我正在学习创建一个应用程序，特别是从数据库（已经创建）获取数据并将其设置为文本。我遵循了几个教程但是我收到了一个错误（代码本身不是错误，但是应用程序崩溃了）。
这是数据库类：
public class ManejadorBaseDeDatos extends SQLiteAssetHelper {
private static String DB_PATH = "data/data/com.xxkuakxxgmail.knowyourchamp/databases/";
private static String DB_NAME = "imcEmbar.db";
private static SQLiteDatabase mDataBase;
private static ManejadorBaseDeDatos sInstance = null;
private static final int DATABASE_VERSION = 1;
public ManejadorBaseDeDatos() {
super(MainActivity.activity, DB_NAME, null, DATABASE_VERSION);
try {
createDataBase();
openDataBase();
} catch (IOException e) {
e.printStackTrace();
}
}
public static ManejadorBaseDeDatos instance() {
if (sInstance == null) {
sInstance = new ManejadorBaseDeDatos();
}
return sInstance;
}
private void createDataBase() throws IOException {
boolean dbExist = checkDataBase();
SQLiteDatabase db_Read = null;
if (dbExist) {
} else {
db_Read = this.getReadableDatabase();
db_Read.close();
try {
copyDataBase();
} catch (IOException e) {
throw new Error("Error");
}
}
}
public boolean checkDataBase()
{
File dbFile = new File(DB_PATH + DB_NAME);
return dbFile.exists();
}
public void copyDataBase() throws IOException {
InputStream myInput = MainActivity.activity.getAssets().open(DB_NAME);
String outFileName = DB_PATH + DB_NAME;
OutputStream myOutput = new FileOutputStream(outFileName);
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer)) > 0) {
myOutput.write(buffer, 0, length);
}
myOutput.flush();
myOutput.close();
myInput.close();
}
private void openDataBase() throws SQLException {
String myPath = DB_PATH + DB_NAME;
mDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
}
public Cursor select(String query) throws SQLException {
return mDataBase.rawQuery(query, null);
}
@Override
public synchronized void close() {
if (mDataBase != null)
mDataBase.close();
super.close();
}
这是我想要调用数据库的类：
public class Imc_Ema extends AppCompatActivity {
Button btnCalcular;
EditText txtIngresoSemanas;
EditText txtResultado;
ManejadorBaseDeDatos manejadorBD;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_imc_adultos);
manejadorBD = ManejadorBaseDeDatos.instance();
btnCalcular = (Button)findViewById(R.id.btnCalcular);
txtIngresoSemanas = (EditText) findViewById(R.id.txtIngresoSemanas);
btnCalcular.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
txtResultado.setText(null);
Double semanas = Double.valueOf(txtIngresoSemanas.getText().toString());
Cursor cursor = manejadorBD.select("SELECT NormaMin FROM imcEmb WHERE Codigo_producto = '" + semanas + "' ");
String consulta = cursor.getString(cursor.getColumnIndex("NormaMin"));
txtResultado.setText(consulta);
}
});
}
}
错误：
12-04 22:07:59.433 13916-13916/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.xxkuakxxgmail.knowyourchamp, PID: 13916
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.xxkuakxxgmail.knowyourchamp/com.xxkuakxxgmail.knowyourchamp.Imc_Ema}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at com.readystatesoftware.sqliteasset.SQLiteAssetHelper.<init>(SQLiteAssetHelper.java:109)
at com.readystatesoftware.sqliteasset.SQLiteAssetHelper.<init>(SQLiteAssetHelper.java:129)
at com.xxkuakxxgmail.knowyourchamp.ManejadorBaseDeDatos.<init>(ManejadorBaseDeDatos.java:0)
at com.xxkuakxxgmail.knowyourchamp.ManejadorBaseDeDatos.instance(ManejadorBaseDeDatos.java:46)
at com.xxkuakxxgmail.knowyourchamp.Imc_Ema.onCreate(Imc_Ema.java:25)
at android.app.Activity.performCreate(Activity.java:6705)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)