen/com.example.myfirstapp/BuildConfig.java

21. If you double click on BuildConfig
If you double click on BuildConfig.java in the Project Explorer you see what’s shown on the overhead following the next one
This is an auto-generated file
It’s worth knowing that it exists, but for the time being the details aren’t important

22. In the previous unit it wasn’t necessary to go through building a configuration step-by-step
We let the configuration come into existence by default instead
In any case, an app that has been brought to the point of being runnable will have a build configuration

24. 3.4 /gen/com.example.myfirstapp/R.java

25. If you double click on R.java in the Project Explorer you see what’s shown on the overhead following the next one
R.java is another auto-generated file
It is too soon for details, but it is worth knowing that R.java is kind of a global container associated with an app

26. Note that it contains final declarations, the declarations of constants associated with the app
It’s also apparent that hexadecimal values are being used
It will be of interest later to see how to make use of the constants defined in the file R

28. 3.5 Android 4.3 and Android Private Libraries

29. The screenshot on the following overhead shows a subset of what you see when you double click on Android 4.3 in the Project Explorer, on the left
(The file R.java is still showing in the editor)
The Android 4.3 folder is a library which contains Android packages and if you were to scroll down further, you would find Java packages
The Android Private Libraries folder is similar
In effect, what you’re seeing is the set of API packages available when creating Android apps

31. 3.6 /bin/res/AndroidManifest.xml
The screenshot on the following overhead shows what you see when you double click on AndroidManifest.xml in the Project Explorer
You may recall that jar files have manifest files
An Android apk file, the result of building a project, is effectively a kind of jar file
Therefore, every completed app will have a manifest file associated with it

33. 3.7 /bin/res/MyFirstApp.apk

34. This section starts with some information taken from Wikipedia, starting on the following overhead

35. APK (file format) From Wikipedia, the free encyclopedia
Android application package file (APK) is the file format used to distribute and install application software and middleware onto Google's Android operating system. To make an APK file, a program for Android is first compiled, and then all of its parts are packaged into one file. This holds all of that program's code (such as .dex files), resources, assets, certificates, and manifest file. As is the case with many file formats, APK files can have any name needed, but must end with the four character, three letter extension, .apk.[1][2][3][4]

36. APK files are ZIP file formatted packages based on the JAR file format, with .apk file extensions. The MIME type associated with APK files is application/vnd.android.package-archive.[5]

37. In short, the apk file is a jar file for an Android app, which packages up the manifest and the compiled source code for distribution
The screenshot on the following overhead shows what you see when you double click on MyFirstApp.apk in the Project Explorer
Not surprisingly, it looks more or less like what you see when you open up a class file in an editor—binary nonsense

39. 3.8 /res/layout/activity_main.xml

40. The screenshot on the following overhead shows what you see when you double click on activity_main.xml under /res/layout in the Project Explorer
As seen before, this is the layout of the output of the app as shown in the development environment
The layout is developed separately from the code logic, so it’s important to be able to find activity_main.xml in order to be able to work with it

42. Consider the contents of the screenshot again:
On the left, there is the Eclipse Project Explorer, with the folders expanded
In the center, there is a view of the graphical layout of the app
This visible graphical layout is defined by the file activity_main.xml

43. Between the explorer and the layout is a palette of graphical tools and components for creating visual layouts for apps
At the bottom of the editor are two tabs, one for Graphical Layout and one simply showing the name of the file, activity_main.xml
Clicking on the activity_main.xml tab shows you the xml source code, as shown on the following overhead

45. The layout file includes layout syntax
It also includes lines like this:
This refers to a resource belonging to the app which is defined elsewhere in the environment
The relationship between resources and references is an important aspect of app development

46. 3.9 /res/menu/main.xml

47. The screenshot on the following overhead shows what you see when you double click on main.xml under /res/menu in the Project Explorer
There is nothing of consequence here for the moment
It is simply included to bring to your attention the fact that the layout you’re familiar with is /res/layout/activity_main.xml, not what you see here

49. 3.10 /res/values/strings.xml

50. The screenshot on the following overhead shows the Project Explorer scrolled down to show /res/values/strings.xml
It also shows what you see when you double click on strings.xml
(Note that of the two tabs at the bottom of the editor screen, you need to be on strings.xml, not Resources, if you want to see the XML source code)

52. What about strings.xml?
strings.xml is the last item on this tour of things to be found in the explorer
Practically speaking, it’s also the most significant at this point
We are finally in the place where we can make a simple, initial modification to the app

53. In the original version of the app, this line appeared in strings.xml:
<string name="hello_world">Hello world!</string>
If you look carefully at the foregoing screenshot, you’ll see that the line has been changed to this:
<string name="hello_world">Good-Bye Cruel World!</string>

54. The code for the app displays a resource by reference
The resource, a string, is defined in strings.xml, separate from the app code
This is a significant feature of Android development that will be dwelled on in following sets of overheads

55. 3.11 Grand Finale

56. At this point, if you’ve made changes to strings
At this point, if you’ve made changes to strings.xml, you can go back to the MainActivity.java, as shown on the following screenshot
A change in the output string of the app requires absolutely no change in the Java source code

58. From MainActivity.java, you can run your application, whether on the emulator or on an attached device
When you click run, you’ll be prompted to save the changes to strings.xml if you didn’t save out of that editor screen
Ta-Da: The following screenshot shows success on the emulator

60. Summary and Mission
This is the end of the initial presentation of the components of an app that can be found in the Project Explorer
High points of what can be found there:
MainActivity.java, the source code
activity_main.xml, the layout
strings.xml, the file containing the string resources for an app
R.java, the file containing resources as defined in the Java code for the app

61. You have two missions, neither of which are graded homework:
1. Create a new Android project and modify it so that its output is not “Hello World”
This should work, and should consist essentially of changing strings.xml
The point of this mission is obviously not the importance of the change
The point is finding strings.xml and reinforcing what relationship it has with the app code

62. 2. Things related to this were mentioned only in passing in this set of overheads, but it’s not too soon for you to conduct a small experiment in preparation for coming attractions
Using the palette of graphical tools for activity_main.xml, drag and drop some new item into the layout for an app

63. Note the contents of R.java before building the project
Then try building the project and consider two things:
A. Do you get error messages?
If so, what are they, and what do they imply?
B. Were there any changes in R.java?

64. The point of the second mission is not necessarily for you to have a firm grasp on what’s going on
The point is just to have you find activity_main.xml, fiddle with the graphical tools palette, and then find R.java in the explorer

65. The End