15 Essential Android Interview Questions*

There are four Java classes related to the use of sensors on the Android platform. List them and explain the purpose of each.

The four Java classes related to the use of sensors on the Android platform areL

  • Sensor: Provides methods to identify which capabilities are available for a specific sensor.
  • SensorManager: Provides methods for registering sensor event listeners and calibrating sensors.
  • SensorEvent: Provides raw sensor data, including information regarding accuracy.
  • SensorEventListener: Interface that defines callback methods that will receive sensor event notifications.

To learn more about sensors, refer to Android developer’s guide.

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What is a ContentProvider and what is it typically used for?

A ContentProvider manages access to a structured set of data. It encapsulates the data and provide mechanisms for defining data security. ContentProvider is the standard interface that connects data in one process with code running in another process.

More information about content providers can be found here in the Android Developer’s Guide.

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Under what condition could the code sample below crash your application? How would you modify the code to avoid this potential problem? Explain your answer.

    Intent sendIntent = new Intent();
    sendIntent.setAction(Intent.ACTION_SEND);
    sendIntent.putExtra(Intent.EXTRA_TEXT, textMessage);
    sendIntent.setType(HTTP.PLAIN_TEXT_TYPE); // "text/plain" MIME type
    startActivity(sendIntent);

An implicit intent specifies an action that can invoke any app on the device able to perform the action. Using an implicit intent is useful when your app cannot perform the action, but other apps probably can. If there is more than one application registered that can handle this request, the user will be prompted to select which one to use.

However, it is possible that there are no applications that can handle your intent. In this case, your application will crash when you invoke startActivity(). To avoid this, before calling startActivity() you should first verify that there is at least one application registered in the system that can handle the intent. To do this use resolveActivity() on your intent object:

    // Verify that there are applications registered to handle this intent
    // (resolveActivity returns null if none are registered)
    if (sendIntent.resolveActivity(getPackageManager()) != null) {
        startActivity(sendIntent);
    } 

See the Android developer’s guide for more information about implicit intents.

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The last callback in the lifecycle of an activity is onDestroy(). The system calls this method on your activity as the final signal that your activity instance is being completely removed from the system memory. Usually, the system will call onPause() and onStop() before calling onDestroy(). Describe a scenario, though, where onPause() and onStop() would not be invoked.

onPause() and onStop() will not be invoked if finish() is called from within the onCreate() method. This might occur, for example, if you detect an error during onCreate() and call finish() as a result. In such a case, though, any cleanup you expected to be done in onPause() and onStop() will not be executed.

Although onDestroy() is the last callback in the lifecycle of an activity, it is worth mentioning that this callback may not always be called and should not be relied upon to destroy resources. It is better have the resources created in onStart() and onResume(), and have them destroyed in onStop() and onPause, respectively.

See the Android developer’s guide for more information about the activity lifecycle.

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Which of the code snippets below is the correct way to check if a Compass sensor is present on the system? Explain your answer.

Answer 1:

    PackageManager m = getPackageManager();
    if (!m.hasSystemFeature(PackageManager.FEATURE_SENSOR_COMPASS)) {
        // This device does not have a compass, turn off the compass feature
    }

Answer 2:

    SensorManager m = getSensorManager();
    if (!m.hasSystemFeature(SensorManager.FEATURE_SENSOR_COMPASS)) {
        // This device does not have a compass, turn off the compass feature
    }

Answer 3:

    Sensor s = getSensor();
    if (!s.hasSystemFeature(Sensor.FEATURE_SENSOR_COMPASS)) {
        // This device does not have a compass, turn off the compass feature
    }

The correct answer is Answer 1, the version that uses PackageManager.

SensorManager and Sensor are part of Android Sensor Framework and are used for direct access and acquisition of raw sensor data. These classes do not provide any method like hasSystemFeature() which is used for evaluation of system capabilities.

Android defines feature IDs, in the form of ENUMs, for any hardware or software feature that may be available on a device. For instance, the feature ID for the compass sensor is FEATURE_SENSOR_COMPASS.

If your application cannot work without a specific feature being available on the system, you can prevent users from installing your app with a <uses-feature> element in your app’s manifest file to specify a non-negotiable dependency.

