Question

我已经使用外部软件在sqlite中创建了一个表，并将其添加到我的项目“ assets”文件夹中。到目前为止，它的工作正常，但是当我尝试从应用程序中插入查询行时，它显示了Null指针异常。下面是我的编码，请帮我找出是否有错误的陈述。

DatabaseOpenHelper.java

public class DatabaseOpenHelper extends SQLiteAssetHelper {
private static final String DATABASE_NAME = "peer.db";
private static final int DATABASE_VERSION = 1;

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

DatabaseAccess.java

public class DatabaseAccess {
private SQLiteOpenHelper openHelper;
private SQLiteDatabase database;
private static DatabaseAccess instance;
private Cursor cursor;
  private DatabaseAccess(Context context) {
    this.openHelper = new DatabaseOpenHelper(context);
}

public static DatabaseAccess getInstance(Context context) {
    if (instance == null) {
        instance = new DatabaseAccess(context);
    }
    return instance;
}

public void open（） {

    this.database = openHelper.getWritableDatabase();
}

public void close() {
    if (database != null) {
        this.database.close();
    }
}

public List<String> getQuotes() {
    List<String> list = new ArrayList<>();
    cursor = database.rawQuery("SELECT * FROM peertable", null);
    cursor.moveToFirst();
    while (!cursor.isAfterLast()) {
        list.add(cursor.getString(0));
        cursor.moveToNext();
    }
    cursor.close();
    return list;
}

}

MainActivity.java

public class MainActivity extends Activity {
public ListView listView;
Context context;
private SQLiteDatabase database;
private static final String COL_1 = "name";
private static final String COL_2 = "number";
private static final String TABLE_NAME = "peertable";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    this.listView = (ListView) findViewById(R.id.listView);

    DatabaseAccess databaseAccess = DatabaseAccess.getInstance(this);
    databaseAccess.open();
    ContentValues values = new ContentValues();
    values.put(COL_1, "Karthik");
    values.put(COL_2, 1237894560);
    database.insert(TABLE_NAME,null,values);

    List<String> data = databaseAccess.getQuotes();
    databaseAccess.close();

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, data);
    this.listView.setAdapter(adapter);
}

}

在MainActivity中，我正在尝试插入查询。

错误日志：

 Process: com.synergy.externaldatabase, PID: 11828
                                               java.lang.RuntimeException: Unable to start activity ComponentInfo{com.synergy.externaldatabase/com.synergy.externaldatabase.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
                                                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2793)
                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2864)
                                                   at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567)
                                                   at android.os.Handler.dispatchMessage(Handler.java:105)
                                                   at android.os.Looper.loop(Looper.java:156)
                                                   at android.app.ActivityThread.main(ActivityThread.java:6523)
                                                   at java.lang.reflect.Method.invoke(Native Method)
                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)
                                                Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
                                                   at com.synergy.externaldatabase.MainActivity.onCreate(MainActivity.java:43)
                                                   at android.app.Activity.performCreate(Activity.java:6915)
                                                   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
                                                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2746)
                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2864) 
                                                   at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567) 
                                                   at android.os.Handler.dispatchMessage(Handler.java:105) 
                                                   at android.os.Looper.loop(Looper.java:156) 
                                                   at android.app.ActivityThread.main(ActivityThread.java:6523) 
                                                   at java.lang.reflect.Method.invoke(Native Method) 
                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942) 
                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)

Answer 1

您无需初始化database，但请在此处使用它：

database.insert(TABLE_NAME,null,values);

在database中声明的DatabaseAccess字段与在MainActivity中声明的字段不同。

Answer 2

应该是

 databaseAccess.insert(TABLE_NAME,null,values);

插入查询时出现空指针异常

2 个答案: