我在尝试从SQLite数据库中提取数据时遇到了问题，我正在尝试创建一个充当问题库的Android应用程序。当我尝试从这个数据库中检索信息时，我得到一个空的问题列表，应用程序运行得很好而没有错误，但问题是为什么我的questionList为空。有人可以帮我弄清楚发生了什么。
扩展SQLiteOpenHelper的QuestionDBHelper代码
package com.cyanidesystems.unitquizapp.data;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
import com.cyanidesystems.unitquizapp.models.Question;
import java.util.ArrayList;
import java.util.List;
import static com.cyanidesystems.unitquizapp.data.QuizQuestionsContract.QuizquestionColumns.*;
public class QuestionDBHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION= 1;
private static final String DATABASE_NAME= "questionBankDB.db";
public static SQLiteDatabase questionDB;
public QuestionDBHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
questionDB=db;
//Creating a string that has the sql statements to create a table with all its columns and properties
final String SQL_CREATE_QUESTION_BANK_TABLE_COMMAND= "CREATE TABLE " + TABLE_NAME + "(" +
COLUMN_SUBJECT + " TEXT NOT NULL, " + COLUMN_YEAR + " TEXT NOT NULL, " + COLUMN_PAPER_NUMBER + " TEXT NOT NULL, " +
COLUMN_QUESTION_ID + " INTEGER PRIMARY KEY, " + COLUMN_QUESTION_STRING + " TEXT NOT NULL, " +
COLUMN_QUEST_OPTION_A + "TEXT NOT NULL, " + COLUMN_QUEST_OPTION_B + " TEXT NOT NULL, " + COLUMN_QUEST_OPTION_C +
" TEXT NOT NULL, " + COLUMN_QUEST_OPTION_D + " TEXT NOT NULL, " + COLUMN_ANSWER + " TEXT NOT NULL )";
db.execSQL(SQL_CREATE_QUESTION_BANK_TABLE_COMMAND);
initQuestions();
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL(" DROP TABLE IF EXISTS "+TABLE_NAME);
onCreate(db);
}
public void initQuestions(){
//initializing the Database with some sample questions
Question q1=new Question("Computer Science","2017","I","00","" +
"What is the meaning of URL","Uniform resource link","Uniform resource locator","uniform risk link",
"universal resource locator","Uniform resource locator");
this.putThisValuesInColumn((q1));
Question q2=new Question("Computer Science","2017","I","01","" +
"Android was acquired by Google in","2000","2003","2005", "2007","2005");
this.putThisValuesInColumn((q2));
Question q3=new Question("Computer Science","2017","I","02","" +
"CPU stands for","central process unit","central powering unit","central processing unit","control power unit","central processing unit");
this.putThisValuesInColumn((q3));
Question q4=new Question("computer science","2017","I","03","What is the meaning of BIOS","Basic input output system","Basic instruction output system","Basic in/out system","bits in/out System","Basic input output system");
this.putThisValuesInColumn((q4));
}
public void putThisValuesInColumn(Question quest){
//this method actually loads the data to be inserted for a particular column
//questionDB=this.getWritableDatabase();
ContentValues values=new ContentValues();
values.put(COLUMN_SUBJECT,quest.getSubject());
values.put(COLUMN_YEAR,quest.getYear());
values.put(COLUMN_PAPER_NUMBER,quest.getPaper_number());
values.put(COLUMN_QUESTION_ID,quest.getQuestID());
values.put(COLUMN_QUESTION_STRING,quest.getQuest_String());
values.put(COLUMN_QUEST_OPTION_A,quest.getOptionA());
values.put(COLUMN_QUEST_OPTION_B,quest.getOptionB());
values.put(COLUMN_QUEST_OPTION_C,quest.getOptionC());
values.put(COLUMN_QUEST_OPTION_D,quest.getOptionD());
values.put(COLUMN_ANSWER,quest.getAnswer());
// loads all of the initialized data into the database table
questionDB.insert(TABLE_NAME,null,values);
}
public ArrayList<Question> getAllQuestions(){
//this method extracts all the questions in the database
//and returns it as a list of question objects
ArrayList<Question> questionList=new ArrayList<>();
//string of selecting quest from DB
String sqlSelectString="SELECT * FROM " + TABLE_NAME;
questionDB=this.getWritableDatabase();
Cursor cursor=questionDB.rawQuery(sqlSelectString,null);
if (cursor.moveToFirst()){
do {
Question question=new Question();
question.setSubject(cursor.getString(0));
question.setYear(cursor.getString(1));
question.setPaper_number(cursor.getString(2));
question.setQuestID(cursor.getString(3));
question.setQuest_String(cursor.getString(4));
question.setOptionA(cursor.getString(5));
