在我的数据库中插入行时出现空指针异常

Question

我正在尝试设置一个基本的SQLite数据库并在其中插入一行。当我尝试插入一行时，我的应用程序失败。我在调用周围放了一个try catch块，我发现它是一个空指针异常。看起来我的数据库永远不会被创建。我正在调用在我的代码顶部创建数据库的对象，所以我不知道为什么没有创建它。这是我试图创建和插入我的数据库的片段。我创建数据库的行是：MapsDatabaseManager db。然后我尝试在try catch块中插入它，我在那里看到空指针。我还在我的类MapsdatabaseManger中创建了实际的数据库。

package com.matt.maps;

import java.util.ArrayList;

import android.app.ListFragment;
import android.os.Bundle;

import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;


public class FragmentList extends ListFragment
{

private final String[] tutorialList = {"RED","GREEN","BLUE",};
//
private ArrayList<ArrayList<Object>> nameList;

// the class that opens or creates the database and makes sql calls to it
MapsDatabaseManager db; 



@Override
public void onCreate(Bundle savedInstanceState)
{
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);



    //prime db with some rows
    try{db.addRow("Matt", "Painting");}
    catch(Exception e)
    {
        Log.e("DB ERROR", e.toString());
        e.printStackTrace();
    }
    //db.addRow("Matt", "Painting");
    //db.addRow("Tiffany", "Delutis");

    //get all rows in an array
    //nameList = db.getAllRowsAsArrays();

    //put array in adapter
    /*
    ArrayAdapter<ArrayList<Object>> adapter2 = new ArrayAdapter<ArrayList<Object>>(getActivity(),
            android.R.layout.simple_list_item_1, nameList);
    setListAdapter(adapter2);
    */

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity(),
            android.R.layout.simple_list_item_1, tutorialList);
    setListAdapter(adapter);

}

@Override
public void onListItemClick(ListView l, View v, int position, long id)
{
    // Do something
    position = position + 1;
    Toast.makeText(v.getContext(), "Selected Item is = "+ position, Toast.LENGTH_LONG).show();  
}

}

package com.matt.maps;

import java.util.ArrayList;

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 MapsDatabaseManager
{
// the Activity or Application that is creating an object from this class.
Context context;

// a reference to the database used by this application/object
private SQLiteDatabase db;

// These constants are specific to the database.  They should be 
// changed to suit your needs.
private final String DB_NAME = "database_name";
private final int DB_VERSION = 1;

// These constants are specific to the database table.  They should be
// changed to suit your needs.
private final String TABLE_NAME = "database_table";
private final String TABLE_ROW_ID = "id";
private final String TABLE_ROW_ONE = "table_row_one";
private final String TABLE_ROW_TWO = "table_row_two";

public MapsDatabaseManager(Context context)
{
    this.context = context;

    // create or open the database
    CustomSQLiteOpenHelper helper = new CustomSQLiteOpenHelper(context);
    this.db = helper.getWritableDatabase();
}




/**********************************************************************
 * ADDING A ROW TO THE DATABASE TABLE
 * 
 * This is an example of how to add a row to a database table
 * using this class.  You should edit this method to suit your
 * needs.
 * 
 * the key is automatically assigned by the database
 * @param rowStringOne the value for the row's first column
 * @param rowStringTwo the value for the row's second column 
 */
public void addRow(String rowStringOne, String rowStringTwo)
{
    // this is a key value pair holder used by android's SQLite functions
    ContentValues values = new ContentValues();
    values.put(TABLE_ROW_ONE, rowStringOne);
    values.put(TABLE_ROW_TWO, rowStringTwo);

    // ask the database object to insert the new data 
    try{db.insert(TABLE_NAME, null, values);}
    catch(Exception e)
    {
        Log.e("DB ERROR", e.toString());
        e.printStackTrace();
    }
}



/**********************************************************************
 * DELETING A ROW FROM THE DATABASE TABLE
 * 
 * This is an example of how to delete a row from a database table
 * using this class. In most cases, this method probably does
 * not need to be rewritten.
 * 
 * @param rowID the SQLite database identifier for the row to delete.
 */
public void deleteRow(long rowID)
{
    // ask the database manager to delete the row of given id
    try {db.delete(TABLE_NAME, TABLE_ROW_ID + "=" + rowID, null);}
    catch (Exception e)
    {
        Log.e("DB ERROR", e.toString());
        e.printStackTrace();
    }
}

/**********************************************************************
 * UPDATING A ROW IN THE DATABASE TABLE
 * 
 * This is an example of how to update a row in the database table
 * using this class.  You should edit this method to suit your needs.
 * 
 * @param rowID the SQLite database identifier for the row to update.
 * @param rowStringOne the new value for the row's first column
 * @param rowStringTwo the new value for the row's second column 
 */ 
public void updateRow(long rowID, String rowStringOne, String rowStringTwo)
{
    // this is a key value pair holder used by android's SQLite functions
    ContentValues values = new ContentValues();
    values.put(TABLE_ROW_ONE, rowStringOne);
    values.put(TABLE_ROW_TWO, rowStringTwo);

    // ask the database object to update the database row of given rowID
    try {db.update(TABLE_NAME, values, TABLE_ROW_ID + "=" + rowID, null);}
    catch (Exception e)
    {
        Log.e("DB Error", e.toString());
        e.printStackTrace();
    }
}

