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POjava.lang.NullPointerException error on a query
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  1. POjava.lang.NullPointerException error on a query
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    copied!<p>i'm ready to develop my second app for android, and I want to use the database. But I'm blocked by an error.</p> <p>This is the class:</p> <pre><code>public class MyDatabase { SQLiteDatabase mDb; DbHelper mDbHelper; Context mContext; private static final String DB_NAME="epsoftsms";//nome del db private static final int DB_VERSION=1; //numero di versione del nostro db public MyDatabase(Context ctx){ mContext=ctx; mDbHelper=new DbHelper(ctx, DB_NAME, null, DB_VERSION); //quando istanziamo questa classe, istanziamo anche l'helper (vedi sotto) } public void open(){ //il database su cui agiamo è leggibile/scrivibile mDb=mDbHelper.getWritableDatabase(); } public void close(){ //chiudiamo il database su cui agiamo mDb.close(); } //i seguenti 2 metodi servono per la lettura/scrittura del db. aggiungete e modificate a discrezione // consiglio:si potrebbe creare una classe Prodotto, i quali oggetti verrebbero passati come parametri dei seguenti metodi, rispettivamente ritornati. Lacio a voi il divertimento public void inserimentoParametri(String parametro,String valore){ //metodo per inserire i dati ContentValues cv=new ContentValues(); cv.put(ParametriMetaData.PARAMETRO, parametro); cv.put(ParametriMetaData.VALORE, valore); mDb.insert(ParametriMetaData.TAB_PARAMETRI, null, cv); } public Cursor listaParametri(){ //metodo per fare la query di tutti i dati return mDb.query(ParametriMetaData.TAB_PARAMETRI, null,null,null,null,null,null); } static class ParametriMetaData { // i metadati della tabella, accessibili ovunque static final String TAB_PARAMETRI = "parametri"; static final String PARAMETRO = "_parametro"; static final String VALORE = "valore"; } private static final String CREA_TAB_PARAMETRI = "CREATE TABLE IF NOT EXISTS " //codice sql di creazione della tabella + ParametriMetaData.TAB_PARAMETRI + " (" + ParametriMetaData.PARAMETRO+ " text primary key, " + ParametriMetaData.VALORE + " text not null);"; private class DbHelper extends SQLiteOpenHelper { //classe che ci aiuta nella creazione del db public DbHelper(Context context, String name, CursorFactory factory,int version) { super(context, name, factory, version); } public SQLiteDatabase getWritableDatabase() { // TODO Auto-generated method stub return null; } @Override public void onCreate(SQLiteDatabase _db) { //solo quando il db viene creato, creiamo la tabella _db.execSQL(CREA_TAB_PARAMETRI); } @Override public void onUpgrade(SQLiteDatabase _db, int oldVersion, int newVersion) { //qui mettiamo eventuali modifiche al db, se nella nostra nuova versione della app, il db cambia numero di versione } } } </code></pre> <p>And this is part of main interested:</p> <pre><code> final MyDatabase db=new MyDatabase(getApplicationContext()); db.open(); //apriamo il db if (db.listaParametri().getCount()==0) { } </code></pre> <p>When I launch the app it return the following errors:</p> <pre><code>06-08 14:04:40.197: E/AndroidRuntime(1149): Uncaught handler: thread main exiting due to uncaught exception 06-08 14:04:40.207: E/AndroidRuntime(1149): java.lang.RuntimeException: Unable to start activity ComponentInfo{it.axiomatic.epsoft.sms/it.axiomatic.epsoft.sms.EpsoftSMSActivity}: java.lang.NullPointerException 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2268) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2284) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.ActivityThread.access$1800(ActivityThread.java:112) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1692) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.os.Handler.dispatchMessage(Handler.java:99) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.os.Looper.loop(Looper.java:123) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.ActivityThread.main(ActivityThread.java:3948) 06-08 14:04:40.207: E/AndroidRuntime(1149): at java.lang.reflect.Method.invokeNative(Native Method) 06-08 14:04:40.207: E/AndroidRuntime(1149): at java.lang.reflect.Method.invoke(Method.java:521) 06-08 14:04:40.207: E/AndroidRuntime(1149): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:782) 06-08 14:04:40.207: E/AndroidRuntime(1149): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:540) 06-08 14:04:40.207: E/AndroidRuntime(1149): at dalvik.system.NativeStart.main(Native Method) 06-08 14:04:40.207: E/AndroidRuntime(1149): Caused by: java.lang.NullPointerException 06-08 14:04:40.207: E/AndroidRuntime(1149): at it.axiomatic.epsoft.sms.MyDatabase.listaParametri(MyDatabase.java:50) 06-08 14:04:40.207: E/AndroidRuntime(1149): at it.axiomatic.epsoft.sms.EpsoftSMSActivity.onCreate(EpsoftSMSActivity.java:30) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123) 06-08 14:04:40.207: E/AndroidRuntime(1149): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2231) 06-08 14:04:40.207: E/AndroidRuntime(1149): ... 11 more </code></pre> <p>What's the problem? Tnx and forgive me for some error, it's my first requesto on Stackoverflow.</p>
 

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