question.setOptionB(cursor.getString(6));
question.setOptionC(cursor.getString(7));
question.setOptionD(cursor.getString(8));
question.setOptionD(cursor.getString(9));
//adding the question into the question list
questionList.add(question);
}while (cursor.moveToNext());
}
return questionList;
}
问题类的外观如何
public class Question {
String subject;
String year;
String paper_number;
String questID;
String quest_String;
String optionA;
String optionB;
String optionC;
String optionD;
String answer;
//class constructor With no Parameters
public Question( ) {
this.subject ="";
this.year = "";
this.paper_number = "";
this.questID = "";
this.quest_String = "";
this.optionA = "";
this.optionB = "";
this.optionC = "";
this.optionD = "";
this.answer = "";
}
//Class Constructor
public Question(String subject, String year, String paper_number, String questID, String quest_String,
String optionA, String optionB, String optionC, String optionD, String answer) {
this.subject = subject;
this.year = year;
this.paper_number = paper_number;
this.questID = questID;
this.quest_String = quest_String;
this.optionA = optionA;
this.optionB = optionB;
this.optionC = optionC;
this.optionD = optionD;
this.answer = answer;
}
//Getters and Setters
public String getSubject() {
return subject;
}
public void setSubject(String subject) {
this.subject = subject;
}
public String getYear() {
return year;
}
public void setYear(String year) {
this.year = year;
}
public String getPaper_number() {
return paper_number;
}
public void setPaper_number(String paper_number) {
this.paper_number = paper_number;
}
public String getQuestID() {
return questID;
}
public void setQuestID(String questID) {
this.questID = questID;
}
public String getQuest_String() {
return quest_String;
}
public void setQuest_String(String quest_String) {
this.quest_String = quest_String;
}
public String getOptionA() {
return optionA;
}
public void setOptionA(String optionA) {
this.optionA = optionA;
}
public String getOptionB() {
return optionB;
}
public void setOptionB(String optionB) {
this.optionB = optionB;
}
public String getOptionC() {
return optionC;
}
public void setOptionC(String optionC) {
this.optionC = optionC;
}
public String getOptionD() {
return optionD;
}
public void setOptionD(String optionD) {
this.optionD = optionD;
}
public String getAnswer() {
return answer;
}
public void setAnswer(String answer) {
this.answer = answer;
}
}
MainActivity.java文件
public class MainActivity extends AppCompatActivity {
//Declaration of Variables
int currentPosition = 0;//this denotes the current position of the question in our List
int numberOfCorrectAnswers = 0;
QuestionDBHelper questionDB=new QuestionDBHelper(this);
ArrayList<Question> quizQuestions = new ArrayList<> ();
//@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ButterKnife.bind(this);
progressBar.setProgress(0);
//quizQuestions = setUpQuestions(new ArrayList<Question>());
//it creates a new instance of an arraylist and passes it to the function
//the function then loads it with data and returns a new loaded list
//which is then assigned to quizQuestions
//setData();
setData();
}
public void setData() {
subjTitle.setText(getString(R.string.Subject,"Computer Science"));
subjYr.setText(getString(R.string.Year,"2017"));
quizQuestions =questionDB.getAllQuestions();
Log.i("size of quizQuestions",quizQuestions.toString());
}
使用信息过滤器记录，显示quizQuestions Arraylist为空
04-23 00:39:39.596 2536-2536/com.cyanidesystems.unitquizapp W/dalvikvm: VFY: unable to resolve virtual method 323: Landroid/content/Context;.getColorStateList (I)Landroid/content/res/ColorStateList;
04-23 00:39:39.772 2536-2536/com.cyanidesystems.unitquizapp I/size of quizQuestions: []
04-23 00:39:39.836 2536-2536/com.cyanidesystems.unitquizapp W/dalvikvm: VFY: unable to find class referenced in signature (Landroid/graphics/drawable/Icon;)
答案 0 :(得分：1)
假设TABLE_NAME正确，请尝试更改代码
if (cursor.moveToFirst()){
do {
// create newObject
questionList.add(question);
} while (cursor.moveToNext());
}
只是
if (cursor.moveToFirst()){
do {
// create newObject
questionList.add(question);
} while (cursor.moveToNext());
}
答案 1 :(得分：0)