SQLite 数据库还有另一个问题。我在这个类的帮助下访问数据库，而且，在这里我创建数据库并预填充它。：
public class DatabaseHandler extends SQLiteOpenHelper {
// All Static variables
// Database Version
private static final int DATABASE_VERSION = 1;
// Database Name
private static final String DATABASE_NAME = "mydatabase";
// Table names
private static final String TABLE_DEVICE = "device";
private static final String TABLE_ICON = "icon";
// Column names of device table
private static final String ID_DEVICE = "idDevice";
private static final String UNIT_NUMBER = "unitNumber";
private static final String DEVICE_ID = "deviceId";
private static final String FK_ICON = "kdIcon";
// Column names of icon table
private static final String ID_ICON = "idIcon";
private static final String CATEGORY = "category";
private static final String ON = "iconOn";
private static final String OFF = "iconOff";
private static final String ON_COOLING = "onCooling";
private static final String ON_HEATING = "onHeating";
private static final String ICON_0 = "icon0";
private static final String ICON_10 = "icon10";
private static final String ICON_20 = "icon20";
private static final String ICON_25 = "icon25";
private static final String ICON_30 = "icon30";
private static final String ICON_40 = "icon40";
private static final String ICON_50 = "icon50";
private static final String ICON_60 = "icon60";
private static final String ICON_75 = "icon75";
private static final String ICON_80 = "icon80";
private static final String ICON_90 = "icon90";
private static final String ICON_100 = "icon100";
public DatabaseHandler(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
String createDeviceTable = "CREATE TABLE " + TABLE_DEVICE + " ("
+ ID_DEVICE + " INTEGER PRIMARY KEY NOT NULL UNIQUE , "
+ UNIT_NUMBER + " INTEGER NOT NULL UNIQUE , " + DEVICE_ID
+ " INTEGER NOT NULL UNIQUE , " + FK_ICON + " INTEGER)";
String createIconTable = "CREATE TABLE " + TABLE_ICON + " (" + ID_ICON
+ " INTEGER PRIMARY KEY NOT NULL UNIQUE , " + CATEGORY
+ " INTEGER NOT NULL , " + ON + " TEXT, " + OFF + " TEXT, "
+ ON_COOLING + " TEXT, " + ON_HEATING + " TEXT, " + ICON_0
+ " TEXT, " + ICON_10 + " TEXT, " + ICON_20 + " TEXT, "
+ ICON_25 + " TEXT, " + ICON_30 + " TEXT, " + ICON_40
+ " TEXT, " + ICON_50 + " TEXT, " + ICON_60 + " TEXT, "
+ ICON_75 + " TEXT, " + ICON_80 + " TEXT, " + ICON_90
+ " TEXT, " + ICON_100 + " TEXT)";
db.execSQL(createDeviceTable);
db.execSQL(createIconTable);
ContentValues values = new ContentValues();
values.put(ID_ICON, 1);
values.put(CATEGORY, 3);
values.put(ON, "lightbulb_0_v2");
values.put(OFF, "lightbulb_100_v2");
db.insert(TABLE_ICON, null, values);
values.clear();
values.put(ID_ICON, 2);
values.put(CATEGORY, 3);
values.put(ON, "lightbulb_0_v1");
values.put(OFF, "lightbulb_max_v1");
db.insert(TABLE_ICON, null, values);
values.clear();
values.put(ID_ICON, 3);
values.put(CATEGORY, 3);
values.put(ON, "electric_outlet_on");
values.put(OFF, "electric_outlet_off");
db.insert(TABLE_ICON, null, values);
values.clear();
values.put(ID_DEVICE, 1);
values.put(UNIT_NUMBER, 35100191);
values.put(DEVICE_ID, 116);
values.put(FK_ICON, 2);
db.insert(TABLE_DEVICE, null, values);
values.clear();
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + TABLE_DEVICE);
db.execSQL("DROP TABLE IF EXISTS " + TABLE_ICON);
// Create tables again
onCreate(db);
}
// It is used to show all availabe icons of category.
