java - Activity 启动时引发空指针异常

Question

我正在尝试在我的应用程序中包含一个小测验。为了测试一下，我尝试了一个作为答案列出的变体。我的主要 Activity 是一个选择器，其中所有选项都通过按钮显示。该应用程序使用一个名为 eGREp.db 的数据库来提供含义、单词列表等功能。由于该部分运行良好，我想创建一个新数据库并单独工作。单击测试时，应该启动测验 Activity 。

我已将 onClick() 字段设置为指向 openQuiz，其具有以下内容

public void openQuiz(View view) {
    // Do something in response to button
    Intent intent = new Intent(this, Quiz.class);
    startActivity(intent);
}

现在应该启动测验 Activity ，但抛出的异常很少。 问答课

package com.example.egrep;

import java.util.List;
import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;

public class Quiz extends Activity {
    List<Question> quesList;
    int score=0;
    int qid=0;
    Question currentQ;
    TextView txtQuestion;
    RadioButton rda, rdb, rdc;
    Button butNext=(Button)findViewById(R.id.button1);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_quiz);
        DbHelper db = new DbHelper(Quiz.this);
        quesList=db.getAllQuestions();
        currentQ=quesList.get(qid);
        txtQuestion=(TextView)findViewById(R.id.textView1);
        rda=(RadioButton)findViewById(R.id.radioButton0);
        rdb=(RadioButton)findViewById(R.id.radioButton1);
        rdc=(RadioButton)findViewById(R.id.radioButton2);
        setQuestionView();
        butNext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                RadioGroup grp=(RadioGroup)findViewById(R.id.radioGroup);
                RadioButton answer=(RadioButton)findViewById(grp.getCheckedRadioButtonId());
                Log.d("yourans", currentQ.getANSWER()+" "+answer.getText());
                if(currentQ.getANSWER().equals(answer.getText()))
                {
                    score++;
                    Log.d("score", "Your score"+score);
                }
                if(qid<5){
                    currentQ=quesList.get(qid);
                    setQuestionView();
                }else{
                    Intent intent = new Intent(Quiz.this, ResultActivity.class);
                    Bundle b = new Bundle();
                    b.putInt("score", score); //Your score
                    intent.putExtras(b); //Put your score to your next Intent
                    startActivity(intent);
                    finish();
                }
            }
        });
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        //  Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.quiz, menu);
        return true;
    }
    private void setQuestionView()
    {
        txtQuestion.setText(currentQ.getQUESTION());
        rda.setText(currentQ.getOPTA());
        rdb.setText(currentQ.getOPTB());
        rdc.setText(currentQ.getOPTC());
        qid++;
    }
}

显示错误

 03-18 15:37:13.834: E/AndroidRuntime(1201): Caused by: java.lang.NullPointerException
03-18 15:37:13.834: E/AndroidRuntime(1201):     at com.example.egrep.DbHelper.addQuestion(DbHelper.java:67)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at com.example.egrep.DbHelper.onCreate(DbHelper.java:35)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:252)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at com.example.egrep.DbHelper.getAllQuestions(DbHelper.java:73)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at com.example.egrep.Quiz.onCreate(Quiz.java:29)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at android.app.Activity.performCreate(Activity.java:5133)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
03-18 15:37:13.834: E/AndroidRuntime(1201):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)

问题类别

package com.example.egrep;

public class Question {
    private int ID;
    private String QUESTION;
    private String OPTA;
    private String OPTB;
    private String OPTC;
    private String ANSWER;
    public Question()
    {
    ID=0;
    QUESTION="";
    OPTA="";
    OPTB="";
    OPTC="";
    ANSWER="";
    }
    public Question(String qUESTION, String oPTA, String oPTB, String oPTC,
    String aNSWER) {

    QUESTION = qUESTION;
    OPTA = oPTA;
    OPTB = oPTB;
    OPTC = oPTC;
    ANSWER = aNSWER;
    }
    public int getID()
    {
    return ID;
    }
    public String getQUESTION() {
    return QUESTION;
    }
    public String getOPTA() {
    return OPTA;
    }
    public String getOPTB() {
    return OPTB;
    }
    public String getOPTC() {
    return OPTC;
    }
    public String getANSWER() {
    return ANSWER;
    }
    public void setID(int id)
    {
    ID=id;
    }
    public void setQUESTION(String qUESTION) {
    QUESTION = qUESTION;
    }
    public void setOPTA(String oPTA) {
    OPTA = oPTA;
    }
    public void setOPTB(String oPTB) {
    OPTB = oPTB;
    }
    public void setOPTC(String oPTC) {
    OPTC = oPTC;
    }
    public void setANSWER(String aNSWER) {
    ANSWER = aNSWER;
    }
}

