Thứ Hai, 27 tháng 5, 2013

Câu hỏi phỏng vấn android

What is Android?

Android is a stack of software for mobile devices which includes an Operating System, middleware and some key applications. The application executes within its own process and its own instance of Dalvik Virtual Machine. Many Virtual Machines run efficiently by a DVM device. DVM executes Java languages byte code which later transforms into .dex format files.

Describe the APK format.

The (Android Packaging Key) APK file is compressed format of the AndroidManifest.xml file, application code (.dex files), resource files, and other files. A project is compiled into a single .apk file.

What is an activity?

The building block of the user interface is the activity. They're in a long-running conversation with the user and may remain active, even when idle, as long as the conversation continues. In terms of desktop development, an Activity is equivalent to a Form.

What are the different phases of the Activity life cycle?

As an activity transitions from state to state, it is notified of the change by calls to the following protected methods:
1) void onCreate(Bundle savedInstanceState)
2) void onStart()
3) void onRestart()
4) void onResume()
5) void onPause()
6) void onStop()
7) void onDestroy()
Taken together, these seven methods define the entire lifecycle of an activity.
protected void onCreate(Bundle savedInstanceState):

This method is called when this activity is initially created. It provides a place to create views, bind data to lists, and perform other operations whose results must exist until onDestroy() is called. Also, this method is called with an android.os.Bundle instance that contains this activity's previously saved state, if this state was saved. Otherwise, null is passed. Finally, onCreate() is always followed by onStart().

protected void onStart():

This method is called when this activity is becoming visible to the user. Also, onStart() is followed by onResume() if this activity is coming to the foreground, or onStop() if this activity is becoming hidden .

protected void onResume():

This method is called when this activity can start to interact with the user -- this activity is now at the top of the activity stack and receives focus. Also, onResume() is always followed by onPause().

protected void onPause():

This method is called when the OS is about to resume a previous activity. The onPause() method is typically used to commit unsaved changes to persistent storage, and stop animations and other time-consuming processor activities. This method should run quickly because the next activity cannot be resumed until onPause() returns. Finally, onPause() is followed by onResume() if this activity is returning to the foreground, or onStop() if this activity is becoming hidden.

protected void onRestart():

This method is called after this activity has been stopped and prior to it being restarted. Also, onRestart() is always followed by onStart().

protected void onDestroy():

This method is called when this activity is being terminated. After this method returns, the process hosting this activity is killed by the OS

What is an AndroidManifest file?

Applications declare their components in a manifest file that's bundled into the Android package, the .apk file that also holds the application's code, files, and resources. The manifest is a structured XML file and is always named AndroidManifest.xml for all applications. It is also used for naming any libraries the application needs to be linked against (besides the default Android library) and identifying any permission the application expects to be granted.

What are Intent filters?

Activities and intent receivers include one or more filters in their manifest to describe what kinds of intents or messages they can handle or want to receive. An intent filter lists a set of requirements, such as data type, action requested, and URI format, that the Intent or message must fulfill. For Activities, Android searches for the Activity with the most closely matching valid match between the Intent and the activity filter. For messages, Android will forward a message to all receivers with matching intent filters.

What language does Android support for application development?

Android applications are written using the Java programming language.

What's the difference between a file, a class and an activity in android?

File - It is a block of arbitrary information, or resource for storing information. It can be of any type.
Class - Its a compiled form of .Java file . Android finally used this .class files to produce an executable apk
Activity - An activity is the equivalent of a Frame/Window in GUI toolkits. It is not a file or a file type it is just a class that can be extended in Android for loading UI elements on view.

What is the significance of the .dex files??

Android programs are compiled into .dex (Dalvik Executable) files, which are in turn zipped into a single .apk file on the device. .dex files can be created by automatically, translating compiled applications written in the Java programming language.

What does ADT stand for??

ADT stands for ANDROID DEVELOPMENT TOOLS The Android SDK includes several tools and utilities to help you create, test, and debug your projects.

What are the different tools in Android? Explain them?

The Android SDK and Virtual Device Manager:

It is used to create and manage Android Virtual Devices (AVD) and SDK packages. The AVD hosts an emulator running a particular build of Android, letting you specify the supported SDK version, screen resolution, amount of SD card storage available, and available hardware capabilities (such as touchscreens and GPS).

The Android Emulator:

An implementation of the Android virtual machine designed to run within a virtual device on your development computer. Use the emulator to test and debug your Android applications.

