java从db类获取列表<string>，并存储在其他类中
我正在我的db类（SQLDatabaseHelper.java）中创建一系列问题，并且有一个方法可以创建一个ListArray供其他类使用：
public List<List<String>> getAllAnswersByQuestion2() {
List<String> array1 = new ArrayList<String>();
List<String> array2 = new ArrayList<String>();
//Clear arrays just in case they still existed and had values
array1.clear();
array2.clear();
SQLiteDatabase db = this.getReadableDatabase();
//Get 5 random values from the questions table
String selectQuery = "SELECT DISTINCT questionDescr, answerSetId FROM " + TABLE_QUESTION + " ORDER BY RANDOM() LIMIT 5";
Cursor c = db.rawQuery(selectQuery, null);
try {
//Ignore any null values in the table, go the first value
if (c!= null && c.moveToFirst())
{
do {
//Create two strings - one for all question descriptions and one for answersetIDs
String questdescr = c.getString(c.getColumnIndex(QDESCR));
String answerSetIds = c.getString(c.getColumnIndex(ASID));
//Put these strings into two separate arrays
array1.add(questdescr);
array2.add(answerSetIds);
//And when all values have been put into arrays......
} while (c.moveToNext());
}
//Create another array to bundle up the two arrays already created
List< List<String> > listArray = new ArrayList< List<String> >();
//Clear this array just in case of residual values
listArray.clear();
listArray.add(array1);
listArray.add(array2);
//Return this array to be picked up in the 'showRandomNextQuestion' method
return listArray;
} finally {
//Close cursor and database to prevent memory leaks
c.close();
db.close();
}
}
我有一个单独的类（QuizQuestionBank.java），它尝试获取返回的ListArray并将其拆分为两个单独的列表以供进一步使用：
import java.util.List;
import 安卓.app.Application;
public class QuizQuestionBank extends Application {
Object db;
@SuppressWarnings("unused")
private List<String> questionStrings;
public List<String> getQuestionStrings() {
SQLDatabaseHelper db = new SQLDatabaseHelper(QuizQuestionBank.this);
//get the data from the database
List<List<String>> listList = db.getAllAnswersByQuestion2();
//Get the question/text/answer Strings
List<String> questionStrings = listList.get(0); //question Strings
return questionStrings;
}
public List<String> setQuestionStrings(List<String> questionStrings) {
return this.questionStrings = questionStrings;
}
}
最后，我有一个MainActivity，它试图从QuizQuestionBank类中获取列表，并使用该列表获取其中的值，然后输出到textview：
private class showNextRandomQuestion1 extends AsyncTask<Void, Void, String> {
@Override
protected String doInBackground(Void... params) {
QuizQuestionBank qb = new QuizQuestionBank();
//question Strings
List<String> questionStrings = qb.getQuestionStrings();
//Generate random index
Random r = new Random();
int rand = Math.abs((r.nextInt() % questionStrings.size()));
String questionString = questionStrings.get(rand);
//remove and square brackets from array of strings
String regex = "\\[|\\]";
aQuestionString = questionString.replaceAll(regex, "");
return aQuestionString;
}
@Override
protected void onPostExecute(String result) {
//take returned question from above and display it
questionView.setText(aQuestionString);
}
}
但是，当调用我的“showNextRandomQuestion1（）”方法时，我当前遇到以下错误：
08-31 19:24:55.637: I/Choreographer(20638): Skipped 63 frames! The application may be doing too much work on its main thread.
08-31 19:24:55.887: I/ActivityManager(20638): Timeline: Activity_idle id: 安卓.os.BinderProxy@41ec2f08 time:939408562
08-31 19:25:01.887: W/dalvikvm(20638): threadid=13: thread exiting with uncaught exception (group=0x41a71d88)
08-31 19:25:02.037: E/AndroidRuntime(20638): FATAL EXCEPTION: AsyncTask #2
08-31 19:25:02.037: E/AndroidRuntime(20638): Process: com.example.quizapp, PID: 20638
08-31 19:25:02.037: E/AndroidRuntime(20638): java.lang.RuntimeException: An error occured while executing doInBackground()
08-31 19:25:02.037: E/AndroidRuntime(20638): at 安卓.os.AsyncTask$3.done(AsyncTask.java:300)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.util.concurrent.FutureTask.run(FutureTask.java:242)
08-31 19:25:02.037: E/AndroidRuntime(20638): at 安卓.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.lang.Thread.run(Thread.java:841)
08-31 19:25:02.037: E/AndroidRuntime(20638): Caused by: java.lang.NullPointerException
08-31 19:25:02.037: E/AndroidRuntime(20638): at 安卓.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:263)
08-31 19:25:02.037: E/AndroidRuntime(20638): at 安卓.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
08-31 19:25:02.037: E/AndroidRuntime(20638): at 安卓.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
08-31 19:25:02.037: E/AndroidRuntime(20638): at com.example.quizapp.SQLDatabaseHelper.getAllAnswersByQuestion2(SQLDatabaseHelper.java:445)
08-31 19:25:02.037: E/AndroidRuntime(20638): at com.example.quizapp.QuizQuestionBank.getQuestionStrings(QuizQuestionBank.java:20)
08-31 19:25:02.037: E/AndroidRuntime(20638): at com.example.quizapp.MainActivity$showNextRandomQuestion1.doInBackground(MainActivity.java:371)
08-31 19:25:02.037: E/AndroidRuntime(20638): at com.example.quizapp.MainActivity$showNextRandomQuestion1.doInBackground(MainActivity.java:1)
08-31 19:25:02.037: E/AndroidRuntime(20638): at 安卓.os.AsyncTask$2.call(AsyncTask.java:288)
08-31 19:25:02.037: E/AndroidRuntime(20638): at java.util.concurrent.FutureTask.run(FutureTask.java:237)
08-31 19:25:02.037: E/AndroidRuntime(20638): ... 4 more
08-31 19:25:02.227: W/dalvikvm(20638): threadid=14: thread exiting with uncaught exception (group=0x41a71d88)
08-31 19:25:02.227: I/Process(20638): Sending signal. PID: 20638 SIG: 9
当我的QuizQuestionBank类试图从db helper类检索listarray时，它似乎遇到了最初的问题。有人知道我需要做些什么来解决这个问题吗？非常感谢
# 1 楼答案
在你的
onPostExecute中试试这个：改变这个
到