DbHelper类

package com.example.egrep;

import java.util.ArrayList;
import java.util.List;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DbHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION = 1;
// Database Name
private static final String DATABASE_NAME = "triviaQuiz";
// tasks table name
private static final String TABLE_QUEST = "quest";
// tasks Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_QUES = "question";
private static final String KEY_ANSWER = "answer"; //correct option
private static final String KEY_OPTA= "opta"; //option a
private static final String KEY_OPTB= "optb"; //option b
private static final String KEY_OPTC= "optc"; //option c
private SQLiteDatabase dbase;
public DbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
String sql = "CREATE TABLE IF NOT EXISTS " + TABLE_QUEST + " ( "
+ KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + KEY_QUES
+ " TEXT, " + KEY_ANSWER+ " TEXT, "+KEY_OPTA +" TEXT, "
+KEY_OPTB +" TEXT, "+KEY_OPTC+" TEXT)";
db.execSQL(sql);
Question q1=new Question("Choose the correct alternative that will continue the same pattern and replace the question mark in the given series." +
" 120, 99, 80, 63, 48, ?","35", "38", "39", "A");
this.addQuestion(q1);
Question q2=new Question("A, P, R, X, S and Z are sitting in a row. S and Z are in the centre. A and P are at the " +
"ends. R is sitting to the left of A. Who is to the right of P ?", "A", "X", "S", "B");
this.addQuestion(q2);
Question q3=new Question("Which of the following is the fastest" +
" writable memory?","RAM", "FLASH","Register","C");
this.addQuestion(q3);
Question q4=new Question("Which of the following device" +
" regulates internet traffic?",    "Router", "Bridge", "Hub","A");
this.addQuestion(q4);
Question q5=new Question("Which of the following is NOT an" +
" interpreted language?","Ruby","Python","BASIC","C");
this.addQuestion(q5);
db.close();
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldV, int newV) {
// Drop older table if existed
db.execSQL("DROP TABLE IF EXISTS " + TABLE_QUEST);
// Create tables again
onCreate(db);
}
// Adding new question
public void addQuestion(Question quest) {
//SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(KEY_QUES, quest.getQUESTION());
values.put(KEY_ANSWER, quest.getANSWER());
values.put(KEY_OPTA, quest.getOPTA());
values.put(KEY_OPTB, quest.getOPTB());
values.put(KEY_OPTC, quest.getOPTC());
// Inserting Row
dbase.insert(TABLE_QUEST, null, values);
}
public List<Question> getAllQuestions() {
List<Question> quesList = new ArrayList<Question>();
// Select All Query
String selectQuery = "SELECT  * FROM " + TABLE_QUEST;
dbase=this.getReadableDatabase();
Cursor cursor = dbase.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
Question quest = new Question();
quest.setID(cursor.getInt(0));
quest.setQUESTION(cursor.getString(1));
quest.setANSWER(cursor.getString(2));
quest.setOPTA(cursor.getString(3));
quest.setOPTB(cursor.getString(4));
quest.setOPTC(cursor.getString(5));
quesList.add(quest);
} while (cursor.moveToNext());
}
// return quest list
return quesList;
}
public int rowcount()
{
int row=0;
String selectQuery = "SELECT  * FROM " + TABLE_QUEST;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
row=cursor.getCount();
return row;
}
}

你能帮我理解为什么抛出这些异常以及上面的代码有什么问题吗？

Answer 1

Button butNext=(Button)findViewById(R.id.button1);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_quiz);

您需要在扩展布局后检索您的Button，否则findViewById将返回null。所以应该是这样

Button butNext;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_quiz); //layout inflated, now you can get your elements
        butNext=(Button)findViewById(R.id.button1);

还学习从 logcat 中读取内容。您会比在这里提问更快地发现错误。以下是重要的几行:

Caused by: java.lang.NullPointerException
03-18 14:41:31.194: E/AndroidRuntime(901):  at android.app.Activity.findViewById(Activity.java:1853)
03-18 14:41:31.194: E/AndroidRuntime(901):  at com.example.egrep.Quiz.<init>(Quiz.java:20)