Dalvik Debug Monitoring Service (DDMS) :

Use the DDMS perspective to monitor and control the Dalvik virtual machines on which you're debugging your applications.

Android Asset Packaging Tool (AAPT) :

Constructs the distributable Android package files (.apk).

Android Debug Bridge,(adb) :

Android Debug Bridge, is a command-line debugging application shipped with the SDK. It provides tools to browse the device, copy tools on the device, and forward ports for debugging.

What is Dalvik Virtual Machine?

The name of Android's virtual machine. The Dalvik VM is an interpreter-only virtual machine that executes files in the Dalvik Executable (.dex) format, a format that is optimized for efficient storage and memory-mappable execution. The virtual machine is register-based, and it can run classes compiled by a Java language compiler that have been transformed into its native format using the included "dx" tool. The VM runs on top of Posix-compliant operating systems, which it relies on for underlying functionality (such as threading and low level memory management). The Dalvik core class library is intended to provide a familiar development base for those used to programming with Java Standard Edition, but it is geared specifically to the needs of a small mobile device.

What is Android Runtime?

Android includes a set of core libraries that provides most of the functionality available in the core libraries of the Java programming language. Every Android application runs in its own process, with its own instance of the Dalvik virtual machine. Dalvik has been written so that a device can run multiple VMs efficiently. The Dalvik VM executes files in the Dalvik Executable (.dex) format which is optimized for minimal memory footprint. The VM is register-based, and runs classes compiled by a Java language compiler that have been transformed into the .dex format by the included "dx" tool.

What is the Open Handset Alliance?

The OHA is a consortium of 84 technology and mobile companies that have joined hands to accelerate innovation in mobile technology and at the same time offer the end users a better, cost-effective and richer mobile experience. Members of this include Google, HTC, Sony, Dell, Intel, Motorola, Qualcomm, Texas Instruments, Samsung, LG, T-Mobile, Nvidia. The OHA was started on 5 November 2007 by Google and 34 other companies. Android is the main software of the alliance.

What is ViewGroup?

A ViewGroup is a special view that can contain other views (called children.) The view group is the base class for layouts and views containers. This class also defines the class ViewGroup.LayoutParams which serves as the base class for layouts parameters.

What is a Service?

A Service is an application component representing either an application's desire to perform a longer-running operation while not interacting with the user or to supply functionality for other applications to use. Services run without a dedicated GUI, but, like Activities and Broadcast Receivers, they still execute in the main thread of the application's process. A Service could be, facility for an application to expose some of its functionality to other applications.

What is the difference between Service and Thread?

Service is like an Activity but has no interface. Probably if you want to fetch the weather for example you won't create a blank activity for it, for this you will use a Service. It is also known as Background Service because it performs tasks in background. A Thread is a concurrent unit of execution. You need to know that you cannot update UI from a Thread. You need to use a Handler for this.

What is an Intent?

An intent is an abstract description of an operation to be performed. Intent provides a facility for performing late runtime binding between the code in different applications. Its most significant use is in the launching of activities, where it can be thought of as the glue between activities Also, Intent is basically a message that is passed between components (such as Activities, Services, Broadcast Receivers, and Content Providers). So, it is almost equivalent to parameters passed to API calls. The fundamental differences between API calls and intents' way of invoking components are: API calls are synchronous while intent-based invocations are asynchronous. API calls are compile time binding while intent-based calls are run-time binding.

What is an Explicit Intent?

In an explicit intent, we actually specify the activity that is required to respond to the intent. In other words, we explicitly designate the target component. This is typically used for application internal messages.

What is an Implicit Intent?

In an implicit intent, the main power of the android design, we just declare an intent and leave it to the platform to find an activity that can respond to the intent. Here, we do not declare the target component and hence is typically used for activating components of other applications seamlessly

What is a Content Provider?

Content Providers are the only way to share data across Android applications. They store and retrieve data thus making it accessible to all. Content Providers give a uniform interface to access the data. Android platform provides default implementations of content providers for data types like audio, video, images, contact information etc.

What is a Toast Notification?

A toast notification is a message that pops up on the surface of the window. It only fills the amount of space required for the message and the user's current activity remains visible and interactive. The notification automatically fades in and out, and does not accept interaction events.

What are the other Notifications?

A Status Bar Notification:
It is used for persistent reminders that come from the background and request the user's response.
A Dialog Notification:
It is Used for Activity-related notifications.

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