java - SQLite 数据库中第 71 行的 NullPointerException

Question

我正在开发一个应用程序，用于从数据库存储和检索数据。通过随机选择id(“id”是表中的主键约束列)来检索数据，存储在光标对象中并与单选按钮一起显示。然后，用户选择这些单选按钮之一，并相应地显示结果。

我已将其实现为(代码:DBHelperDisplay.java):

package course.examples.jumboquest;

import java.util.ArrayList;
import java.util.Random;
//import java.io.*;
//import java.lang.*;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
//import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
//import android.content.Intent;
import android.os.CountDownTimer;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.DatabaseUtils;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteDatabase;

public class DBHelperDisplay extends ActionBarActivity {

TextView tv; 
DBHelper myDB;
RadioGroup radioChoices;
RadioButton rbtChoice; 
Button btSubmit;
String choice1;
String choice2;
String choice3;
String choice4;
String strAns;
CustomTimer cdt;
TextView quest;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.dbhelper_display);
    
    cdt = new CustomTimer(20000, 1000);
    cdt.start();
    
    myDB = new DBHelper(this);
    
    myDB.insertQuestion(1, "who is the team member whose name starts with s?", "Vinita", "Akanksha", "Swati", "Megha", "Swati");
    myDB.insertQuestion(2, "who is the team member whose name starts with m?", "Vinita", "Akanksha", "Swati", "Megha", "Megha");
    myDB.insertQuestion(3, "who is the team member whose name starts with a?", "Vinita", "Akanksha", "Swati", "Megha", "Akanksha");
    myDB.insertQuestion(4, "who is the team member whose name starts with v?", "Vinita", "Akanksha", "Swati", "Megha", "Vinita");
    myDB.insertQuestion(5, "who is the team member whose name ends with i?", "Vinita", "Akanksha", "Swati", "Megha", "Swati");
    
    Cursor rs = myDB.getData();
    rs.moveToFirst();
    String Question = rs.getString(rs.getColumnIndex(DBHelper.Col_Ques));
    choice1 = rs.getString(rs.getColumnIndex(DBHelper.Col_Choice1));
    choice2 = rs.getString(rs.getColumnIndex(DBHelper.Col_Choice2));
    choice3 = rs.getString(rs.getColumnIndex(DBHelper.Col_Choice3));
    choice4 = rs.getString(rs.getColumnIndex(DBHelper.Col_Choice4));
    strAns = rs.getString(rs.getColumnIndex(DBHelper.Col_Ans));
    
    
    tv = (TextView) findViewById(R.id.timertxt);
    
  quest = (TextView) findViewById(R.id.quest);
  quest.setText(Question);
    
    //final TextView ans = (TextView) findViewById(R.id.ans);
    
    Button btClear = (Button)findViewById(R.id.btClear);
    btClear.setText("CLEAR");     // Line 71
   
   addListenerRadioChoices() ;
    
   btClear.setOnClickListener(new View.OnClickListener(){
     public void onClick(View v) {
        //ans.setText("");
            
       }
  });
 }
   public void addListenerRadioChoices(){ 
    
    radioChoices = (RadioGroup) findViewById(R.id.radioChoices);
        
    ((RadioButton) radioChoices.getChildAt(0)).setText(choice1); 
    ((RadioButton) radioChoices.getChildAt(1)).setText(choice2);
    ((RadioButton) radioChoices.getChildAt(2)).setText(choice3);
    ((RadioButton) radioChoices.getChildAt(3)).setText(choice4);
        
    btSubmit = (Button)findViewById(R.id.btSubmit); 
    btSubmit.setText("SUBMIT");
    btSubmit.setOnClickListener(new View.OnClickListener(){
     public void onClick(View v) {
        
        int selected = radioChoices.getCheckedRadioButtonId();
        rbtChoice = (RadioButton) findViewById(selected); 
        String Ans = rbtChoice.getText().toString();
         if(Ans.equalsIgnoreCase(strAns)){
            
            cdt.cancel();
            //ans.setText(" ANSWER");
            
         }
   
        
    }
   });
  } 

public class CustomTimer extends CountDownTimer{
    //TextView ed;
    public CustomTimer(long millisInFuture, long countDownInterval){
        super(millisInFuture, countDownInterval);
    }
    @Override
    public void onTick(long millisUntilFinished){
        //current = millisUntilFinished/1000;
        tv.setText("Time Left:" + millisUntilFinished/1000);
    }
    @Override
    public void onFinish() {
        tv.setText("Time Up - lost the game!");
    }
}

public class DBHelper extends SQLiteOpenHelper {
    
    public static final String Database_Name = "Questions.db"; 
    public static final String Table_Name = "Comics"; 
    public static final String Col_ID = "id"; 
    public static final String Col_Ques = "question"; 
    public static final String Col_Choice1 = "choice1"; 
    public static final String Col_Choice2 = "choice2"; 
    public static final String Col_Choice3 = "choice3"; 
    public static final String Col_Choice4 = "choice4"; 
    public static final String Col_Ans = "answer"; 
    
    public DBHelper(Context context){
        super(context, Database_Name, null, 4);
    }
    @Override 
    public void onCreate(SQLiteDatabase db){
        // TODO Auto-generated method stub  
       String   CREATE_TABLE = "CREATE TABLE " + Table_Name + "( " + Col_ID + " INTEGER PRIMARY KEY, " + Col_Ques + " TEXT, " + Col_Choice1 + " TEXT, "
                                + Col_Choice2 + " TEXT, " + Col_Choice3 + " TEXT, " + Col_Choice4 + " TEXT, " + Col_Ans + " TEXT );" ;
       db.execSQL(CREATE_TABLE);
       
