7-storage.pdf

Files and storage Android can read/write files from two locations: – internal and external storage. – Both are persistent storage; data remains after power-off / reboot. internal storage: Built into the device. – guaranteed to be present – typic...

Files and storage Android can read/write files from two locations: – internal and external storage. – Both are persistent storage; data remains after power-off / reboot. internal storage: Built into the device. – guaranteed to be present – typically smaller (~1-4 gb) – can't be expanded or removed – specific and private to each app – wiped out when the app is uninstalled  File (link) and Streams (link) java.io.File - Objects that represent a file or directory. – methods: canRead, canWrite, create, delete, exists, getName, getParent, getPath, isFile, isDirectory, lastModified, length, listFiles, mkdir, mkdirs, renameTo java.io.InputStream, OutputStream - Stream objects represent flows of data bytes from/to a source or destination. – Could come from a file, network, database, memory,... – Normally not directly used; they only include low-level methods for reading/writing a byte (character) at a time from the input. – Instead, a stream is often passed as parameter to other objects like java.util.Scanner, java.io.BufferedReader, java.io.PrintStream to do the actual reading / writing.  Using internal storage (link) An activity has methods you can call to read/write files: – getFilesDir() - returns internal directory for your app – getCacheDir() - returns a "temp" directory for scrap files – getResources().openRawResource(R.raw.id) - read an input file from res/raw/ – openFileInput("name", mode) - opens a file for reading – openFileOutput("name", mode) - opens a file for writing You can use these to read/write files on the device. – many methods return standard java.io.File objects – some return java.io.InputStream or OutputStream objects, which can be used with standard classes like Scanner, BufferedReader, and PrintStream to read/write files (see Java API)  Internal storage example 1 // read a file, and put its contents into a TextView // (assumes hello.txt file exists in res/raw/ directory) Scanner scan = new Scanner( getResources().openRawResource(R.raw.hello)); String allText = ""; // read entire file while (scan.hasNextLine()) { String line = scan.nextLine(); allText += line; } myTextView.setText(allText); scan.close();  Internal storage example 2 // write a short text file to the internal storage PrintStream output = new PrintStream openFileOutput("out.txt", MODE_PRIVATE)); output.println("Hello, world!"); output.println("How are you?"); output.close();... // read the same file, and put its contents into a TextView Scanner scan = new Scanner( openFileInput("out.txt", MODE_PRIVATE)); String allText = ""; // read entire file while (scan.hasNextLine()) { String line = scan.nextLine(); allText += line; } myTextView.setText(allText); scan.close();  External storage external storage: Card that is inserted into the device. (such as a MicroSD card) – can be much larger than internal storage (~8-32 gb) – can be removed or transferred to another device if needed – may not be present, depending on the device – read/writable by other apps and users; not private to your app – not wiped when the app is uninstalled, except in certain cases  External storage permission If your app needs to read/write the device's external storage, you must explicitly request permission to do so in your app's AndroidManifest.xml file. – On install, the user will be prompted to confirm your app permissions....  Using external storage Methods to read/write external storage: – getExternalFilesDir("name") - returns "private" external directory for your app with the given name – Environment.getExternalStoragePublicDirectory(name) - returns public directory for common files like photos, music, etc. pass constants for name such as Environment.DIRECTORY_ALARMS, DIRECTORY_DCIM, DIRECTORY_DOWNLOADS, DIRECTORY_MOVIES, DIRECTORY_MUSIC, DIRECTORY_NOTIFICATIONS, DIRECTORY_PICTURES, DIRECTORY_PODCASTS, DIRECTORY_RINGTONES You can use these to read/write files on the external storage. – the above methods return standard java.io.File objects – these can be used with standard classes like Scanner, BufferedReader, and PrintStream to read/write files (see Java API)  External storage example // write short data to app-specific external storage File outDir = getExternalFilesDir(null); // root dir File outFile = new File(outDir, "example.txt"); PrintStream output = new PrintStream(outFile); output.println("Hello, world!"); output.close(); // read list of pictures in external storage File picsDir = Environment.getExternalStoragePublicDirectory( Environment.DIRECTORY_PICTURES); for (File file : picsDir.listFiles()) {... }  Checking if storage is available public boolean isExternalStorageWritable() { return Environment.MEDIA_MOUNTED.equals( Environment.getExternalStorageState()); } public boolean isExternalStorageReadable() { return isExternalStorageWritable() || Environment.MEDIA_MOUNTED_READ_ONLY.equals( Environment.getExternalStorageState()); }  Accessing web data (link) To read data from the web, first request the INTERNET permission in your AndroidManifest.xml: Then you can use the standard java.net.URL class to connect to a file or page at a given URL and read its data: URL url = new URL("http://foobar.com/example.txt"); Scanner scan = new Scanner(url.openStream()); while (scan.hasNextLine()) { String line = scan.nextLine();... }