Android-Interview-Questions

Collaboration Rules

Make sure your question is new and unique - not just rephrasing a previously existing question. If possible, include a link to the solution/topic. Questions should be fairly straightforward and completely technical (no "how many golf balls are in America" questions.) If there is a new topic that you think should be included, then include it!

Topics:

General Developer Questions
Core Java
Core Android
Android Design and XML
Android Networking
Databases

General Developer Questions

How familiar you are with the Android and Google Guidelines?
Describe Test-Driven Development. [info]
Explain unit tests versus functional tests.
Describe Scrum and Kanban.
What project management tools have you used?
How do you ensure that you are working efficiently?
Do you have basic familiarity with working on the command line i.e. Gradle, Ant, or the Java Compiler?

Core Java

Object-Oriented Programming

What are the main 3 Object Oriented Programing (OOP) concepts?
- composition, inheritance & delegation
- Encapsulation
- interface
Explain object serialization and how to implement it in Java.
- Serialization is the conversion of an object to a series of bytes, so that the object can be easily saved to persistent storage or streamed across a communication link. The byte stream can then be deserialized - converted into a replica of the original object. [ref]
Explain anonymous classes. [info]
- A class without name, enable you to make your code more concise, declare & instantiate a class at the same time
Describe the differences between abstract classes and interfaces. [info]
- Interfaces: a group of methods with empty bodies
- Abstract class: a class that is declared abstract -- it may or may not contains abstract methods. Abstract class can't be instantiated but can be subclassed. An abstract method is a method that is declared without an implementation, if a class includes abstract methods, the class itself must be declared abstract. Its subclass must provide the implementation of the abstract method.
Explain what a Singleton class is and how to create one in Java [info]
- Only one instance across the system
- The easiest way to create a Singleton is by enum, enum fields are compile time constants, but they are instances of their enum type. And, they're constructed when the enum type is referenced for the first time. (see Effective Java)
```
public enum MySingleton {
    INSTANCE;
    private MySingleton() {
        System.out.println("Here");
    }
}
// output:
Here
INSTANCE
```
Why should the equals() and hashCode() methods often be overridden together? [info]
- Without hashCode(), your object wouldn't behave properly when used in conjunction with class such as HashMap, HashSet and Hashtable
- Whenever a.equals(b), then a.hashCode() must be same as b.hashCode().
How do you properly override the equals() method? For example, what considerations should be taken when checking for equality? [info]
Difference between final, finally and finalize?
- final: keyword to define that an entity can only be assigned once
- finally: The finally block always executes when the try block exits.
- finalize(): called when an object is about to be Garbage collected
In Java, does the finally block gets executed if we insert a return statement inside the try block of a try-catch-finally?
- yes, finally will be executed even if there is return in try and catch block.[ref]
Explain method overloading & overriding.
- overloading: same method signature but different parameters
- overriding: same method signature & parameters, e.g parent & child situation
What is memory leak and how does Java handle it?
- Possible Android memory leak cases: http://blog.nimbledroid.com/2016/05/23/memory-leaks.html
  - static Activity
  - static View
  - Inner class
  - Anonymous class, maintain a reference to the class that they were declared inside (AsyncTask leak when it continues to execute in the background when the activity has been destroyed)
  - Handlers & Threads, runnable can keep the reference to the activity, as long as the message hasn't be handled before activity was destroyed, it will lead to memory leaks
  - Sensor Manager: service runs in their own process & assist app to perform certain actions, if the context want to be notified, it will need to register itself as a listener which cause the service to maintain a reference to the activity, if one forgets to unregister the listener, the leak will occur

Data Structures

What are the use cases and differences of arrays and ArrayLists?
What are the use cases and differences of a HashSet and a TreeSet?

Build Tools

Have you used any Ant, Maven, Gradle features for your project?

Programming Paradigms

Explain event-driven programming in Java [info]
What is Java's Garbage Collection and how does it help you as a developer?
How can you typecast in Java? [info]
Explain Java's try-catch-finally paradigm [info]

Core Android

How does the Android notification system work?
- Push notification: Firebase Cloud Messaging, 3rd party services
How can two distinct Android apps interact? (several answers)
Describe Activities. [info]
- single focused thing a user can do, unit of Android dev
What are the four states of the Activity Lifecycle? [active/running, paused, stopped, destroyed]
- visible in foreground
- pause, partially visible, can be killed in extreme low memory situation (pre-Honeycomb)
- stopped, no longer visible, still retain state, often killed when memory is needed else where
- destroyed
What are the seven callback methods of an Activity used to perform operations when the Activity transitions between states? [onCreate(), onStart(), onResume(), onPause(), onStop(), onRestart(), onDestroy()]
- entire life time, between onCreate() and onDestroy()
- visible life time, between onStart() and onStop()
- foreground life time, between onResume() and onPause()
What is the difference between a fragment and an activity? Explain the relationship between the two.
What is the difference between Serializable and Parcelable? Which is the best approach in Android?
- Serializable: standard java, slow, reflection is used
- Parcelable: designed for android, faster, more complicated in implementation but more explicit
What are "launch modes"? [ref]
- instruction on how activity should be launched
  - standard
  - singleTop
  - singleTask (not appropriate for most applications)
  - singleInstance (not appropriate for most applications)
What are Intents? [info]
- Intent is a messaging object that you can use to require an action from another app component.
What is an Implicit Intent? [info]

specify an action that can invoke any app on device that is able to perform the action

What is an Explicit Intent? [info]
- use to launch the specific app component
Describe three common use cases for using an Intent.
- start an activity
- start a service
- deliver a broadcast
What is a Service? [info]
- an application component that can perform long running operation in the background, it runs main thread by default!
- methods to override:
  - onStartCommand()
  - onBind()
  - onCreate()
  - onDestroy()
- Never call onStartCommand() directly, use startService()
What is a ContentProvider and what is it typically used for? [info]
- manage data access between processes so you can provide your data to other application securely
What is a Fragment? [info]
- A Fragment represents a behavior or a portion of user interface in an Activity. You can combine multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities.
What is ADB?
- Android Debug Bridge, versatile command tool, a client, a daemon, a server
What is ANR?
- Android Not Responding
What is AndroidManifest.xml used for? Give examples of what kind of data you would add to it. [info]
Describe how broadcasts and intents work to be able to pass messages around your app.[info]
What is the Dalvik Virtual Machine?
What are different ways to store data in your Android app? [info]
Android appplication components [info]
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?
What is the difference between Service and IntentService? How is each used?
What is a Sticky Intent?
What is AIDL? [info]
What is dependency injection?
What are the different protection levels in permission? [info]
How would you preserve Activity state during a screen rotation?

Android Design and XML

Explain the differences and similarities of Relative Layout and Linear Layout.
Explain the differences and similarities of List Views and Grid Views.
Describe how to implement XML namespaces.
Explain how to present different styles/drawables for a button depending on the state of the button (pressed, selected, etc.) using XML (no Java) [info]
for layout_width and layout_height, what's the difference between match_parent and wrap_content?
How do you implement Google's new Material Design in an Android application? [info]
Difference between View.GONE and View.INVISIBLE?

Android Networking

Have you use an HTTP Library, which, why, did you like it?
Describe how REST APIs work.
What are some typical methods of HTTP request/responses? [GET, POST, PUT, PATCH, DELETE, UPDATE]

Databases

Why does Android use SQLite?
What libraries have you used for interacting with databases and why did you choose them?
What are contract classes? [info]
How do you use the BaseColumns interface to describe your data schema? [info]

jinqian / Android-Interview-Questions