當我實例化我的SQLiteOpenHelper的子類時，應用程序不啓動

Question

我開始在Android開發中，當我實例化SQLiteOpenHelper的子類時出現問題，因爲當我這樣做時，我的應用程序簡單無法啓動。

這是我的活動。

package com.almostGames.sera; 

import android.os.Bundle; 
import android.app.Activity; 
import android.support.v4.view.ViewPager; 
import android.view.View; 
import android.view.Window; 
import android.view.WindowManager; 
import android.widget.Button; 

public class SeraActivity extends Activity { 

Button btJogar, btPlacares, btDesafios, btHP; 

ImagensDbHelper mDbHelper = new ImagensDbHelper(getApplicationContext()); 


@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    requestWindowFeature(Window.FEATURE_NO_TITLE); 
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
     WindowManager.LayoutParams.FLAG_FULLSCREEN); 

    chamarTela1(); //metodo que chama a tela1 


} 

public void chamarTela1() { 
    setContentView(R.layout.home); //chama a tela 
    btJogar = (Button) findViewById(R.id.btnJogar); 
    btJogar.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      chamarTela2(); // Chama a tela2 ao clicar no botao 
     } 
    }); 
} 

public void chamarTela2() { 
    setContentView(R.layout.seleciona_livros); //chama a tela 
    ViewPager galeria = (ViewPager) findViewById(R.id.galeria); 
    GaleriaImagensAdapter adapter = new GaleriaImagensAdapter(this); 
    galeria.setAdapter(adapter); 
} 
} 

這裏是我的合同類

package com.almostGames.sera; 

import android.database.sqlite.*; 
import android.provider.BaseColumns; 

public final class ImagensContrato { 


// Construtor vazio 
public ImagensContrato() {} 


/* Definição do conteúdo da tabela*/ 
public static abstract class ImagensEntry implements BaseColumns { 
    public static final String TABLE_NAME = "imagens"; 
} 

private String SQL_CREATE_ENTRIES = 
    "CREATE TABLE sera " + 
     " (_id INTEGER(11) NOT NULL," + 
     " livro VARCHAR(25) NOT NULL," + 
     " resposta VARCHAR(30) NOT NULL," + 
     " acertou INTEGER(11) NOT NULL," + 
     " localizacao BLOB," + 
     " PRIMARY KEY (_id)," + 
     " UNIQUE (_id))"; 

private static final String SQL_DELETE_ENTRIES = 
    "DROP TABLE IF EXISTS " + ImagensEntry.TABLE_NAME; 

public String getSqlCreateEntries() { 
    return SQL_CREATE_ENTRIES; 
} 

public String getSqlDeleteEntries() { 
    return SQL_DELETE_ENTRIES; 
} 

} 

這是我的數據庫幫助

package com.almostGames.sera; 


import android.content.Context; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteOpenHelper; 


public class ImagensDbHelper extends SQLiteOpenHelper { 
// If you change the database schema, you must increment the database version. 
public static final int DATABASE_VERSION = 1; 
public static final String DATABASE_NAME = "sera.db"; 
ImagensContrato contrato = new ImagensContrato(); 

public ImagensDbHelper(Context context) { 
    super(context, DATABASE_NAME, null, DATABASE_VERSION); 
} 

public void onCreate(SQLiteDatabase db) { 
    db.execSQL(contrato.getSqlCreateEntries()); 
} 

public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    // This database is only a cache for online data, so its upgrade policy is 
    // to simply to discard the data and start over 
    db.execSQL(contrato.getSqlDeleteEntries()); 
    onCreate(db); 
} 

public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    onUpgrade(db, oldVersion, newVersion); 
} 

} 

這裏是異常

08-27 22:26:06.784: E/AndroidRuntime(25370): FATAL EXCEPTION: main 
08-27 22:26:06.784: E/AndroidRuntime(25370): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.almostGames.sera/com.almostGames.sera.SeraActivity}: java.lang.NullPointerException 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2064) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2178) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.ActivityThread.access$700(ActivityThread.java:141) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1271) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.os.Handler.dispatchMessage(Handler.java:99) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.os.Looper.loop(Looper.java:137) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.ActivityThread.main(ActivityThread.java:5118) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at java.lang.reflect.Method.invokeNative(Native Method) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at java.lang.reflect.Method.invoke(Method.java:511) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at dalvik.system.NativeStart.main(Native Method) 
08-27 22:26:06.784: E/AndroidRuntime(25370): Caused by: java.lang.NullPointerException 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at com.almostGames.sera.SeraActivity.<init>(SeraActivity.java:15) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at java.lang.Class.newInstanceImpl(Native Method) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at java.lang.Class.newInstance(Class.java:1319) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.Instrumentation.newActivity(Instrumentation.java:1053) 
08-27 22:26:06.784: E/AndroidRuntime(25370): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2055) 
08-27 22:26:06.784: E/AndroidRuntime(25370): ... 11 more 

我做這個教程（ http://developer.android.com/training/basics/data-storage/databases.html），但是當我實例化我的DbHelper時，應用程序無法啓動。 PS：我收到消息：「不幸已停止」。

Answer 1

ImagensDbHelper mDbHelper = new ImagensDbHelper(getApplicationContext()); 

此行崩潰，因爲它之前還沒有創造onCreate（）方法和活動被稱爲getApplicationContext（）返回null。因此，嘗試將此初始化移入onCreate（）方法，如下所示：

ImagensDbHelper mDbHelper; 


@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    requestWindowFeature(Window.FEATURE_NO_TITLE); 
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
     WindowManager.LayoutParams.FLAG_FULLSCREEN); 

    mDbHelper = new ImagensDbHelper(getApplicationContext()); 
    chamarTela1(); //metodo que chama a tela1 
}