Android apps Flashcards
What is an application
The Application class in Android is the base class within an Android app that contains all other components such as activities and services. The Application class, or any subclass of the Application class, is instantiated before any other class when the process for your application/package is created.
What is context
The Application class in Android is the base class within an Android app that contains all other components such as activities and services. The Application class, or any subclass of the Application class, is instantiated before any other class when the process for your application/package is created.
What is application Context
The Application class in Android is the base class within an Android app that contains all other components such as activities and services. The Application class, or any subclass of the Application class, is instantiated before any other class when the process for your application/package is created.
What is Activity Context
The Application class in Android is the base class within an Android app that contains all other components such as activities and services. The Application class, or any subclass of the Application class, is instantiated before any other class when the process for your application/package is created.
What is ArmV7
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.
What is BuildType in Gradle?
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.
Explain the build process in Android
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.
What is Android Application Architecture?
Android application architecture has the following components:
Services − It will perform background functionalities
Intent − It will perform the inter connection between activities and the data passing mechanism
Resource Externalization − strings and graphics
Notification − light, sound, icon, notification, dialog box and toast
Content Providers − It will share the data between applications
Describe Activities
Activities are basically containers or windows to the user interface.
What is the lifecyce of an activity
OnCreate(): This is when the view is first created. This is normally where we create views, get data from bundles etc.
OnStart(): Called when the activity is becoming visible to the user. Followed by onResume() if the activity comes to the foreground, or onStop() if it becomes hidden.
OnResume(): Called when the activity will start interacting with the user. At this point your activity is at the top of the activity stack, with user input going to it.
OnPause(): Called as part of the activity lifecycle when an activity is going into the background, but has not (yet) been killed.
OnStop(): Called when you are no longer visible to the user.
OnDestroy(): Called when the activity is finishing
OnRestart(): Called after your activity has been stopped, prior to it being started again
What’s the difference between onCreate() and onStart()?
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().
Scenario in which only onDestroy is called for an activity without onPause() and onStop()?
If finish() is called in the OnCreate method of an activity, the system will invoke onDestroy() method directly.
Why would you do the setContentView() in onCreate() of Activity class?
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.
onSavedInstanceState() and onRestoreInstanceState() in activity?
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.
Launch modes in Android?
Standard: It creates a new instance of an activity in the task from which it was started. Multiple instances of the activity can be created and multiple instances can be added to the same or different tasks.
Eg: Suppose there is an activity stack of A -> B -> C.
Now if we launch B again with the launch mode as “standard”, the new stack will be A -> B -> C -> B.
SingleTop: It is the same as the standard, except if there is a previous instance of the activity that exists in the top of the stack, then it will not create a new instance but rather send the intent to the existing instance of the activity.
Eg: Suppose there is an activity stack of A -> B.
Now if we launch C with the launch mode as “singleTop”, the new stack will be A -> B -> C as usual.
Now if there is an activity stack of A -> B -> C.
If we launch C again with the launch mode as “singleTop”, the new stack will still be A -> B -> C.
SingleTask: A new task will always be created and a new instance will be pushed to the task as the root one. So if the activity is already in the task, the intent will be redirected to onNewIntent() else a new instance will be created. At a time only one instance of activity will exist.
Eg: Suppose there is an activity stack of A -> B -> C -> D.
Now if we launch D with the launch mode as “singleTask”, the new stack will be A -> B -> C -> D as usual.
Now if there is an activity stack of A -> B -> C -> D.
If we launch activity B again with the launch mode as “singleTask”, the new activity stack will be A -> B. Activities C and D will be destroyed.
SingleInstance: Same as single task but the system does not launch any activities in the same task as this activity. If new activities are launched, they are done so in a separate task.
Eg: Suppose there is an activity stack of A -> B -> C -> D. If we launch activity B again with the launch mode as “singleInstance”, the new activity stack will be:
Task1 — A -> B -> C
Task2 — D
How does the activity respond when the user rotates the screen?
When the screen is rotated, the current instance of activity is destroyed a new instance of the Activity is created in the new orientation. The onRestart() method is invoked first when a screen is rotated. The other lifecycle methods get invoked in the similar flow as they were when the activity was first created.
How to prevent the data from reloading and resetting when the screen is rotated?
The most basic approach would be to use a combination of ViewModels and onSaveInstanceState() . So how we do we that?
Basics of ViewModel: A ViewModel is LifeCycle-Aware. In other words, a ViewModel will not be destroyed if its owner is destroyed for a configuration change (e.g. rotation). The new instance of the owner will just re-connected to the existing ViewModel. So if you rotate an Activity three times, you have just created three different Activity instances, but you only have one ViewModel.
So the common practice is to store data in the ViewModel class (since it persists data during configuration changes) and use OnSaveInstanceState to store small amounts of UI data.
For instance, let’s say we have a search screen and the user has entered a query in the Edittext. This results in a list of items being displayed in the RecyclerView. Now if the screen is rotated, the ideal way to prevent resetting of data would be to store the list of search items in the ViewModel and the query text user has entered in the OnSaveInstanceState method of the activity.
Mention two ways to clear the back stack of Activities when a new Activity is called using intent
Mention two ways to clear the back stack of Activities when a new Activity is called using intent
The first approach is to use a FLAG_ACTIVITY_CLEAR_TOP flag. The second way is by using FLAG_ACTIVITY_CLEAR_TASK and FLAG_ACTIVITY_NEW_TASK in conjunction.
