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当我试图从SQLite数据库获取数据时，我总是收到一个空指针异常，我不知道如何解决这个问题。代码：当试图从SQL数据库检索数据时收到空指针异常android
活动：
public class Draft_Budget extends Activity {
String[] budget;
DBHelper budDb;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.draft_budget);
ActionBar actionBar = getActionBar();
actionBar.show();
populateListViewFromDB();
registerListClickCallback();
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
budget =
getResources().getStringArray(R.array.Budget_freq);
Spinner s1 = (Spinner) findViewById(R.id.spinner2);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_single_choice, budget);
s1.setAdapter(adapter);
s1.setOnItemSelectedListener(new OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2, long arg3) {
int index = arg0.getSelectedItemPosition();
Toast.makeText(getBaseContext(), "you have selected item: " + budget[index], Toast.LENGTH_SHORT).show();
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
}
});
}
@Override
protected void onDestroy(){
super.onDestroy();
closeDB();
}
private void openDB(){
budDb = new DBHelper(this);
budDb.open();
}
private void closeDB() {
budDb.close();
}
public void add_expense(View view){
Intent intent = new Intent(this, Setup_expenses.class);
intent.putExtra("expense_Id",0);
startActivity(intent);
populateListViewFromDB();
}
public void clearexp(View v){
budDb.deleteAll();
populateListViewFromDB();
}
private void populateListViewFromDB(){
Cursor cursor = budDb.getAllRows();
startManagingCursor(cursor);
String[] fromFeildNames = new String[]
{DBHelper.KEY_label, DBHelper.KEY_cost, DBHelper.KEY_cost};
int[] toViewIDs = new int[]
{R.id.expense_name, R.id.expense_Id, R.id.expense_cost};
SimpleCursorAdapter exCursorAdapter = new SimpleCursorAdapter(
this,
R.layout.view_expense_entry,
cursor,
fromFeildNames,
toViewIDs);
ListView expenselist = (ListView) findViewById(R.id.list_expense);
expenselist.setAdapter(exCursorAdapter);
}
private void registerListClickCallback(){
ListView budlist = (ListView) findViewById(R.id.list_expense);
budlist.setOnItemClickListener(new AdapterView.OnItemClickListener(){
@Override
public void onItemClick(AdapterView<?> parent, View viewClicked, int position, long idInDB){
updateItemForId(idInDB);
displayToastForId(idInDB);
}
});
}
private void updateItemForId(long idInDB){
Cursor cursor = budDb.getRow(idInDB);
if (cursor.moveToFirst()){
long idDB = cursor.getLong(DBHelper.COL_ROWID);
String name = cursor.getString(DBHelper.COL_label);
int cost = cursor.getInt(DBHelper.COL_cost);
budDb.updateRow(idDB, name, cost);
}
cursor.close();
populateListViewFromDB();
}
private void displayToastForId(long idInDB){
Cursor cursor = budDb.getRow(idInDB);
if (cursor.moveToFirst()){
long idDB = cursor.getLong(DBHelper.COL_ROWID);
String label = cursor.getString(DBHelper.COL_label);
int cost = cursor.getInt(DBHelper.COL_cost);
String message = "ID: " + idDB + "\n" + "Category: " + label + "\n" + "Cost: " + cost;
Toast.makeText(Draft_Budget.this, message, Toast.LENGTH_LONG).show();
}
cursor.close();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
super.onCreateOptionsMenu(menu);
CreateMenu(menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item){
return MenuChoice(item);
}
private void CreateMenu(Menu menu)
{
MenuItem mnu1 = menu.add(0, 0, 0, "Item 1");
{
mnu1.setIcon(R.drawable.ic_action_overflow);
mnu1.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);
}
}
private boolean MenuChoice(MenuItem item)
{
switch (item.getItemId()) {
case 0:
Toast.makeText(this, "You clicked on Item 1", Toast.LENGTH_LONG).show();
return true;
}
return false;
}
public void done(View view) {
Intent done = new Intent(this, Home_page.class);
done.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(done);
}
数据库提供商：
public class DBHelper {
private static final String TAG = "DBHelper";
public static final String KEY_ROWID = "_id";
public static final int COL_ROWID = 0;
public static final String KEY_label = "Label";
public static final String KEY_cost = "Cost";
public static final int COL_label = 1;
public static final int COL_cost = 2;
public static final String[] ALL_KEYS = new String[] {KEY_ROWID, KEY_label, KEY_cost};
public static final int DATABASE_VERSION = 5;
public static final String DATABASE_NAME = "Expense.db";
public static final String DATABASE_TABLE = "expenseTable";
public static final String DATABASE_CREATE_SQL = "create table " + DATABASE_TABLE + " (" + KEY_ROWID + " integer primary key autoincrement, "
+ KEY_label + " text not null, " + KEY_cost + " integer not null" + ");";
final Context context;
DatabaseHelper myDBHelper;
SQLiteDatabase db;
public DBHelper(Context ctx){
this.context = ctx;
myDBHelper = 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_SQL);
} catch (SQLException e){
e.printStackTrace();
}
}
@Override
public void onUpgrade(SQLiteDatabase _db, int oldversion, int newversion) {
Log.w(TAG, "Upgarding application's database from version " + oldversion + " to " + newversion + " , which will destroy all old data!");
