【发布时间】：2022-01-25 09:10:23
【问题描述】：
我正在 Android Studio 中创建一个测验应用程序，因此我创建了一个包含问题的数据库。在帮助程序类中，我有一个方法 getAllQuestions 将问题从数据库传输到数组列表，因此我可以提取 Main 中的 Questions 和 Buttons 中的 setText。但是，当我运行我的应用程序时，按钮和问题是空的，如果列表本身为空，应用程序不会抛出 nullpointer 之类的异常。
DB_Helper 方法：
private void addQuestions(QuestionsDataBase Questions) {
ContentValues cv = new ContentValues();
cv.put(QuestionTable.Column_Question,Questions.getQuestion());
cv.put(QuestionTable.Column_Option1,Questions.getOption1());
cv.put(QuestionTable.Column_Option2,Questions.getOption2());
cv.put(QuestionTable.Column_Option3,Questions.getOption3());
cv.put(QuestionTable.Column_Option4,Questions.getOption4());
cv.put(QuestionTable.Column_Correct_Ans,Questions.getQuestion());
db.insert(QuestionTable.Table_Name,null,cv);
}
公共 ArrayList getAllQuestions(){
ArrayList<QuestionsDataBase> questionsList = new ArrayList<>();
db = getReadableDatabase();
String[] Projection ={
QuestionTable._ID,
QuestionTable.Column_Question,
QuestionTable.Column_Option1,
QuestionTable.Column_Option2,
QuestionTable.Column_Option3,
QuestionTable.Column_Option4,
QuestionTable.Column_Correct_Ans
};
Cursor c = db.query(QuestionTable.Table_Name,
Projection,
null,
null,
null,
null,
null);
if(c.moveToFirst()){
do{
QuestionsDataBase questions = new QuestionsDataBase();
questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Question)));
questions.setOption1(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option1)));
questions.setOption2(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option2)));
questions.setOption3(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option3)));
questions.setOption4(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option4)));
questions.setCorrectAns(c.getInt(c.getColumnIndexOrThrow(QuestionTable.Column_Correct_Ans)));
questionsList.add(questions);
questionsList.add(questions);
}while(c.moveToNext());
}
c.close();
return questionsList;
}
MainActivity 中的方法：
private void fecthDB(){
DB_Helper db = new DB_Helper(this);
questionList = db.getAllQuestions();
startQuiz();
}
private void startQuiz() {
questionTotalCount = questionList.size(); // the total amount of questions in the current quizactivity( is set to 10)
Collections.shuffle(questionList); // shuffle the questions form the database that are stored in QuestionList
showQuestions();
nextbutton.setOnClickListener(view -> {
if(!answered){
if(option1.isChecked() || option2.isChecked() || option3.isChecked() || option4.isChecked())
QuizOperations();
}
});
}
private void showQuestions() // 显示数据库中的问题和选项 {
rbGroup.clearCheck();
if(questionCounter< questionTotalCount) // if not all the questions yet answered set text to new questions
{
currentQuestions = questionList.get(questionCounter);
questionCount.setText(currentQuestions.getQuestion());
option1.setText(currentQuestions.getOption1());
option2.setText(currentQuestions.getOption2());
option3.setText(currentQuestions.getOption3());
option4.setText(currentQuestions.getOption4());
questionCounter ++; // +1 to question counter
answered = false; // tye quiz is not yet completely answered while the current question is smaller then total
nextbutton.setText("Next");
questionCount.setText("Questions: " + questionCounter + "/" + questionTotalCount); // how many questions answered out of 10
}
else{// if all the questions are answered
handler.postDelayed(() -> {
Intent intent = new Intent(getApplicationContext(),QuizActivity.class); // open the next activity
}, 500); // wait for a half second
}
}
【问题讨论】：