What’s the difference between FLAG_ACTIVITY_CLEAR_TASK and FLAG_ACTIVITY_CLEAR_TOP?
FLAG_ACTIVITY_CLEAR_TASK is used to clear all the activities from the task including any existing instances of the class invoked. The Activity launched by intent becomes the new root of the otherwise empty task list. This flag has to be used in conjunction with FLAG_ ACTIVITY_NEW_TASK.
FLAG_ACTIVITY_CLEAR_TOP on the other hand, if set and if an old instance of this Activity exists in the task list then barring that all the other activities are removed and that old activity becomes the root of the task list. Else if there’s no instance of that activity then a new instance of it is made the root of the task list. Using FLAG_ACTIVITY_NEW_TASK in conjunction is a good practice, though not necessary.
Describe content providers
A ContentProvider provides data from one application to another, when requested. It manages access to a structured set of data. It provides mechanisms for defining data security. ContentProvider is the standard interface that connects data in one process with code running in another process.
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 provider object receives data requests from clients, performs the requested action, and returns the results.
Access data using Content Provider:
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.
Describe services
A Service is an application component that can perform long-running operations in the background, and it doesn’t provide a user interface. It can run in the background, even when the user is not interacting with your application. These are the three different types of services:
Foreground Service: 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.
Background Service: 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.
Bound Service: 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.
Difference between Service & Intent Service
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.
Difference between AsyncTasks & Threads?
Thread should be used to separate long running operations from main thread so that performance is improved. But it can’t be cancelled elegantly and it can’t handle configuration changes of Android. You can’t update UI from Thread.
AsyncTask can be used to handle work items shorter than 5ms in duration. With AsyncTask, you can update UI unlike java Thread. But many long running tasks will choke the performance.
Difference between Service, Intent Service, AsyncTask & Threads
Android service is a component that is used to perform operations on the background such as playing music. It doesn’t has any UI (user interface). The service runs in the background indefinitely even if application is destroyed.
AsyncTask allows you to perform asynchronous work on your user interface. It performs the blocking operations in a worker thread and then publishes the results on the UI thread, without requiring you to handle threads and/or handlers yourself.
IntentService is a base class for Services that handle 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 thread is a single sequential flow of control within a program. Threads can be thought of as mini-processes running within a main process.
What are Handlers?
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.
What is a Job Scheduling?
Job Scheduling api, as the name suggests, allows to schedule jobs while letting the system optimize based on memory, power, and connectivity conditions. The JobScheduler supports batch scheduling of jobs. The Android system can combine jobs so that battery consumption is reduced. JobManager makes handling uploads easier as it handles automatically the unreliability of the network. It also survives application restarts. Some scenarios:
Tasks that should be done once the device is connect to a power supply
Tasks that require network access or a Wi-Fi connection.
Task that are not critical or user facing
Tasks that should be running on a regular basis as batch where the timing is not critical
You can click on this link to learn more about Job Schedulers.
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 Activity, 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.
Note: AsyncTasks by default run on a single thread using a serial executor, meaning it has only 1 thread and each task runs one after the other.
What is the onTrimMemory() method?
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. Reference
Android Bound Service
A bound service is a service that allows other android components (like activity) to bind to it and send and receive data. A bound service is a service that can be used not only by components running in the same process as local service, but activities and services, running in different processes, can bind to it and send and receive data.
When implementing a bound service we have to extend Service class but we have to override onBind method too. This method returns an object that implements IBinder, that can be used to interact with the service
Implementing Android bound service with Android Messenger
Service based on Messenger can communicate with other components in different processes, known as Inter Process Communication (IPC), without using AIDL.
A service handler: this component handles incoming requests from clients that interact with the service itself.
A Messenger: this class is used to create an object implementing IBinder interface so that a client can interact with the service.
Example Implementation: Link
AIDL vs Messenger Queue
mentioned in the response, Messenger Queue builds us a queue and the data/messages are passed between 2 or more processes sequential. But in case of AIDL the messages are passed in parallel.
AIDL is for the purpose when you’ve to go application level communication for data and control sharing, a scenario depicting it can be : An app requires list of all contacts from Contacts app (content part lies here) plus it also wants to show the call’s duration and you can also disconnect it from that app (control part lies here).
In Messenger queues you’re more IN the application and working on threads and processes to manage the queue having messages so no Outside services interference here.
Messenger is needed if you want to bind a remote service (e.g. running in another process).
What is a ThreadPool? And is it more effective than using several separate Threads?
Creating and destroying threads has a high CPU usage, so when we need to perform lots of small, simple tasks concurrently, the overhead of creating our own threads can take up a significant portion of the CPU cycles and severely affect the final response time. ThreadPool consists of a task queue and a group of worker threads, which allows it to run multiple parallel instances of a task.
Difference between Serializable and Parcelable?
Serialization is the process of converting an object into a stream of bytes in order to store an object into memory, so that it can be recreated at a later time, while still keeping the object’s original state and data.
How to disallow serialization? We can declare the variable as transient.
Serializable is a standard Java interface. Parcelable is an Android specific interface where you implement the serialization yourself. It was created to be far more efficient than Serializable (The problem with this approach is that reflection is used and it is a slow process. This mechanism also tends to create a lot of temporary objects and cause quite a bit of garbage collection.).
What is a singleton class in Android?
A singleton class is a class which can create only an object that can be shared all other classes.
