Learn Java for Android Challenge: Iteration

You’ve read about how iteration works in Java. Test your new skills with this challenge: five progressively difficult exercises that help you solidify your knowledge of the Java programming language and Android development. That’s right, Android too! You may need to refer to other Android tutorials that we’ve published on Mobiletuts+, but if you can complete this challenge successfully you will know you are progressing nicely in your Java and Android SDK understanding.

Setup

To prepare for this challenge, you’ll want to start with a basic Android application. Simply create an Android application within Eclipse and edit its default Activity, specifically the onCreate() method, to test the code from each of these challenges.

If what we’ve just asked of you is already too challenging, we would recommend taking a step back. Start with some of the Android tutorials, such as Introduction to Android Development or Beginning Android: Getting Started with Fortune Crunch. Once you’ve mastered setting up an Android project, return and try these exercises.

Getting Started: Working with String Array Resources

At first, we considered using a simple string array for you to use to complete these iteration challenges:

String aColors[] = {"Red", "Orange", "Yellow", "Green", "Blue", "Indigo", "Violet"};

However, there’s a much better way to store fixed arrays of values in Android: as resources. To create a string array resource, you must first create String resources for each value. Next, create a String Array resource using those String resources as elements. Use the <string-array> tag to combine String resources into an array resource using child <item> tags for each element. For instance, here’s an array of colors inside an Android resource file:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="red">Red</string>
    <string name="orange">Orange</string>
    <string name="yellow">Yellow</string>
    <string name="green">Green</string>
    <string name="blue">Blue</string>
    <string name="indigo">Indigo</string>
    <string name="violet">Violet</string>
    <string-array name="colorsArray">
        <item>@string/red</item>
        <item>@string/orange</item>
        <item>@string/yellow</item>
        <item>@string/green</item>
        <item>@string/blue</item>
        <item>@string/indigo</item>
        <item>@string/violet</item>
    </string-array>
</resources>

To load this array resource in your Activity class, use the getStringArray() method of the Resources object. For instance:

String aColors[] = getResources().getStringArray(R.array.colorsArray);

Challenge #1: Warm-Up Challenge

Now you’re ready to get started. Load the string array from the resources, as discussed above. Then, iterate through the array’s contents using a for() loop. Print each string to the Android LogCat debug log using the Log.v() method.

Extra points if you use the shorthand version of for() loops, discussed in Learn Java for Android Development: Working with Arrays.

Find the answer to this challenge in the challengeOne() method of the downloadable project.

Challenge #2: Stretch Your Skills

Iterate the same array as Challenge #1, but use a different iteration mechanism. For example, use a while() loop instead. Print each string to the Android LogCat debug log using the Log.v() method.

Find the answer to this challenge in the challengeTwo() method of the downloadable project.

Challenge #3: Reverse!

Iterate the same array backwards. Print each string to the Android LogCat debug log using the Log.v() method.

HINT: Challenge #2 can help.

Find the answer to this challenge in the challengeThree() method of the downloadable project.

Challenge #4: It’s All About Character

Next, go back to the for() loop you created in Challenge #1. Update it to print out the individual characters of each String as well. This challenge will require an inner for() loop.

HINT: You can use the toCharArray() method of the String class to retrieve a character array.

The answer to this challenge is in the challengeFour() method of the downloadable project.

Challenge #5: Reflect on How Far You’ve Come

For this final challenge, you’re going to need a bit of understanding about Java reflection. Use reflection to iterate through the declared fields within the android.os.Build class using a for() loop. Print each field name to the Android LogCat debug log using the Log.v() method.

HINT: Our short tutorial on Java reflection will teach you everything you need to know to complete this challenge.

We’ve provided two different solutions for this challenge. The first solution assumes that the package is imported and the compiler knows about the class. The second solution does not make this assumption. These solutions are found in the challengeFiveA() and challengeFiveB() methods of the downloadable project.

Conclusion

Android developers use iteration techniques on a regular basis to solve coding problems. Iteration is frequently used to iterate arrays, data structures like lists, or database content using cursors. Feel free to post your alternative answers (or any questions) in the comments section.

Best of luck!