数据库| if（c.moveToFirst（））| NPE

Question

所以我已经开始创建一个数据库，当我试图获取AllRecords时，应用程序会抛出一个空指针。

DBAdapter db = new DBAdapter(this); 

-

db.open();
        Cursor c = db.getAllRecords();
        if (c.moveToFirst()) //<--- NPE
        {
            do {          
                DisplayRecord(c);
            } while (c.moveToNext());
        }
        db.close();

我无法弄清楚为什么会这样做。我的插入语句是：

db.open();        
        long id = db.insertRecord("Peanuts");        
        id = db.insertRecord("Coconuts");
        id = db.insertRecord("Almonds");
        db.close();

我不明白为什么这会返回null

编辑：每个请求的整个数据库代码

import android.content.ContentValues;
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 DBAdapter {
public static final String KEY_ROWID = "id";
public static final String KEY_ITEM = "item";

private static final String TAG = "DBAdapter";

private static final String DATABASE_NAME = "InventoryDB";
private static final String DATABASE_TABLE = "inventory";
private static final int DATABASE_VERSION = 1;

private static final String DATABASE_CREATE =
    "create table if not exists inventory (id integer primary key autoincrement, "
    + "item VARCHAR not null);";

private final Context context;    

private DatabaseHelper DBHelper;
private SQLiteDatabase db;

public DBAdapter(Context ctx) 
{
    this.context = ctx;
    DBHelper = new DatabaseHelper(context);
}

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

    @Override
    public void onCreate(SQLiteDatabase db) 
    {
        try {
            db.execSQL(DATABASE_CREATE);    
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
    {
        Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                + newVersion + ", which will destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS inventory");
        onCreate(db);
    }
}    

//---opens the database---
public DBAdapter open() throws SQLException 
{
    db = DBHelper.getWritableDatabase();
    return this;
}

//---closes the database---    
public void close() 
{
    DBHelper.close();
}

//---insert a record into the database---
public long insertRecord(String item) 
{
    ContentValues initialValues = new ContentValues();
    initialValues.put(KEY_ITEM, item);
    return db.insert(DATABASE_TABLE, null, initialValues);
}

//---deletes a particular record---
public boolean deleteContact(long rowId) 
{
    return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}

//---retrieves all the records---
public Cursor getAllRecords() 
{
    return db.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_ITEM}, null, null, null, null, null, null);
}

//---retrieves a particular record---
public Cursor getRecord(long rowId) throws SQLException 
{
    Cursor mCursor =
            db.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
            KEY_ITEM}, 
            KEY_ROWID + "=" + rowId, null, null, null, null, null);
    if (mCursor != null) {
        mCursor.moveToFirst();
    }
    return mCursor;
}

//---updates a record---
public boolean updateRecord(long rowId, String item) 
{
    ContentValues args = new ContentValues();
    args.put(KEY_ITEM, item);

    return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}

}