Android Developer Interview Preparation Guide

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A foreground service performs some operation that is noticeable to the user. For example, we can use a foreground service to play an audio track. A Notification must be displayed to the user.

A service is bound when an application component binds to it by calling bindService(). A bound service offers a client-server interface that allows components to interact with the service, send requests, receive results. A bound service runs only as long as another application component is bound to it.

If you want someone to perform any Intent operation at future point of time on behalf of you, then we will use Pending Intent.

A background service performs an operation that isn’t directly noticed by the user. In Android API level 26 and above, there are restrictions to using background services and it is recommended to use WorkManager in these cases.

This context is tied to the lifecycle of an application. The application context can be used where you need a context whose lifecycle is separate from the current context or when you are passing a context beyond the scope of an activity.

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

There are 3 CPU architectures in Android. ARMv7 is the most common as it is optimised for battery consumption. ARM64 is an evolved version of that that supports 64-bit processing for more powerful computing. ARMx86, is the least used for these three, since it is not battery friendly. It is more powerful than the other two.

Activities are basically containers or windows to the user interface. Services is a component that is used to perform operations on the background. It does not have an UI.

‘ANR’ in Android is ‘Application Not Responding.’ It means when the user is interacting with the activity, and the activity is in the onResume() method, a dialog appears displaying “application not responding.”

It happens because we start a heavy and long running task like downloading data in the main UI thread. The solution of the problem is to start your heavy tasks in the backbround using Async Task class.

Handlers are objects for managing threads. It receives messages and writes code on how to handle the message. They run outside of the activity’s lifecycle, so they need to be cleaned up properly or else you will have thread leaks.
Handlers allow communicating between the background thread and the main thread.
A Handler class is preferred when we need to perform a background task repeatedly after every x seconds/minutes.

☛ First step involves compiling the resources folder (/res) using the aapt (android asset packaging tool) tool. These are compiled to a single class file called R.java. This is a class that just contains constants.
☛ Second step involves the java source code being compiled to .class files by javac, and then the class files are converted to Dalvik bytecode by the “dx” tool, which is included in the sdk ‘tools’. The output is classes.dex.
☛ The final step involves the android apkbuilder which takes all the input and builds the apk (android packaging key) file.

To develop a mobile application, Android developers require some tools and this requirement is satisfied by “Android SDK” which is a set of tools that are used for developing or writing apps.

It has a Graphical User Interface which emulates the Android environment. This emulator acts as an actual mobile device on which the developers write their code and then debug/test the same code to check if anything is wrong.

We need to register a LocalBroadcastReceiver in the activity. And send a broadcast with the data using intents from the background service. As long as the activity is in the foreground, the UI will be updated from the background. Ensure to unregister the broadcast receiver in the onStop() method of the activity to avoid memory leaks. We can also register a Handler and pass data using Handlers.

Start by making sure your Android application has the necessary read access permissions. Then, get access to the ContentResolver object by calling getContentResolver() on the Context object, and retrieving the data by constructing a query using ContentResolver.query().
The ContentResolver.query() method returns a Cursor, so you can retrieve data from each column using Cursor methods.

This context is available in an activity. This context is tied to the lifecycle of an activity. The activity context should be used when you are passing the context in the scope of an activity or you need the context whose lifecycle is attached to the current context.

The onCreate() method is called once during the Activity lifecycle, either when the application starts, or when the Activity has been destroyed and then recreated, for example during a configuration change.
The onStart() method is called whenever the Activity becomes visible to the user, typically after onCreate() or onRestart().

☛ 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.

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.

Java is the widely used language for Android development.

It also supports C/C++ and when used with Android SDK, it improves the performance speed too.

Build types define properties that Gradle uses when building and packaging your Android app.
☛ A build type defines how a module is built, for example whether ProGuard is run.
☛ A product flavour defines what is built, such as which resources are included in the build.
☛ Gradle creates a build variant for every possible combination of your project’s product flavours and build types.

As onCreate() of an Activity is called only once, this is the point where most initialisation should go. It is inefficient to set the content in onResume() or onStart() (which are called multiple times) as the setContentView() is a heavy operation.

Sticky Intents allows communication between a function and a service. sendStickyBroadcast() performs a sendBroadcast(Intent) known as sticky, i.e. the Intent you are sending stays around after the broadcast is complete, so that others can quickly retrieve that data through the return value of registerReceiver(BroadcastReceiver, IntentFilter). For example, if you take an intent for ACTION_BATTERY_CHANGED to get battery change events: When you call registerReceiver() for that action — even with a null BroadcastReceiver — you get the Intent that was last Broadcast for that action. Hence, you can use this to find the state of the battery without necessarily registering for all future state changes in the battery.

Description of the intent. For instance, ACTION_CALL — used to perform calls

onTrimMemory(): Called when the operating system has determined that it is a good time for a process to trim unneeded memory from its process. This will happen for example when it goes in the background and there is not enough memory to keep as many background processes running as desired.
Android can reclaim memory for from your app in several ways or kill your app entirely if necessary to free up memory for critical tasks. To help balance the system memory and avoid the system’s need to kill your app process, you can implement the ComponentCallbacks2 interface in your Activity classes. The provided onTrimMemory() callback method allows your app to listen for memory related events when your app is in either the foreground or the background, and then release objects in response to app lifecycle or system events that indicate the system needs to reclaim memory.

OnRestoreInstanceState() - When activity is recreated after it was previously destroyed, we can recover the saved state from the Bundle that the system passes to the activity. Both the onCreate() and onRestoreInstanceState() callback methods receive the same Bundle that contains the instance state information. But because the onCreate() method is called whether the system is creating a new instance of your activity or recreating a previous one, you must check whether the state Bundle is null before you attempt to read it. If it is null, then the system is creating a new instance of the activity, instead of restoring a previous one that was destroyed.
onSaveInstanceState() - is a method used to store data before pausing the activity.