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Learning Android Application Programming: Creating an Android User Interface

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This chapter shows how to create an Android application. By doing so it shows you how to code with the Android activity lifecycle, how to code a simple user interface, and how to specify user preferences.
This chapter is from the book
  • Life is like a riding a bicycle, you don’t fall off unless you stop pedaling.
  • —Claude Pepper

Now it’s time to begin coding the On Your Bike application. This Android app will act as a bicycle computer—a device, usually clipped on the handlebars, that helps you keep track of the length and time of your ride. By creating this application, you will learn more about how to code with the Android activity lifecycle, how to code a simple user interface, and how to specify user preferences.

Refactoring Your Code

Because of project time pressures, you often need to make quick changes to code. Over time, these little changes add up, and, as a result, you need to revisit the code before the project is complete. This is known as technical debt. The code base becomes fragile, and it’s easy to introduce bugs and more difficult to maintain the code. It’s important to have a spring cleaning every now and then to fix the most obvious issues.

It makes sense to rearrange the code at a time when you’re not trying to change its functionality, a process referred to as refactoring. Of course, it’s also much easier to change functionality when you have clean, refactored code.

When you’re undertaking a major refactoring, don’t forget to back up your code first, or, better still, keep your code under version control. But don’t despair if you get lost and make a mistake with your code: You can always download the code for the chapter from the On Your Bike website (http://www.androiddevbook.com) or from GitHub (https://github.com/androiddevbook/onyourbike).

The simplest form of refactoring is to rename packages, classes, methods, and variables. You might do this for several reasons.

  • Renaming a class, method, or variable will increase the readability or understanding of the existing code.
  • Naming wasn’t consistent across the application.
  • A method’s functionality has changed, and it now does something a little different from what its original name indicated. It makes sense to rename the method to something more descriptive.
  • You can move duplicate blocks of code into a single new method. This can help implement the Don’t Repeat Yourself (DRY) principle, whose primary purpose is to prevent the repetition of information.
  • You can break larger methods into several smaller methods so that they can be reused. This will also make the code more readable.

Always remember, your code should be human readable first, and machine readable second. If you’ve ever had to work on other people’s code or returned to code you wrote months ago, you’ll be thankful for that readability. If you don’t follow this principle, it can result in substantial frustration. You may end up cursing yourself—or the original developer.

Now let’s refactor your ongoing project to better describe it. Follow these steps.

  1. In the Package Explorer view, do the following.

    • Expand the /src directory.
    • Right-click the com.androiddevbook.onyourbike.chapter3 package.
    • Select Refactor > Rename.
    • Change the end of the package name from chapter3 to chapter4, as shown in Figure 4.1. Keep the Update references checkbox checked.

      Figure 4.1

      Figure 4.1 Rename Package dialog box in Eclipse

    • Click Preview to check the changes that will take place. You will see that the import statements in MainActivity will change and that the package will be renamed.
    • Click OK to apply the changes. Ignore any compiler errors that are shown.
  2. Perform the same procedure (by right-clicking the filename and selecting Refactor > Rename) with the MainActivity class, and rename it TimerActivity.
  3. Locate the \res\layout\activity_main.xml file, and rename it activity_timer.xml.
  4. Change the call to the setContentView method in TimerActivity to pass the new activity identifier:

  5. After you save the TimerActivity.java file, the compilation error will be resolved.
  6. Open \res\values\strings.xml, and change the following lines to reflect a new application name and a new title.

    • Change the value of the string node with an attribute app_name to the following:

      <string name="app_name">On Your Bike - Chapter 4</string>
    • Change the name attribute title_activity_main to title_activity_timer, and the node value to the following:

      <string name="title_activity_timer">Timer</string>
  7. Double-click on the error in the Problem view to open the AndroidManifest.xml file. Change the following.

    • Change the package name to match the new package:

    • Change the activity name to match the new activity class:

    • Change the activity label to match the new string resource:

  8. From the Refactor menu, select Rename, and rename the className constant in TimerActivity. It is better practice to define a variable treated as a constant with uppercase letters and make it private so that it is not visible outside the class:

    private static String CLASS_NAME;

    Eclipse will automatically rename all references to the constant.

  9. Rename the project On Your Bike Chapter 4 by right-clicking on the project name, selecting Refactor -> Rename, entering the new name, and clicking OK. It is a good idea to clean your project after making all the changes to make sure that everything has been recompiled and to double-check that there are no errors. You do this by selecting Project > Clean.
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