public List<String> getAllIconsOfOldCategory(int oldCategory) {
List<String> iconsList = new ArrayList<String>();
String SqlSelectIconsQuery = "";
// if (oldCategory == 3)
SqlSelectIconsQuery = "SELECT " + ON + " FROM " + TABLE_ICON;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(SqlSelectIconsQuery, null);
if (cursor.moveToFirst()) {
do {
iconsList.add(cursor.getString(0));
Log.v("BAZA", cursor.getString(0));
} while (cursor.moveToNext());
}
cursor.close();
db.close();
return iconsList;
// return null;
}
public String getIcon(int deviceId, int unite_number, int status, int oldCategory) {
SQLiteDatabase db = this.getReadableDatabase();
// if (oldCategory == 3)
String SqlFindIconId = "SELECT " + FK_ICON + " FROM " + TABLE_DEVICE
+ " WHERE " + UNIT_NUMBER + " = " + unite_number + " AND "
+ DEVICE_ID + " = " + deviceId;
Cursor cursor = db.rawQuery(SqlFindIconId, null);
if(cursor !=null){
String icon;
if(status == 0){
icon = "SELECT "+ON+" FROM "+TABLE_ICON+ " WHERE "+ID_ICON +" = "+ cursor.getString(0);
}else{
icon = "SELECT "+OFF+" FROM "+TABLE_ICON+ " WHERE "+ID_ICON +" = "+ cursor.getString(0);
}
db.close();
return db.rawQuery(SqlFindIconId, null).getString(0);
}
db.close();
return null;
}
}
现在，您可以看到有两种方法。第一，getAllIconsOfOldCategory(int oldCategory)第二 getIcon(int deviceId, int unite_number, int status, int oldCategory)。在我使用第一个方法的应用程序中，一切正常......但是当我尝试使用第二种方法时，应用程序在getIcon()方法开始执行之前抛出 NullPointerException 并且应用程序被强制关闭。
所以，下面我将提供这个类，我在其中使用这个 Databashandler 类及其方法。在这里，我们称之为classA.java。因此，classA（这扩展为另一个类）用于渲染 listView 和 listView 的每一行。（[ImageView][Text][SwitchButton]-->这是 listView 中的一行）。这里我有onCreate()方法：我认为这个方法没有问题，因为它不会显示任何错误，并且当我按下 imageView 时它会起作用（您将在下面看到更多解释）。
private DatabaseHandler db;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
db = new DatabaseHandler(classA.this
.getParent());
}
因此，在这个类中，有一种renderDevice()方法用于渲染列表视图中的所有行，并返回视图。我的目标是，检查数据库中是否有图像，显示这个，如果没有，则显示预定义的一个。但是，它不起作用......直到我调用db.getIcon()方法的那一行都可以。然后它显示错误。而且该方法甚至没有开始执行。
ImageView device_status_image = (ImageView) lightView
.findViewById(R.id.device_status_image);
Settings settings = Settings.getInstance();
String icon = db.getIcon(light.id, settings.getLastPkAccessPoint(), status, light.oldCategory);
if ( icon != null) {
int resId = getResources().getIdentifier(icon, "drawable",
getPackageName());
device_status_image.setImageResource(resId);
} else
device_status_image.setImageResource(R.drawable.normal_icon);
}
但是，此外，我已经开始onClickListener了device_status_image。这里一切正常......所以，它打开了一个对话框。这意味着这ClassA.this.getParent()很好并且有效。你可以看到，我打电话给db.getAllIconsOfOldCategory() 我设置新适配器的地方。
device_status_image.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
final Dialog dialog = new Dialog(ClassA.this
.getParent());
dialog.setContentView(R.layout.list_view_icon);
listView = (ListView) dialog.findViewById(R.id.list_view);
IconAdapter adapter = new IconAdapter(inflater, light.id,
light.oldCategory, light.name, db
.getAllIconsOfOldCategory(light.oldCategory));
listView.setAdapter(adapter);
dialog.show();
Button bCancel = (Button) dialog.findViewById(R.id.bCancel);
bCancel.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
dialog.dismiss();
}
});
}
});
所以，我尝试添加String icon = db.getIcon(light.id, settings.getLastPkAccessPoint(), status, light.oldCategory);到 OnClickListener 中，只是为了检查方法是否在这里执行（我通过调试检查）并且它有效，方法getIcon()执行了......但是，当我把它放回它需要的地方时。惊喜，惊喜，它不再起作用了..
而且，这里是日志文件，如果它对你有帮助的话。谢谢！它很长。
09-16 01:35:59.167: E/AndroidRuntime(28019): FATAL EXCEPTION: main
09-16 01:35:59.167: E/AndroidRuntime(28019): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mysite.android/com.mysite.mobile.LightsActivity}: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mysite.android/com.mysite.mobile.classA}: java.lang.NullPointerException
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2194)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.startActivityNow(ActivityThread.java:1991)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:135)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:347)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:703)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabHost.setCurrentTab(TabHost.java:350)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:154)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:540)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.view.View.performClick(View.java:3549)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.view.View$PerformClick.run(View.java:14393)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.os.Handler.handleCallback(Handler.java:605)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.os.Handler.dispatchMessage(Handler.java:92)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.os.Looper.loop(Looper.java:154)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.main(ActivityThread.java:4945)
09-16 01:35:59.167: E/AndroidRuntime(28019): at java.lang.reflect.Method.invokeNative(Native Method)
09-16 01:35:59.167: E/AndroidRuntime(28019): at java.lang.reflect.Method.invoke(Method.java:511)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
09-16 01:35:59.167: E/AndroidRuntime(28019): at dalvik.system.NativeStart.main(Native Method)
09-16 01:35:59.167: E/AndroidRuntime(28019): Caused by: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mysite.android/com.mysite.mobile.classA}: java.lang.NullPointerException
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2194)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.startActivityNow(ActivityThread.java:1991)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:135)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:347)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:703)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabHost.setCurrentTab(TabHost.java:350)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.widget.TabHost.addTab(TabHost.java:240)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.mysite.mobile.LightsActivity.changeDeviceTabs(LightsActivity.java:34)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.mysite.mobile.LightsActivity.onCreate(LightsActivity.java:19)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.Activity.performCreate(Activity.java:4531)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1071)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
09-16 01:35:59.167: E/AndroidRuntime(28019): ... 18 more
09-16 01:35:59.167: E/AndroidRuntime(28019): Caused by: java.lang.NullPointerException
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.mysite.mobile.classA.renderDevice(classA.java:89)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.mysite.mobile.MyViewDeviceActivity.renderDevices(MyViewDeviceActivity.java:58)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.mysite.mobile.MyViewDeviceActivity.onCreate(MyViewDeviceActivity.java:30)
09-16 01:35:59.167: E/AndroidRuntime(28019): at com.mysite.mobile.classA.onCreate(classA.java:38)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.Activity.performCreate(Activity.java:4531)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1071)
09-16 01:35:59.167: E/AndroidRuntime(28019): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
09-16 01:35:59.167: E/AndroidRuntime(28019): ... 29 more