/**********************************************************************
 * RETRIEVING A ROW FROM THE DATABASE TABLE
 * 
 * This is an example of how to retrieve a row from a database table
 * using this class.  You should edit this method to suit your needs.
 * 
 * @param rowID the id of the row to retrieve
 * @return an array containing the data from the row
 */
public ArrayList<Object> getRowAsArray(long rowID)
{
    // create an array list to store data from the database row.
    // I would recommend creating a JavaBean compliant object 
    // to store this data instead.  That way you can ensure
    // data types are correct.
    ArrayList<Object> rowArray = new ArrayList<Object>();
    Cursor cursor;

    try
    {
        // this is a database call that creates a "cursor" object.
        // the cursor object store the information collected from the
        // database and is used to iterate through the data.
        cursor = db.query
        (
                TABLE_NAME,
                new String[] { TABLE_ROW_ID, TABLE_ROW_ONE,TABLE_ROW_TWO },
                TABLE_ROW_ID + "=" + rowID,
                null, null, null, null, null
        );

        // move the pointer to position zero in the cursor.
        cursor.moveToFirst();

        // if there is data available after the cursor's pointer, add
        // it to the ArrayList that will be returned by the method.
        if (!cursor.isAfterLast())
        {
            do
            {
                rowArray.add(cursor.getLong(0));
                rowArray.add(cursor.getString(1));
                rowArray.add(cursor.getString(2));
            }
            while (cursor.moveToNext());
        }

        // let java know that you are through with the cursor.
        cursor.close();
    }
    catch (SQLException e) 
    {
        Log.e("DB ERROR", e.toString());
        e.printStackTrace();
    }

    // return the ArrayList containing the given row from the database.
    return rowArray;
}




/**********************************************************************
 * RETRIEVING ALL ROWS FROM THE DATABASE TABLE
 * 
 * This is an example of how to retrieve all data from a database
 * table using this class.  You should edit this method to suit your
 * needs.
 * 
 * the key is automatically assigned by the database
 */

public ArrayList<ArrayList<Object>> getAllRowsAsArrays()
{
    // create an ArrayList that will hold all of the data collected from
    // the database.
    ArrayList<ArrayList<Object>> dataArrays = new ArrayList<ArrayList<Object>>();

    // this is a database call that creates a "cursor" object.
    // the cursor object store the information collected from the
    // database and is used to iterate through the data.
    Cursor cursor;

    try
    {
        // ask the database object to create the cursor.
        cursor = db.query(
                TABLE_NAME,
                new String[]{TABLE_ROW_ID, TABLE_ROW_ONE, TABLE_ROW_TWO},
                null, null, null, null, null
        );

        // move the cursor's pointer to position zero.
        cursor.moveToFirst();

        // if there is data after the current cursor position, add it
        // to the ArrayList.
        if (!cursor.isAfterLast())
        {
            do
            {
                ArrayList<Object> dataList = new ArrayList<Object>();

                dataList.add(cursor.getLong(0));
                dataList.add(cursor.getString(1));
                dataList.add(cursor.getString(2));

                dataArrays.add(dataList);
            }
            // move the cursor's pointer up one position.
            while (cursor.moveToNext());
        }
    }
    catch (SQLException e)
    {
        Log.e("DB Error", e.toString());
        e.printStackTrace();
    }

    // return the ArrayList that holds the data collected from
    // the database.
    return dataArrays;
}




/**********************************************************************
 * THIS IS THE BEGINNING OF THE INTERNAL SQLiteOpenHelper SUBCLASS.
 * 
 * I MADE THIS CLASS INTERNAL SO I CAN COPY A SINGLE FILE TO NEW APPS 
 * AND MODIFYING IT - ACHIEVING DATABASE FUNCTIONALITY.  ALSO, THIS WAY 
 * I DO NOT HAVE TO SHARE CONSTANTS BETWEEN TWO FILES AND CAN
 * INSTEAD MAKE THEM PRIVATE AND/OR NON-STATIC.  HOWEVER, I THINK THE
 * INDUSTRY STANDARD IS TO KEEP THIS CLASS IN A SEPARATE FILE.
 *********************************************************************/

/**
 * This class is designed to check if there is a database that currently
 * exists for the given program.  If the database does not exist, it creates
 * one.  After the class ensures that the database exists, this class
 * will open the database for use.  Most of this functionality will be
 * handled by the SQLiteOpenHelper parent class.  The purpose of extending
 * this class is to tell the class how to create (or update) the database.
 * 
 * @author Randall Mitchell
 *
 */
private class CustomSQLiteOpenHelper extends SQLiteOpenHelper
{
    public CustomSQLiteOpenHelper(Context context)
    {
        super(context, DB_NAME, null, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db)
    {
        // This string is used to create the database.  It should
        // be changed to suit your needs.
        String newTableQueryString = "create table " +
                                    TABLE_NAME +
                                    " (" +
                                    TABLE_ROW_ID + " integer primary key autoincrement not null," +
                                    TABLE_ROW_ONE + " text," +
                                    TABLE_ROW_TWO + " text" +
                                    ");";
        // execute the query string to the database.
        db.execSQL(newTableQueryString);
    }


    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
    {
        // NOTHING TO DO HERE. THIS IS THE ORIGINAL DATABASE VERSION.
        // OTHERWISE, YOU WOULD SPECIFIY HOW TO UPGRADE THE DATABASE.
    }
}

}

Answer 1

您有一个非常简单的错误：您忘记初始化MapsDatabaseManager，因此db变量将为null。请在此处试用此代码：

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    db = new MapsDatabaseManager(this);

    //prime db with some rows
    try {
        db.addRow("Matt", "Painting");
    } catch(Exception e) {
        Log.e("DB ERROR", e.toString());
        e.printStackTrace();
    }

    // ...