我正在使用解析云数据库为Android创建一个测验应用程序。我在我的一个活动中得到一个空指针异常。这是我的代码..pls help..i还包括我的日志......如果需要，我会提供进一步的细节
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;
public class QuestionActivity extends Activity implements OnClickListener{
private Question currentQ;
private Exam currentExam;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_question);
currentExam = ((MyApp)getApplication()).getCurrentExam(); //app crashes here
currentQ = currentExam.getNextQuestion();
Button nextBtn = (Button) findViewById(R.id.nextBtn);
nextBtn.setOnClickListener(this);
setQuestions();
}
private void setQuestions() {
//set the question text from current question
String question = currentQ.getQuestion() + "?";
TextView qText = (TextView) findViewById(R.id.question);
qText.setText(question);
//set the available options
TextView option1 = (TextView) findViewById(R.id.answer1);
option1.setText( currentQ.getOptionOne() );
TextView option2 = (TextView) findViewById(R.id.answer2);
option2.setText( currentQ.getOptionTwo() );
TextView option3 = (TextView) findViewById(R.id.answer3);
option3.setText( currentQ.getOptionThree() );
TextView option4 = (TextView) findViewById(R.id.answer4);
option4.setText( currentQ.getOptionFour() );
}
@Override public void onClick(View v) {
//validate a checkbox has been selected
if (!checkAnswer()) return;
// check if end of game
if (currentExam.isExamOver()){
Toast.makeText(getApplicationContext(),"finish",Toast.LENGTH_LONG).show();
finish();
} else {
Intent i = new Intent(this, QuestionActivity.class);
startActivity(i);
finish();
}
}
@Override public boolean onKeyDown(int keyCode, KeyEvent event) {
switch (keyCode) {
case KeyEvent.KEYCODE_BACK : return true;
}
return super.onKeyDown(keyCode, event);
}
private boolean checkAnswer() {
RadioGroup options = (RadioGroup)findViewById(R.id.group1);
Integer selected = options.getCheckedRadioButtonId();
if ( selected < 0){
return false;
} else {
if (currentQ.getCorrectAnswer() == selected+1 ) {
currentExam.incrementRightAnswers();
} else {
currentExam.incrementWrongAnswers();
}
return true;
}
}
}
继承我的日志
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: FATAL EXCEPTION: main
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: Process: me.ashif.preparation, PID: 2909
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{me.ashif.preparation/me.ashif.preparation.QuestionActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'me.ashif.preparation.Exam me.ashif.preparation.Exam.getCurrentExam()' on a null object reference
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.app.ActivityThread.- wrap11(ActivityThread.java)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.os.Looper.loop(Looper.java:148)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5417)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'me.ashif.preparation.Exam me.ashif.preparation.Exam.getCurrentExam()' on a null object reference
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at me.ashif.preparation.QuestionActivity.onCreate(QuestionActivity.java:35)
01-15 09:46:04.508 2909-2909/me.ashif.preparation E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:6237)
我的app类代码
import android.app.Application;
import android.widget.Toast;
import com.parse.FindCallback;
import com.parse.Parse;
import com.parse.ParseACL;
import com.parse.ParseException;
import com.parse.ParseObject;
import com.parse.ParseQuery;
import java.util.List;
/**
* Created by ashif on 15/1/16.
*/
public class MyApp extends Application {
private Exam currentExam;
public void setCurrentExam(Exam currentGame) {
this.currentExam = currentExam;
}
public Exam getCurrentExam() {
return currentExam;
}
@Override
public void onCreate() {
super.onCreate();
ParseObject.registerSubclass(Question.class);
Parse.enableLocalDatastore(this);
Parse.initialize(this, "NsvQwd2jpTunUlpsb4SrkzzKOXoPYwiWEOBgg9Ly", "HdSfQm9uDNyCyzxQBlH6jPmy6nvbmxxttuCtAVRz");
ParseACL defaultACL = new ParseACL();
defaultACL.setPublicReadAccess(true);
ParseACL.setDefaultACL(defaultACL, true);
ParseQuery<Question> query = ParseQuery.getQuery("Question");
final Application app = this;
query.findInBackground(new FindCallback<Question>() {
@Override public void done(List<Question> questions, ParseException e) {
if (e == null) {
//We have updated questions from the cloud so we will cache them all
ParseObject.unpinAllInBackground("QUESTIONS");
ParseObject.pinAllInBackground("QUESTIONS", questions);
} else {
//alert the user that unable to fetch questions -this should be more robust in production apps
Toast.makeText(app,
"Error updating questions - please make sure you have internet connection",
Toast.LENGTH_LONG).show();
}
}
});
}
}
检查我的清单
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="me.ashif.preparation" >
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:name=".MyApp"
android:supportsRtl="true"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/app_name"
android:theme="@style/AppTheme.NoActionBar" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name="me.ashif.preparation.QuestionActivity"
android:label="@string/app_name"
/>
<meta-data
android:name="com.parse.APPLICATION_ID"
android:value="NsvQwd2jpTunUlpsb4SrkzzKOXoPYwiWEOBgg9Ly" />
<meta-data
android:name="com.parse.CLIENT_KEY"
android:value="HdSfQm9uDNyCyzxQBlH6jPmy6nvbmxxttuCtAVRz" />
</application>
答案 0 :(得分：2)
根据日志，NullPointerException位于onCreate函数下方的{}}位置
currentExam = ((MyApp)getApplication()).getCurrentExam();
您是否在清单文件的申请标记中声明了MyApp？
答案 1 :(得分：2)
与日志一样：
尝试调用虚方法'me.ashif.preparation.Exam 关于空对象引用的me.ashif.preparation.Exam.getCurrentExam（）'
表示可能忘记在MyApp AndroidManifest.xml个application tag文件中添加android:name课程，其中<application
android:label="@string/app_name"
android:name=".MyApp">
....
</application>
属性为：
MyApp同样在currentExam类Exam中，currentQ = currentExam.getNextQuestion();
类的对象不会在任何地方初始化，所以请确保它已初始化：
NPE行将抛出<ul>
<li ng-repeat="album in albums">
<input type="checkbox" ng-model="album.selected" value={{album.name}} ng-click="save()"/> {{album.name}}
</li>
答案 2 :(得分：0)
确保您在AndroidManifest中定义了MyApp：
<application
android:name="MyApp"
答案 3 :(得分：0)
在清单中声明应用程序标记，它将解决问题
<application
android:name="me.ashif.preparation.MyApp"