Android SQLiteDatabase 空指针异常 [英] Android SQLiteDatabase Null Pointer Exception

问题描述

我刚刚开始为 Android 开发.到目前为止，它出人意料地顺利航行，但现在我在实现 SQLite 数据库时遇到了一些问题.我的代码基于 Google 的记事本示例(运行良好)，但我不断收到空指针异常，这似乎与 getWritableDatabase() 方法有关.

I've just started developing for Android. So far it's been surprisingly smooth sailing, but now I've run into some problems implementing an SQLite database. My code is based on Google's Notepad example (which runs perfectly well), but I keep getting Null Pointer Exceptions thrown at me and it seems to be related to the getWritableDatabase()-method.

有问题的类是 QuizDbAdapter:

The offending class is QuizDbAdapter:

package com.mypackage.myapp;

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

public class QuizDbAdapter {
   public static final String KEY_ROWID =         "_id";
   public static final String KEY_QUESTION =      "question";
   public static final String KEY_ANSWER1 =      "answer1";
   public static final String KEY_ANSWER2 =      "answer2";
   public static final String KEY_ANSWER3 =      "answer3";
   public static final String KEY_ANSWER4 =      "answer4";
   public static final String KEY_CORRECT_ANSWER =   "correct_answer";
   public static final String KEY_IMAGE =         "image";
   public static final String KEY_CATEGORIES =      "categories";
   public static final String KEY_IMAGEQUESTION =   "image_question";
   public static final String KEY_EXPLANATION =   "explanation";

   private static final String TAG = "QuizDbAdapter";
   private DatabaseHelper databaseHelper;
   private SQLiteDatabase database;

   private static final String DATABASE_CREATE =
      "create table questions (_id integer primary key autoincrement, "
        + "question text not null, answer1 text not null, answer2 text not null,"
        + "answer3 text not null, answer4 text not null, correct_answer integer not null,"
        + "image text not null, categories text not null, image_question boolean not null,"
        + "explanation text not null);"
        + "create table roadsigns (_id integer primary key autoincrement, "
        + ");";

   private static final String DATABASE_NAME = "mydb.sqlite";
   private static final int DATABASE_VERSION = 1;
   private static final String TABLE_QUESTIONS = "questions";

   private final Context context;

   private static class DatabaseHelper extends SQLiteOpenHelper {
      DatabaseHelper(Context c) {
         super(c, DATABASE_NAME, null, DATABASE_VERSION);
      }

      @Override
      public void onCreate(SQLiteDatabase db) {
         db.execSQL(DATABASE_CREATE);
      }

      @Override
      public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
         db.execSQL("DROP TABLE IF EXISTS questions");
            onCreate(db);
      }
   }

   public QuizDbAdapter(Context c) {
      this.context = c;
   }

   public QuizDbAdapter open() throws SQLException {
      databaseHelper = new DatabaseHelper(context);
      database = databaseHelper.getWritableDatabase();
      return this;
   }

   public void close() {
      databaseHelper.close();
   }

   public Cursor fetchAllQuestions() {
      return database.query(TABLE_QUESTIONS, new String[] { KEY_ROWID, KEY_QUESTION,
            KEY_ANSWER1, KEY_ANSWER2, KEY_ANSWER3, KEY_ANSWER4, KEY_CORRECT_ANSWER,
            KEY_IMAGE, KEY_CATEGORIES, KEY_IMAGEQUESTION, KEY_EXPLANATION },
            null, null, null, null, null);
   }

   public Cursor fetchQuestion(long rowId) throws SQLException {
      Cursor cursor = database.query(true, TABLE_QUESTIONS, new String[] { KEY_ROWID,
            KEY_QUESTION, KEY_ANSWER1, KEY_ANSWER2, KEY_ANSWER3, KEY_ANSWER4,
            KEY_CORRECT_ANSWER, KEY_IMAGE, KEY_CATEGORIES, KEY_IMAGEQUESTION,
            KEY_EXPLANATION }, KEY_ROWID + "=" + rowId, null, null, null, null, null);
      if (cursor != null)
         cursor.moveToFirst();
      return cursor;
   }
}

每当我尝试使用

QuizDbHelper databaseHelper = new QuizDbHelper(this);
databaseHelper.open(); // <-- X(

应用程序阻塞并提供一个空指针异常，指示对 open() 的调用.

the application chokes and serves up a Null Pointer Exception indicating the call to open().

堆栈跟踪:

10-06 23:24:02.104: ERROR/AndroidRuntime(369): FATAL EXCEPTION: main
10-06 23:24:02.104: ERROR/AndroidRuntime(369): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mypackage.myapp/com.mypackage.myapp.QuestionActivity}: java.lang.NullPointerException
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.ActivityThread.access$2300(ActivityThread.java:125)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.os.Looper.loop(Looper.java:123)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.ActivityThread.main(ActivityThread.java:4627)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at java.lang.reflect.Method.invokeNative(Native Method)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at java.lang.reflect.Method.invoke(Method.java:521)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at dalvik.system.NativeStart.main(Native Method)
10-06 23:24:02.104: ERROR/AndroidRuntime(369): Caused by: java.lang.NullPointerException
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at com.mypackage.myapp.Quiz.newQuiz(Quiz.java:88)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at com.mypackage.myapp.QuestionActivity.onCreate(QuestionActivity.java:21)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
10-06 23:24:02.104: ERROR/AndroidRuntime(369):     ... 11 more

推荐答案

Quiz.newQuiz 是静态方法吗?如果是这种情况，this 不可用(因此是空指针).

Is Quiz.newQuiz a static method? If that is the case, this is not available (and as such a null pointer).

这篇关于Android SQLiteDatabase 空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！