Sistema Operativo Android , utilizzo del database SQLite

Il sistema operativo Android è molto avanzato, quasi simile a quello di un normale pc.
Si basa sul kernel Linux 2.6 e permette di sviluppare applicazioni java di alta complessità.
Al contrario di altri sistemi operativi (per telefoni), Android può utilizzare per la persistenza dei dati il database SQLite.

Esempio di un gerenic Dao Per Android

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class GenericDAO extends SQLiteOpenHelper {

private static final String TAG = "GenericDAO";
private static SQLiteDatabase db;
private static String dName;
private static String tName;
private static String sql;
public static final String KEY_ID = "_id";

private static GenericDAO instance;

private GenericDAO(Context ctx, String dbName, String sql, String tableName, int ver){
super(ctx, dbName, null, ver);
Log.i(TAG, "Creating or opening database [ " + dbName + " ].");
GenericDAO.sql = sql;
dName = dbName;
tName = tableName;
}

public static GenericDAO getInstance(Context ctx, String dbName, String sql, String tableName, int ver){
if(instance == null){
instance = new GenericDAO(ctx, dbName, sql, tableName, ver);
try{
Log.i(TAG, "Creating or opening the database [ " + dbName + " ].");
db = instance.getWritableDatabase();
}catch(SQLiteException se){
Log.e(TAG, "Cound not create and/or open the database [ " + dbName + " ] that will be used for reading and writing.", se);
}
}
return instance;
}

public void close(){
if(instance != null){
Log.i(TAG, "Closing the database [ " + dName + " ].");
db.close();
instance = null;
}
}

@Override
public void onCreate(SQLiteDatabase db){
Log.i(TAG, "Trying to create database table if it isn't existed [ " + sql + " ].");
try{
db.execSQL(sql);
}catch(SQLException se){
Log.e(TAG, "Cound not create the database table according to the SQL statement [ " + sql + " ].", se);
}
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
Log.i(TAG, "Upgrading database from version " + oldVersion + " to " + newVersion + ", which will destroy all old data");
try{
db.execSQL("DROP TABLE IF EXISTS " + tName);
}catch(SQLException se){
Log.e(TAG, "Cound not drop the database table [ " + tName + " ].", se);
}
onCreate(db);
}

public long insert(String table, ContentValues values){
return db.insert(table, null, values);
}

public Cursor get(String table, String[] columns){
return db.query(table, columns, null, null, null, null, null);
}

public Cursor get(String table, String[] columns, long id){
Cursor cursor =db.query(true, table, columns, KEY_ID + "=" + id, null, null, null, null, null);
if (cursor != null) {
cursor.moveToFirst();
}
return cursor;
}

public int delete(String table) {
return db.delete(table, "1", null);
}

public int delete(String table, long id) {
return db.delete(table, KEY_ID + "=" + id, null);
}

public int update(String table, long id, ContentValues values) {
return db.update(table, values, KEY_ID + "=" + id, null);
}
}

Modello java Notes.java

public class Notes {

public static final String DATABASE_NAME = "data";
public static final String DATABASE_TABLE = "notes";
public static final int DATABASE_VERSION = 1;
public static final String TABLE_CREATE =
"create table notes (_id integer primary key autoincrement, "
+ "title text not null, body text not null);";

public static final String COL_TITLE = "title";
public static final String COL_BODY = "body";

private int id;
private String title;
private String body;

public int getId() {
return id;
}
public String getTitle() {
return title;
}
public String getBody() {
return body;
}
public void setId(int id) {
this.id = id;
}
public void setTitle(String title) {
this.title = title;
}
public void setBody(String body) {
this.body = body;
}


}

Utilizzo del Dao

private String testCRUD(){

String result="";
Cursor cursor = null;
String[] columns = new String[] {GenericDAO.KEY_ID, Notes.COL_TITLE, Notes.COL_BODY};

GenericDAO dao = GenericDAO.getInstance(this, Notes.DATABASE_NAME, Notes.TABLE_CREATE, Notes.DATABASE_TABLE, Notes.DATABASE_VERSION);

if(dao != null){

ContentValues values = new ContentValues();
values.put(Notes.COL_TITLE, "aaa");
values.put(Notes.COL_BODY, "bbb");
dao.insert(Notes.DATABASE_TABLE, values);

values = new ContentValues();
values.put(Notes.COL_TITLE, "ccc");
values.put(Notes.COL_BODY, "ddd");
dao.insert(Notes.DATABASE_TABLE, values);

cursor = dao.get(Notes.DATABASE_TABLE, columns);


int idColumn = cursor.getColumnIndex(GenericDAO.KEY_ID); 
int titleColumn = cursor.getColumnIndex(Notes.COL_TITLE); 
int bodyColumn = cursor.getColumnIndex(Notes.COL_BODY); 
.....................................................................
.....................................................................
.....................................................................
result += " now update the second record.";

values = new ContentValues();
values.put(Notes.COL_TITLE, "eee");
values.put(Notes.COL_BODY, "fff");
dao.update(Notes.DATABASE_TABLE, 2, values);

cursor.requery();
cursor.close();

result += " now delete first record.";

dao.delete(Notes.DATABASE_TABLE, 1);

result += " now delete all records.";

dao.delete(Notes.DATABASE_TABLE);

dao.close();
}

return result;

}

Android mette a disposizione dei programmatori librerie, tutorial, esempi di codice e un plugin per Eclipse.
Quest'ultimo è stato scelto da Google come ambiente di sviluppo ufficiale.