Files

• Persistence


  A strategy that allows the reusing of volatile objects and other data items by storing them into a permanent storage system such as disk files and databases
• File IO management in Android

  • Streams
  • Scanner
  • PrintWriter
• Permanent file storage locations in Android

  • Internal main memory
  • External SD card
• Internal files

  Also called Resource Files or Embedded Files, they share space with other application's resources such as code, icons, pictures, music, etc.
• Factors determining permanent data storage destination

  • Size (small/large)
  • Location (internal/external)
  • Accessibility (private/public)
• Persistent environment options


  • Store private primitive data in key-value pairs
  • Store private data on the device's main memory
  • Store public data on the shared external storage
  • Store structured data in a private/public database
  • Store data on the web
• Shared Preferences


  Good for handling a handful of items, data is saved as <Key, Value> pairs where the key is a string and the value must be a primitive data type, they are permanent unlike Java Maps
• Shared Preferences usage


  Typically used to keep state information and shared data among several activities of an application
• Using Shared Preferences API

  1. Get Shared Preferences object

  2. Get Shared Preferences Editor object

  3. Put data using putXxx methods

  4. Commit changes

  5. Retrieve data using getXxx methods
• Shared Preference containers are saved as XML files in the application's internal memory space
• Android applications may include resource elements such as those in res/drawable, res/raw, res/menu, res/style
• Reading an internal resource file

  1. Get resource ID

  2. Open input stream on resource

  3. Create BufferedReader to read lines

  4. Append lines to string buffer

  5. Close reader and stream

  6. Display text in TextView
• Android requires resource file names to be in lowercase form
• If the resource file is not found, it raises a NotFoundException condition
• Conventional IO housekeeping operations should be issued to close the reader and stream objects
• Raw file


  An arbitrary dataset stored in its original raw format (such as .docx, pdf, gif, jpeg, etc)
• Accessing raw files

  1. Through an InputStream acting on a R.raw.filename resource entity

  2. Android requires resource file names to be in lowercase form

  3. getResources().openRawResource(fileResourceId) creates an InputStream object that sends the bytes from the selected resource file to an input buffer

  4. If the resource file is not found it raises a NotFoundException condition
• Extracting lines from input buffer

  A BufferedReader object is responsible for extracting lines from the input buffer and assembling a string which finally will be shown to the user in a textbox
• Protocol expects that conventional IO house keeping operations should be issued to close the reader and stream objects
• Internal resource file


  Private and cannot be seen by other apps residing in main memory
• The internal resource file (notes.txt) is stored in the phone's internal memory under the name: /data/data/cis470.matos.fileresources/files/notes.txt
• openFileInput(FILE_NAME)


  Opens a private file linked to this Context's application package for reading
• Reading from internal file

  1. InputStream is used to open the file

  2. BufferedReader is used to extract lines from the input stream and assemble them into a string

  3. The string is then set as the text of the EditText
• Writing to internal file

  OutputStreamWriter is used to write the text from the EditText to the file using openFileOutput(FILE_NAME, 0)
• The file's path is: /data/data/packageName/FileName
• You may delete an existing resource file using the conventional .delete() method
• SD cards


  • Offer larger capacity and portability
  • Many devices allow SD cards to be easily removed and reused in another device
  • Ideal for keeping music, pictures, ebooks, and video files
• Use the File Explorer tool to locate files in your device (or emulator) in the mnt/sdcard/ folder
• When dealing with external files you need to request permission to read and write to the SD card by adding the following clauses to your AndroidManifest.xml:
• It is a better practice to determine the SD location using Environment.getExternalStorageDirectory().getAbsolutePath() rather than using a specific path
• Environment.getExternalStorageDirectory().getPath()


  Method to determine the path to the external SD card
• PrintWriter

  Object used to send data tokens to disk using methods: print(), println(), printf()
• Scanner


  Accepts whitespace separated tokens and converts them to their corresponding types using methods: next(), nextInt(), nextDouble(), etc.
• You want to use the method Environment.getExternalStorageDirectory().getPath() to determine the path to the external SD card
• A PrintWriter object is used to send data tokens to disk using any of the following methods: print(), println(), printf()
• A Scanner accepts whitespace separated tokens and converts them to their corresponding types using methods: next(), nextInt(), nextDouble(), etc.
• Persistence


  A strategy that allows the reusing of volatile objects and other data items by storing them into a permanent storage system such as disk files and databases
• File IO management in Android

  Includes the familiar IO Java classes: Streams, Scanner, PrintWriter, and so on
• Permanent files


  Can be stored internally in the device's main memory (usually small, but not volatile) or externally in the much larger SD card

  
• Internal files


  Also called: Resource Files or Embedded Files