However, if you just want to disable specific elements of your application when a feature is missing, you can use the PackageManager class. PackageManager is used for retrieving various kinds of information related to the application packages that are currently installed on the device.

To learn more about compatibility and handling different types of devices or sensors please refer to the Android developer’s guide.

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Describe three common use cases for using an Intent.

Common use cases for using an Intent include:

  • To start an activity: You can start a new instance of an Activity by passing an Intent to startActivity() method.
  • To start a service: You can start a service to perform a one-time operation (such as download a file) by passing an Intent to startService().
  • To deliver a broadcast: You can deliver a broadcast to other apps by passing an Intent to sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast().

More information about intents can be found in Android developer’s guide.

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Suppose that you are starting a service in an Activity as follows:

Intent service = new Intent(context, MyService.class);             
startService(service);

where MyService accesses a remote server via an Internet connection.

If the Activity is showing an animation that indicates some kind of progress, what issue might you encounter and how could you address it?

Responses from a remote service via the Internet can often take some time, either due to networking latencies, or load on the remote server, or the amount of time it takes for the remote service to process and respond to the request.

As a result, if such a delay occurs, the animation in the activity (and even worse, the entire UI thread) could be blocked and could appear to the user to be “frozen” while the client waits for a response from the service. This is because the service is started on the main application thread (or UI thread) in the Activity.

The problem can (and should) be avoided by relegating any such remote requests to a background thread or, when feasible, using an an asynchronous response mechanism.

Note well: Accessing the network from the UI thread throws a runtime exception in newer Android versions which causes the app to crash.

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Normally, in the process of carrying out a screen reorientation, the Android platform tears down the foreground activity and recreates it, restoring each of the view values in the activity’s layout.

In an app you’re working on, you notice that a view’s value is not being restored after screen reorientation. What could be a likely cause of the problem that you should verify, at a minimum, about that particular view?

You should verify that it has a valid id. In order for the Android system to restore the state of the views in your activity, each view must have a unique ID, supplied by the android:id attribute.

More information is available here.

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What is DDMS? Describe some of its capabilities.

DDMS is the Dalvik Debug Monitor Server that ships with Android. It provides a wide array of debugging features including:

  • port-forwarding services
  • screen capture
  • thread and heap information
  • network traffic tracking
  • incoming call and SMS spoofing
  • simulating network state, speed, and latency
  • location data spoofing
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What is the relationship between the life cycle of an AsyncTask and an Activity? What problems can this result in? How can these problems be avoided?

An AsyncTask is not tied to the life cycle of the Activity that contains it. So, for example, if you start an AsyncTask inside an Activity and the user rotates the device, the Activity will be destroyed (and a new Activity instance will be created) but the AsyncTask will not die but instead goes on living until it completes.

Then, when the AsyncTask does complete, rather than updating the UI of the new Activity, it updates the former instance of the Activity (i.e., the one in which it was created but that is not displayed anymore!). This can lead to an Exception (of the type java.lang.IllegalArgumentException: View not attached to window manager if you use, for instance, findViewById to retrieve a view inside the Activity).

There’s also the potential for this to result in a memory leak since the AsyncTask maintains a reference to the Activty, which prevents the Activity from being garbage collected as long as the AsyncTask remains alive.

For these reasons, using AsyncTasks for long-running background tasks is generally a bad idea . Rather, for long-running background tasks, a different mechanism (such as a service) should be employed.

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What is an Intent? Can it be used to provide data to a ContentProvider? Why or why not?

The Intent object is a common mechanism for starting new activity and transferring data from one activity to another. However, you cannot start a ContentProvider using an Intent.

When you want to access data in a ContentProvider, you must instead use the ContentResolver object in your application’s Context to communicate with the provider as a client. The ContentResolver object communicates with the provider object, an instance of a class that implements ContentProvider. The provider object receives data requests from clients, performs the requested action, and returns the results.

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What is the difference between a fragment and an activity? Explain the relationship between the two.

An activity is typically a single, focused operation that a user can perform (such as dial a number, take a picture, send an email, view a map, etc.). Yet at the same time, there is nothing that precludes a developer from creating an activity that is arbitrarily complex.

Activity implementations can optionally make use of the Fragment class for purposes such as producing more modular code, building more sophisticated user interfaces for larger screens, helping scale applications between small and large screens, and so on. Multiple fragments can be combined within a single activity and, conversely, the same fragment can often be reused across multiple activities. This structure is largely intended to foster code reuse and facilitate economies of scale.

