我一直在尝试开发一个应用程序，在其中我在数据库中添加项目的arraylist并在listview中显示它们，但是在尝试插入数据库时我得到空指针异常。 这是日志
07-25 13:41:22.826: E/DB ERROR(306): java.lang.NullPointerException
07-25 13:41:22.826: W/System.err(306): java.lang.NullPointerException
07-25 13:41:22.936: W/System.err(306): at com.demo.itemscanner.ItemScannerDatabaseManager.addRow(ItemScannerDatabaseManager.java:91)
07-25 13:41:22.936: W/System.err(306): at com.demo.itemscanner.ItemScannerDatabaseManager$CustomSQLiteOpenHelper.onCreate(ItemScannerDatabaseManager.java:64)
07-25 13:41:22.936: W/System.err(306): at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:106)
07-25 13:41:22.936: W/System.err(306): at com.demo.itemscanner.ItemScannerDatabaseManager.<init>(ItemScannerDatabaseManager.java:38)
07-25 13:41:22.936: W/System.err(306): at com.demo.itemscanner.ItemScannerActivity.onCreate(ItemScannerActivity.java:33)
07-25 13:41:22.936: W/System.err(306): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
07-25 13:41:22.936: W/System.err(306): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
07-25 13:41:22.936: W/System.err(306): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-25 13:41:22.946: W/System.err(306): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-25 13:41:22.946: W/System.err(306): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-25 13:41:22.946: W/System.err(306): at android.os.Handler.dispatchMessage(Handler.java:99)
07-25 13:41:22.946: W/System.err(306): at android.os.Looper.loop(Looper.java:123)
07-25 13:41:22.946: W/System.err(306): at android.app.ActivityThread.main(ActivityThread.java:4627)
07-25 13:41:22.946: W/System.err(306): at java.lang.reflect.Method.invokeNative(Native Method)
07-25 13:41:22.946: W/System.err(306): at java.lang.reflect.Method.invoke(Method.java:521)
07-25 13:41:22.946: W/System.err(306): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-25 13:41:22.946: W/System.err(306): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-25 13:41:22.946: W/System.err(306): at dalvik.system.NativeStart.main(Native Method)
以下是与之关联的代码：
ItemScannerDatabaseManager.java
package com.demo.itemscanner;
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 ItemScannerDatabaseManager {
// 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 = "item_scanner";
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 = "item_table";
private final String ITEM_ID = "id";
private final String ITEM_NAME = "item_name";
private final String ITEM_PRICE = "item_price";
private final String CURRENT_STOCK = "current_stock";
private final String IMAGE = "image";
public ItemScannerDatabaseManager(Context context) {
this.context = context;
// create or open the database
CustomSQLiteOpenHelper helper = new CustomSQLiteOpenHelper(context);
this.db = helper.getWritableDatabase();
}
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
+ "("
+ ITEM_ID + " integer primary key autoincrement, "
+ ITEM_NAME + " text not null, "
+ ITEM_PRICE + " integer,"
+ CURRENT_STOCK + " integer,"
+ IMAGE + " text "
+ ");";
// execute the query string to the database.
db.execSQL(newTableQueryString);
addRow();
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
onCreate(db);
}
}
/**********************************************************************
* ADDING A ROW TO THE DATABASE TABLE
*
* the key is automatically assigned by the database
* @param name the value for the item name and so on..
*/
public void addRow()
{
try{// this is a key value pair holder used by android's SQLite functions
ContentValues values = new ContentValues();
values.put(ITEM_NAME, "Ivory Silk Clutch");
values.put(ITEM_PRICE, "999");
values.put(CURRENT_STOCK, "50");
values.put(IMAGE, R.drawable.ivory_silk);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "Gold Clutch");
values.put(ITEM_PRICE, "799");
values.put(CURRENT_STOCK, "10");
values.put(IMAGE, R.drawable.gold);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "Bridal Clutch");
values.put(ITEM_PRICE, "899");
values.put(CURRENT_STOCK, "20");
values.put(IMAGE, R.drawable.bridal);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "Black Clutch");
values.put(ITEM_PRICE, "799");
values.put(CURRENT_STOCK, "20");
values.put(IMAGE, R.drawable.black);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "evening Clutch");
values.put(ITEM_PRICE, "999");
values.put(CURRENT_STOCK, "20");
values.put(IMAGE, R.drawable.evening_clutch);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "grey Clutch");
values.put(ITEM_PRICE, "699");
values.put(CURRENT_STOCK, "20");
values.put(IMAGE, R.drawable.grey);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "cream Clutch");
values.put(ITEM_PRICE, "999");
values.put(CURRENT_STOCK, "15");
values.put(IMAGE, R.drawable.cream);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "White Tubular Clutch");
values.put(ITEM_PRICE, "999");
values.put(CURRENT_STOCK, "25");
values.put(IMAGE, R.drawable.white_tubular_clutch);
db.insert(TABLE_NAME, null, values);
values.put(ITEM_NAME, "Party Clutch");
values.put(ITEM_PRICE, "999");
values.put(CURRENT_STOCK, "20");
values.put(IMAGE, R.drawable.party);
db.insert(TABLE_NAME, null, values);
}
catch(Exception e)
{
Log.e("DB ERROR", e.toString());
e.printStackTrace();
}
}
/**********************************************************************
* RETRIEVING ALL ROWS FROM THE DATABASE TABLE
* the key is automatically assigned by the database
*/
public ArrayList<Item> getAllItemsAsArrays()
{
// create an ArrayList that will hold all of the data collected from
// the database.
