我正在创建一个优惠券应用，优惠券存储在数据库中，并与本地广告进行比较，让用户知道何时使用优惠券。我创建了一个数据库，其中包含所有优惠券以及用户个人库的优惠券。我很难让图书馆显示在GridView中。 gridview适配器，数据库助手和活动代码的代码都在下面以及我收到的错误。请帮忙。
LibraryListener.java
public class LibraryListener extends Activity{
GridView grid;
private static final String TABLE_NAME = "Library";
//set table column names as constants
private static final String LIBRARY_ID = "_id";
private static final String UPC = "UPC";
private static final String DESCRIPTION = "Description";
private static final String DISCOUNT = "Discount";
private static final String MIN_REQ = "MinReq";
private static final String DOUBLE = "Double";
private static final String IMAGE = "Image";
private static final String EXPIRATION = "Expiration";
private static final String QTY = "Qty";
private static final String[] TABLE_COLUMNS = new String[]{UPC,DESCRIPTION,DISCOUNT,MIN_REQ,DOUBLE,IMAGE,EXPIRATION,QTY};
private SQLiteDatabase database;
private ArrayList<String> libraryUPC;
private ArrayList<String> libraryImages;
private ArrayList<String> libraryDescription;
private ArrayList<Double> libraryDiscount;
private ArrayList<Integer> libraryMinReq;
private ArrayList<String> libraryDouble;
private ArrayList<String> expirationDate;
private ArrayList<Integer> libraryQty;
private ArrayList<String> imageFilePath;
DatabaseHelper dbOpenHelper = new DatabaseHelper(this);
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.gridview);
GridView gridView = (GridView)findViewById(R.id.gridview);
try {
dbOpenHelper.createDataBase();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
dbOpenHelper.openDatabase();
fillLibrary();
dbOpenHelper.close();
gridView.setAdapter(new GridViewAdapter(this, libraryDescription,imageFilePath));
//setUpList();
}
// Fill the library with items from database
private void fillLibrary() {
libraryImages = new ArrayList<String>();
Cursor cursor = database.query(TABLE_NAME, TABLE_COLUMNS, null, null, null, null, LIBRARY_ID);
cursor.moveToFirst();
if(!cursor.isAfterLast()) {
do {
//UPC,DESCRIPTION,DISCOUNT,MIN_REQ,DOUBLE,IMAGE,EXPIRATION,QTY
String upc = cursor.getString(0);
libraryUPC.add(upc);
String description = cursor.getString(1);
libraryDescription.add(description);
double discount = cursor.getDouble(2);
libraryDiscount.add(discount);
int min = cursor.getInt(3);
libraryMinReq.add(min);
String dbl = cursor.getString(4);
libraryDouble.add(dbl);
String image = cursor.getString(5);
libraryImages.add(image);
String expiration = cursor.getString(6);
expirationDate.add(expiration);
int qty = cursor.getInt(7);
libraryQty.add(qty);
} while (cursor.moveToNext());
}
cursor.close();
}
}
DatabaseHelper.java
public class DatabaseHelper extends SQLiteOpenHelper{
private static String DB_PATH = "/data/data/com.example.extremesavings/databases/";
private static final String DATABASE_NAME="ExtremeSavings";
private static final int DATABASE_VERSION=1;
private final Context myContext;
private SQLiteDatabase db;
String description;
double discount;
String minimum;
String dbl;
String imagefilePath;
public DatabaseHelper(Context c){
super(c, DATABASE_NAME, null, 1);
this.myContext = c;
}
private void showInfoDialog(Context context, String title, String information) {
new AlertDialog.Builder (context)
.setMessage(information)
.setTitle(title)
.setPositiveButton("OK", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
}
}).show();
}
public void createDataBase() throws IOException{
boolean dbExist = checkDataBase();
if(dbExist){
showInfoDialog(myContext, "Alert", "Database already exists");
}else{
//create empty database into the default system path
this.getWritableDatabase();
try {
//overwrite empty database with internal database
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
}
/*
* Check if the database already exist to avoid re-copying the file
* each time you open the application.
*/
public boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DATABASE_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
return checkDB != null ? true : false;
}
/**
* Copies your database from your local assets-folder to the just created empty database in the
* system folder, from where it can be accessed and handled.
* This is done by transfering bytestream.