A fragment is essentially a modular section of an activity, with its own lifecycle and input events, and which can be added or removed at will. It is important to remember, though, that a fragment’s lifecycle is directly affected by its host activity’s lifecycle; i.e., when the activity is paused, so are all fragments in it, and when the activity is destroyed, so are all of its fragments.

More information is available here in the Android Developer’s Guide.

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What is difference between Serializable and Parcelable ? Which is best approach in Android ?

Serializable is a standard Java interface. You simply mark a class Serializable by implementing the interface, and Java will automatically serialize it in certain situations.

Parcelable is an Android specific interface where you implement the serialization yourself. It was created to be far more efficient than Serializable, and to get around some problems with the default Java serialization scheme.

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What are “launch modes”? What are the two mechanisms by which they can be defined? What specific types of launch modes are supported?

A “launch mode” is the way in which a new instance of an activity is to be associated with the current task.

Launch modes may be defined using one of two mechanisms:

  • Manifest file. When declaring an activity in a manifest file, you can specify how the activity should associate with tasks when it starts. Supported values include:

    • standard (default). Multiple instances of the activity class can be instantiated and multiple instances can be added to the same task or different tasks. This is the common mode for most of the activities.
    • singleTop. The difference from standard is, if an instance of the activity already exists at the top of the current task and the system routes the intent to this activity, no new instance will be created because it will fire off an onNewIntent() method instead of creating a new object.
    • singleTask. A new task will always be created and a new instance will be pushed to the task as the root. However, if any activity instance exists in any tasks, the system routes the intent to that activity instance through the onNewIntent() method call. In this mode, activity instances can be pushed to the same task. This mode is useful for activities that act as the entry points.
    • singleInstance. Same as singleTask, except that the no activities instance can be pushed into the same task of the singleInstance’s. Accordingly, the activity with launch mode is always in a single activity instance task. This is a very specialized mode and should only be used in applications that are implemented entirely as one activity.
  • Intent flags. Calls to startActivity() can include a flag in the Intent that declares if and how the new activity should be associated with the current task. Supported values include:

    • FLAG_ACTIVITY_NEW_TASK. Same as singleTask value in Manifest file (see above).
    • FLAG_ACTIVITY_SINGLE_TOP. Same as singleTop value in Manifest file (see above).
    • FLAG_ACTIVITY_CLEAR_TOP. If the activity being started is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it are destroyed and this intent is delivered to the resumed instance of the activity (now on top), through onNewIntent(). There is no corresponding value in the Manifest file that produces this behavior.

More information about launch modes is available here.

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What is the difference between Service and IntentService? How is each used?

Service is the base class for Android services that can be extended to create any service. A class that directly extends Service runs on the main thread so it will block the UI (if there is one) and should therefore either be used only for short tasks or should make use of other threads for longer tasks.

IntentService is a subclass of Service that handles asynchronous requests (expressed as “Intents”) on demand. Clients send requests through startService(Intent) calls. The service is started as needed, handles each Intent in turn using a worker thread, and stops itself when it runs out of work. Writing an IntentService can be quite simple; just extend the IntentService class and override the onHandleIntent(Intent intent) method where you can manage all incoming requests.

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* There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work.
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Pablo Pera
United States
Pablo has taken more than 10 mobile apps from idea to millions of users in the market, as the lead engineer of multiple teams in various companies, including his own. Before focusing full-time on Android apps, he worked for Google and CERN (home of the LHC particle accelerator) as well as various tech startups in NY. He has invaluable experience in all areas related to an Android app from design and coding to acquisition and growth.
Ken Roe
United States
Ken is a highly skilled Software Engineer with over twenty years experience. He has been running a small company that develops and sells mobile apps, and is especially good at Python, Objective-C, and Swift. He is currently working on a PhD in Computer Science at Johns Hopkins University.
Antoon Groenewoud
Germany
Antoon is a top programmer and designer with experience ranging from entrepreneur to freelancer to CTO. He is active within the games industry with a knack for novel, complex solutions, and the ability to quickly integrate into any team and adapt to the problem at hand be it UI/UX or back-end or anything in between.