    }   
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
          // TODO Auto-generated method stub
          db.execSQL("DROP TABLE IF EXISTS Comics");
          onCreate(db);
    }
    
    public boolean insertQuestion(int id, String question, String choice1, String choice2, String choice3, String choice4, String answer){
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
                    
        contentValues.put("id",id);
        contentValues.put("question",question);
        contentValues.put("choice1",choice1);
        contentValues.put("choice2",choice2);
        contentValues.put("choice3",choice3);
        contentValues.put("choice4",choice4);
        contentValues.put("answer",answer);
        db.insert("Comics", null, contentValues);
        db.close();
        return true;
    }
    public Cursor getData(){
    //public void getData(){
        SQLiteDatabase db = this.getReadableDatabase();
        int numRows = (int)DatabaseUtils.queryNumEntries(db,Table_Name);
        //System.out.println(numRows);
        int min = 1;
        int max = numRows;
        Random r = new Random();
        int id = r.nextInt(max - min + 1) + min;
    //  Cursor res= db.rawQuery("Select * from Comics", null);
    //  ArrayList<String> array_list = new ArrayList<String>();
        Cursor res= db.rawQuery("Select * from Comics where id = '" + id + "'", null);
        /*res.moveToFirst();
          while(res.isAfterLast() == false){
          array_list.add(res.getString(res.getColumnIndex(Col_ID)));
          res.moveToNext();
          }
          //quest.setText("");
         StringBuilder text = new StringBuilder();
          for(int i= 0; i<array_list.size();i++){
              text.append(array_list.get(i).toString() + "\n");
          }
         quest.setText(text.toString());*/
    return res;
    }
}
}

我的dbhelper_display.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<TextView
    android:id="@+id/timertxt"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"/>

<TextView
    android:id="@+id/quest"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

<RadioGroup
    android:id="@+id/radioChoices"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" >

    <RadioButton
        android:id="@+id/rbtChoice1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

    <RadioButton
        android:id="@+id/rbtChoice2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <RadioButton
        android:id="@+id/rbtChoice3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

    <RadioButton
        android:id="@+id/rbtChoice4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
    
</RadioGroup>

 <Button
            android:id="@+id/btnSubmit"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
 <Button
            android:id="@+id/btnClear"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>

<TextView
    android:id="@+id/ans"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

</LinearLayout>

我在实现上述代码时遇到 NullPointerException。我已附上相同的 LOgCat。

记录猫结果:

10-24 07:57:44.567: D/dalvikvm(1498): GC_FOR_ALLOC freed 91K, 5% free 3265K/3432K, paused 80ms, total 81ms
10-24 07:57:45.107: D/gralloc_goldfish(1498): Emulator without GPU emulation detected.
10-24 07:57:51.387: D/AndroidRuntime(1498): Shutting down VM
10-24 07:57:51.387: W/dalvikvm(1498): threadid=1: thread exiting with uncaught exception (group=0xb1a47ba8)
10-24 07:57:51.437: E/AndroidRuntime(1498): FATAL EXCEPTION: main
10-24 07:57:51.437: E/AndroidRuntime(1498): Process: course.examples.jumboquest, PID: 1498
10-24 07:57:51.437: E/AndroidRuntime(1498): java.lang.RuntimeException: Unable to start activity   ComponentInfo{course.examples.jumboquest/course.examples.jumboquest.DBHelperDisplay}: java.lang.NullPointerException
10-24 07:57:51.437: E/AndroidRuntime(1498):     at   android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.os.Handler.dispatchMessage(Handler.java:102)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.os.Looper.loop(Looper.java:136)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.ActivityThread.main(ActivityThread.java:5017)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at java.lang.reflect.Method.invokeNative(Native Method)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at java.lang.reflect.Method.invoke(Method.java:515)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at dalvik.system.NativeStart.main(Native Method)
10-24 07:57:51.437: E/AndroidRuntime(1498): Caused by: java.lang.NullPointerException
10-24 07:57:51.437: E/AndroidRuntime(1498):     at course.examples.jumboquest.DBHelperDisplay.onCreate(DBHelperDisplay.java:71)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.Activity.performCreate(Activity.java:5231)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
10-24 07:57:51.437: E/AndroidRuntime(1498):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
10-24 07:57:51.437: E/AndroidRuntime(1498):     ... 11 more
10-24 07:57:55.767: I/Process(1498): Sending signal. PID: 1498 SIG: 9

执行数据库时出现什么错误？此外，早些时候它还显示错误“游标索引请求 -1，大小为 1”。我做了一些更改，现在显示上述错误。如果同样的错误再次出现，应该怎么做？我是 Android 中的 SQLite 数据库新手。

Answer 1

我强烈怀疑这是一个拼写错误，您的 dbhelper_display.xml 布局文件中没有带有 android:id="@+id/btClear 的按钮。基于事实如果您能够编译，我怀疑您在不同的文件中拥有该 ID，但在 dbhelper_display.xml 中没有。

从您发布的 XML 代码中，我看到了这一点:

android:id="@+id/btnClear"

Java 代码是这样说的:

Button btClear = (Button)findViewById(R.id.btClear);

使这些保持一致，它就会起作用。另请检查您的 btnSubmit，我相信它也有同样的问题。