Download Data Management

CS499 – Mobile Application
Development
Fall 2013
Programming the Android Platform
Data Management
Data Management
• Files (internal/external)
• SQLite database
• SharedPreferences (not discussed but a couple
of examples included online)
File
• Represents a file system entity identified by a
pathname
• Classified as internal or external
– Internal memory (on the device) usually used for
application private files
– External memory (removable media) used for public
files
– Cache files – temporary files
• Examples:
– DataManagementFileInternalMemory
– DataManagementFileExternalMemory
File API
• boolean isDIrectory()
– return true if this file represents a directory
• String getAbsolutePath()
– returns the absolute path to this file
• boolean setReadable(boolean readable)
– sets read permission on this file
• MANY others – see documentation
Writing an Internal Memory File
// Open file with ContextWrapper.openFileOutput()
FileOutputStream fos =
openFileOutput(filename,MODE_PRIVATE);
PrintWriter pw = new PrintWriter(
new BufferedWriter( new OutputStreamWriter(fos)));
// Write to file
pw.println(…);
//Close file
pw.close();
Reading an Internal Memory File
// Open file with ContextWrapper.openFileOutput()
FileInputStream fis =
openFileOutput(filename);
PrintWriter fr = new PrintWriter(
new BufferedReader( new InputStreamReader(fis)));
// Read from file
while (null != (line=fr.readLine())) {
// process data
}
//Close file
fr.close();
External Memory Files
• Removable media may appear/disappear without
warning
• String Environment.getExternalStorageState()
– MEDIA_MOUNTED – present & mounted with
read/write access
– MEDIA_MOUNTED_READ_ONLY
– MEDIA_REMOVED – not present
• Need permission to write external files
– in AndroidManifest.xml:
<uses-permission android:name=
“android.permission.WRITE_EXTERNAL_STORAGE” />
Writing an External Memory File
public class FileWriteAndReadActivity extends Activity {
public void onCreate(Bundle savedState) {
…
if (Environment.MEDIA_MOUNTED.equals(
Environment.getExternalStorageState())) {
File outFile = new File(getExternalFilesDir(
Environment.DIRECTORY_PICTURES),fileName);
try {
BufferedOutputStream os =
new BufferedOutputStream(new FileOutputStream(outFile));
BufferedInputStream is =
new BufferedInputStream(getResources()
.openRawResource(R.drawable.icon));
copy(is,os);
} catch (FileNotFoundException e) {}
}
}
Writing an External Memory File
private void copy(InputStream is, OutputStream os) {
final byte[] buf = new byte[1024];
int numBytes;
try {
while (-1 != (numBytes = is.read(buf))) {
os.write(buf,0,numBytes);
}
} catch (IOException e) { … }
} finally {
try {
is.close();
os.close();
} catch (IOException e) {}
…
SQLite
• SQLite provides in-memory database available
to the app that created it
• Designed to operate within a very small
footprint
• Implements most of SQL92
• Supports ACID transactions
– ACID: atomic, consistent, isolated & durable
• Will need to use a content provider to make
data available to rest of the system.
Databases
• This is not a class in databases (CS450) and
you aren’t required to know about databases
to use SQLite
• Data organized in tables where
– each row will hold a single element that we are
interested in. Each element has a unique
identifier called the key
– each column is a field of the given elements in the
table
Example: GradesDB
GNumber
LastName
FirstName
Exam1grade
Exam2grade
G12345678
Smith
Bob
80
80
G00000001
Jones
Davey
70
90
G01010101
White
Betty
90
85
G87654321
Doe
Jane
20
100
G99999999
Doe
John
20
50
G88888888
Smith
Bob
80
85
SQL – Structured Query Language
• Standard way to make database requests
(creation, queries, insertion, deletion)
• Based on relational algebra and tuple relational
calculus
• Fairly standardized
SELECT LastName, FirstName FROM GradesDB
WHERE Gnumber = G99999999
• In Android, requests formulated as method calls
with multiple parameters, but has the notation at
the core.
Opening a Database
• Recommended method relies on a helper
class called SQLiteOpenHelper
• Create a subclass SQLiteOpenHelper
– Override onCreate()
– Execute CREATE TABLE command
• Use Constructor to instantiate subclass
• Use SQLiteOpenHelper methods to open &
return underlying database
Opening a Database
public class DatabaseOpenHelper extends SQLiteOpenHelper {
final private static String CREATE_CMD =
“CREATE TABLE artists(“
+ “_id “+”INTEGER PRIMARY KEY AUTOINCREMENT, “
+ “name “+”TEXT NOT NULL)”;
public DatabaseOpenHelper(Context context) {
super(context,”artist_db”,null,1);
}
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_CMD);
}
…
Using a Database
public class DatabaseExampleActivity extends ListActivity {
final static String[] columns={“_id”, “name”};
static SQLiteDatabase db = null;
public void onCreate(Bundle savedState) {
…
DatabaseOpenHelper dbHelper = new DatabaseOpenHelper();
db = dbHelper.getWritableDatabase();
insertArtists();
Cursor c = readArtists();
deleteLadyGaga();
setListAdapter(new SimpleCursorAdapter(
this, R.layout.list_layout,c,columns,
new int[]{R.id._id,R.id.name}));
}
Insertion
private void insertArtists() {
ContentValues values = new ContentValues();
values.put(“name”,”Lady Gaga”);
db.insert(“artists”,null,values);
values.clear();
values.put(“name”,”Johnny Cash”);
db.insert(“artists”,null,values);
values.clear();
values.put(“name”,”Ludwig von Beethoven”);
db.insert(“artists”,null,values);
}
Deletion & Querying
private int deleteLadyGaga() {
return db.delete(“artists”,”name =?”,
new String[]{“Lady Gaga”});
}
private Cursor readArtists() {
// SELECT * from artists
// i.e. return all rows that match
return db.query(“artists”, new String[]{“_id”,”name”},
null, new String[]{},null,null,null);
}
Querying
public Cursor query(String table,
String columns[],
String selection,
String selectionArgs,
String groupBy,
String having,
String orderBy,
String limit)
The parameters are designed to allow queries
with the full power of SQL
Query Examples
• Return all rows where there value matches a given
parameter
String[] COLUMNS = new STRING{“FirstName”,”LastName”};
String selection = “Gnumber = ?”;
String[] args = {“G99999999”};
Cursor result = db.query(“GradesDB”,COLUMNS, selection,
args,null,null,null,null);
• Return rows where some field has a particular range
String[] COLUMNS = new STRING{“FirstName”,”LastName”};
String selection = “Exam1Grade < ? AND Exam2Grade < ?”;
String[] args = {“70”,”70”};
Cursor result = db.query(“GradesDB”,COLUMNS, selection,
args,null,null,null,null);
Cursor class
• Provides random read-write access to the
result set returned by a database query
• http://developer.android.com/reference/andr
oid/database/Cursor.html
• moveToFirst(), moveToNext(),
moveToPosition(int) – iterate through
• getString(int index), getInt(int index) – column
value associated with a particular row
• CursorAdapter lets you put info into a ListView
Upgrading
Content Providers
Examining Databases
• Databases stored in
/data/data/<package name>/databases
• Can examine database with sqlite3