* */
private void copyDataBase() throws IOException{
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DATABASE_NAME);
// Path to the just created empty db
String outFileName = DB_PATH + DATABASE_NAME;
//Open the empty db as the output stream
OutputStream myOutput = new FileOutputStream(outFileName);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
}
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
public void openDatabase(){
//Open the database
String myPath = DB_PATH + DATABASE_NAME;
db = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
}
@Override
public synchronized void close() {
if(db != null)
db.close();
super.close();
}
//search table for UPC code
public boolean findUpc(String tableName, String upc)
{
String Query = "Select * from " + tableName + " where UPC = " + upc;
Cursor cursor = db.rawQuery(Query, null);
if(cursor.getCount() <= 0){
cursor.close();
return false;
}
cursor.close();
//test to see if coupon is found in database
Toast toast = Toast.makeText(myContext, "Coupon found in Database", Toast.LENGTH_LONG);
toast.show();
return true;
}
//Add coupon from database to library
public void addToLibrary(String upc, String expirationDate, int qty)
{
openDatabase();
//search library to see if coupon already exists in table
boolean inLibrary = findUpc("Library",upc);
if (!inLibrary){
String selectQuery = "Select * from CouponDatabase where UPC = " + upc;
Cursor cursor = db.rawQuery(selectQuery, null);
cursor.moveToFirst();
if(!cursor.isAfterLast()) {
do {
//DESCRIPTION,DISCOUNT,MIN_REQ,DOUBLE,IMAGE,QTY
description = cursor.getString(2);
discount = cursor.getDouble(3);
minimum = cursor.getString(4);
dbl = cursor.getString(5);
imagefilePath = "R.drawable." + cursor.getString(6);
} while (cursor.moveToNext());
}
cursor.close();
//create string to insert data into library table
String insertQuery = "INSERT into Library (UPC, Description, Discount, MinReq, Double, Image, Expiration, Qty)"
+ "(" + upc + ", " + description + ", " + discount + ", " + minimum + ", " + dbl + ", " + imagefilePath + ", " + expirationDate + ", " + qty + ")";
//insert data into Library table
db.execSQL(insertQuery);
Toast toast = Toast.makeText(myContext, "Item added to Library", Toast.LENGTH_LONG);
toast.show();
}
}
@Override
public void onCreate(SQLiteDatabase arg0) {
// TODO Auto-generated method stub
}
@Override
public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) {
// TODO Auto-generated method stub
}
}
07-15 23：17：34.485：E / AndroidRuntime（13806）：致命异常：主要 07-15 23：17：34.485：E / AndroidRuntime（13806）：进程：com.example.extremesavings，PID：13806 07-15 23：17：34.485：E / AndroidRuntime（13806）：java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.extremesavings / com.example.extremesavings.LibraryListener}：java.lang.NullPointerException：Attempt调用虚方法'android.database.Cursor android.database.sqlite.SQLiteDatabase.query（java.lang.String，java.lang.String []，java.lang.String，java.lang.String []，java。空对象引用上的lang.String，java.lang.String，java.lang.String）' 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2306） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2366） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.ActivityThread.access $ 800（ActivityThread.java:149） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1284） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.os.Handler.dispatchMessage（Handler.java:102） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.os.Looper.loop（Looper.java:135） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.ActivityThread.main（ActivityThread.java:5297） 07-15 23：17：34.485：E / AndroidRuntime（13806）：at java.lang.reflect.Method.invoke（Native Method） 07-15 23：17：34.485：E / AndroidRuntime（13806）：at java.lang.reflect.Method.invoke（Method.java:372） 07-15 23：17：34.485：E / AndroidRuntime（13806）：at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:908） 07-15 23：17：34.485：E / AndroidRuntime（13806）：at com.android.internal.os.ZygoteInit.main（ZygoteInit.java:703） 07-15 23：17：34.485：E / AndroidRuntime（13806）：引起：java.lang.NullPointerException：尝试调用虚方法'android.database.Cursor android.database.sqlite.SQLiteDatabase.query（java.lang。在null对象引用上的String，java.lang.String []，java.lang.String，java.lang.String []，java.lang.String，java.lang.String，java.lang.String）' 07-15 23：17：34.485：E / AndroidRuntime（13806）：at com.example.extremesavings.LibraryListener.fillLibrary（LibraryListener.java:67） 07-15 23：17：34.485：E / AndroidRuntime（13806）：at com.example.extremesavings.LibraryListener.onCreate（LibraryListener.java:57） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.Activity.performCreate（Activity.java:6020） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1105） 07-15 23：17：34.485：E / AndroidRuntime（13806）：在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2259） 07-15 23：17：34.485：E / AndroidRuntime（13806）：... 10 more
答案 0 :(得分：0)
我认为在创建活动之前实例化dbOpenHelper时会发生错误。你能尝试在onCreate方法中加入 dbOpenHelper = new DatabaseHelper（this）; 吗？