ArrayList<Item> dataArrays = new ArrayList<Item>();
Item item = new Item();
// 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[] { ITEM_ID, ITEM_NAME, ITEM_PRICE, CURRENT_STOCK,IMAGE},
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
{
item.setId(cursor.getLong(0));
item.setTitle(cursor.getString(1));
item.setPrice(cursor.getLong(2));
item.setCurrentStock(cursor.getLong(3));
item.setProductImage(Integer.parseInt(cursor.getString(4)));
dataArrays.add(item);
}
// move the cursor's pointer up one position.
while (cursor.moveToNext());
}
cursor.close();
}
catch (SQLException e)
{
Log.e("DB Error", e.toString());
e.printStackTrace();
}
// return the ArrayList that holds the data collected from
// the database.
return dataArrays;
}
}
ItemScannerActivity.java
package com.demo.itemscanner;
import java.util.ArrayList;
import android.app.Activity;
import android.net.Uri;
import android.os.Bundle;
import android.util.Log;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TableLayout;
import android.widget.TableRow;
import android.widget.TextView;
public class ItemScannerActivity extends Activity {
ItemScannerDatabaseManager db;
TableLayout dataTable;
//ArrayList<Item> itemList = new ArrayList<Item>();
private ArrayAdapter<Item> listAdapter ;
@Override
public void onCreate(Bundle savedInstanceState) {
try {
// Android specific calls
super.onCreate(savedInstanceState);
setContentView(R.layout.dataview);
ListView list = (ListView)findViewById(R.id.ListViewCatalog);
// create the database manager object
db = new ItemScannerDatabaseManager(this);
for(int i=0; i<db.getAllItemsAsArrays().size();i++){
listAdapter.add(db.getAllItemsAsArrays().get(i));
}
list.setAdapter(listAdapter);
} catch (Exception e) {
Log.e("ERROR", e.toString());
e.printStackTrace();
}
}
@Override
public void onResume() {
super.onResume();
}
@Override
public void onPause() {
super.onPause();
}
@Override
public void onDestroy() {
super.onDestroy();
}
}
Item.java
package com.demo.itemscanner;
public class Item {
private long id;
private String title;
private int productImage;
private double price;
private boolean selected;
private double currentStock;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public int getProductImage() {
return productImage;
}
public void setProductImage(int productImage) {
this.productImage = productImage;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
public boolean isSelected() {
return selected;
}
public void setSelected(boolean selected) {
this.selected = selected;
}
public double getCurrentStock() {
return currentStock;
}
public void setCurrentStock(double currentStock) {
this.currentStock = currentStock;
}
/*public Item(String title, Drawable productImage, double currentStatus,
double price) {
this.title = title;
this.productImage = productImage;
this.currentStatus = currentStatus;
this.price = price;
}*/
}
xml文件， dataview.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#ffffff"
android:orientation="vertical" >
<TextView
android:id="@+id/TextView01"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="5dp"
android:text="Product Catalog"
android:textColor="#000000"
android:textSize="24sp" >
</TextView>
<ListView
android:id="@+id/ListViewCatalog"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:background="#ffffff"
android:cacheColorHint="#ffffff"
android:clickable="true" >
</ListView>
</LinearLayout>
看起来这个表没有在数据库中创建，它在db.insert行中返回空指针异常（TABLE_NAME，null，values）;
请告诉我哪里出错，任何帮助将不胜感激。 - 谢谢
答案 0 :(得分：1)
引发NullPointerException，因为此时db的数据库addRow（ItemScannerDatabaseManager方法中使用的数据库）为null。如果我是你，我会添加另一个addRow方法，如下所示：
public void addRow(SQLiteDatabase theDb) {
// Setup the ContentValues object
ContentValues values = new ContentValues();
//.. add stuff to values
// insert in to the database
theDb.insert(TABLE_NAME, null, values);
}
然后，您可以在onCreate的{{1}}方法中使用此方法 ，如下所示：
SQLiteOpenHelper