//Destroy
_db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
//Recreate database
onCreate(_db);
}
}
public DBHelper open(){
db = myDBHelper.getWritableDatabase();
return this;
}
public void close(){
myDBHelper.close();
}
//Add set of values to database
public long insertRow(int Cost, String Label) {
db = myDBHelper.getWritableDatabase();
ContentValues intialvalues = new ContentValues();
intialvalues.put(KEY_label, Label);
intialvalues.put(KEY_cost, Cost);
return db.insert(DATABASE_TABLE, null, intialvalues);
}
//Delete
public boolean deleteRow(long rowId){
return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}
public void deleteAll(){
Cursor c = getAllRows();
long rowId = c.getColumnIndexOrThrow(KEY_ROWID);
if (c.moveToFirst()) {
do {
deleteRow(c.getLong((int) rowId));
} while (c.moveToNext());
}
c.close();
}
//Return data
public Cursor getAllRows() throws SQLException{
db = myDBHelper.getReadableDatabase();
return db.query(DATABASE_TABLE, ALL_KEYS , null, null, null, null, null);
}
//Get specific row
public Cursor getRow(long rowId) throws SQLException{
Cursor c = db.query(true, DATABASE_TABLE, new String[] {KEY_ROWID, KEY_label, KEY_cost}, KEY_ROWID + "=" + rowId, null, null, null, null, null);
if (c != null){
c.moveToFirst();
}
return c;
}
//Change row to equal new data
public boolean updateRow(long rowId, String Label, int Cost){
db = myDBHelper.getWritableDatabase();
ContentValues newvalues = new ContentValues();
newvalues.put(KEY_label, Label);
newvalues.put(KEY_cost, Cost);
return db.update(DATABASE_TABLE, newvalues, KEY_ROWID + "=" + rowId, null) > 0;
}
该活动的布局文件：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="@string/Expense"
android:id="@+id/textView"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:background="#939393"
android:textColor="#ffffff" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="+Add Expense"
android:id="@+id/button2"
android:onClick="add_expense"
android:layout_below="@+id/textView"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:nestedScrollingEnabled="false" />
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="@string/Income"
android:id="@+id/textView2"
android:textColor="#ffffff"
android:background="#939393"
android:layout_centerVertical="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="+Add Income"
android:textSize="6pt"
android:id="@+id/button3"
android:onClick="Set_income"
android:layout_below="@+id/textView2"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<Spinner
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/spinner2"
android:drawSelectorOnTop="true"
android:layout_above="@+id/button"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="Done"
android:id="@+id/button"
android:onClick="done"
android:layout_alignParentBottom="true" />
<ListView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/list_expense"
android:layout_below="@+id/button2"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_above="@+id/textView2" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Clear expenses:"
android:id="@+id/clearexp"
android:onClick="clearexp"
android:layout_below="@+id/textView"
android:layout_toRightOf="@+id/button2"
android:layout_toEndOf="@+id/button2"
android:layout_marginLeft="92dp"
android:layout_marginStart="92dp" />
</RelativeLayout>
，并为ListView：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/expense_Id"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:visibility="gone"/>
<TextView
android:id="@+id/expense_name"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:paddingLeft="6dip"
android:paddingTop="6dip"
android:textSize="14pt"/>
<TextView
android:id="@+id/expense_cost"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:paddingRight="6dip"
android:paddingBottom="6dip"
android:textSize="14pt"/>
</LinearLayout>
任何想法如何解决这个问题？
*编辑日志：
06-10 14:56:51.098 2110-2110/com.example.thomaseleutheriades.personalwallet E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.thomaseleutheriades.personalwallet, PID: 2110
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.thomaseleutheriades.personalwallet/com.example.thomaseleutheriades.personalwallet.Draft_Budget}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2429)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1342)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5341)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:825)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:641)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.thomaseleutheriades.personalwallet.Draft_Budget.populateListViewFromDB(Draft_Budget.java:89)
at com.example.thomaseleutheriades.personalwallet.Draft_Budget.onCreate(Draft_Budget.java:37)
at android.app.Activity.performCreate(Activity.java:5343)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1088)
at
发布您的logcat。 –
change int index = arg0.getSelectedItemPosition（）;索引= arg2。顺便说一句，需要Logcat来调查空对象。 – NamNH
在调用'populateListViewFromDB（）'之前，你不打开数据库，所以我猜'budDB'是'null'。调用'openDB（）'前'populateListViewFromDB（）'看看是否能解决问题 – Titus