NullPointerException当我尝试在Android Studio上运行我的Android程序时

Question

由于某种原因，每当我运行我的应用程序时，我都会收到NullPointerException。我认为这个问题在newProblem()方法中，但我不知道问题是什么。请帮助我解决这个问题，这实际上是我的第二款Android应用。

的Java：

package com.anuraagy.quiz; 

import android.app.Activity; 
import android.os.Bundle; 
import android.view.Menu; 
import android.content.*; 
import android.view.MenuItem; 
import android.widget.*; 
import android.view.*; 
import java.util.*; 

import java.text.AttributedCharacterIterator; 


public class Quiz extends Activity { 

    //Create all the private button fields 
    private Button submitButton; 
    private EditText inputText; 
    private TextView viewText; 

    private int number1; 
    private int number2; 
    private int inputValue; 
    private int totalValue; 
    private String numberString; 
    private ArrayList runningTotal; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_quiz); 

     Button submit = (Button)findViewById(R.id.button); 

     TextView viewText = (TextView)findViewById(R.id.textView4); 

     EditText inputText = (EditText)findViewById(R.id.numberInputed); 
//  boolean hello = mathString(); 
     newProblem(); 
     submit.setOnClickListener(new View.OnClickListener() { 
      //Onclick button function 
      public void onClick(View view) { 
       if(runningTotal.size() < 20) { 
        if (checkAnswer()) { 
         newProblem(); 
        } 
//     } else { 
//      wrongAnswer(); 
//     } 
       } 
      } 
     }); 

    } 


    @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; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

    private void newProblem() 
    { 
     int number1 = (int)(Math.random()*12 + 1); 
     int number2 = (int)(Math.random()*12 + 1); 
     int totalValue = number1 * number2; 

     String numberString = number1 + " * " + number2; 
     viewText.setText(numberString); 


    } 

    //Check the answer submitted 
    private boolean checkAnswer() 
    { 
     if(number1*number2 == Integer.parseInt(inputText.getText().toString())) 
     { 
      runningTotal.add("true"); 
      return true; 
     } 
     else 
     { 
      runningTotal.add("false"); 
      return false; 
     } 
    } 
// private boolean mathString() 
// { 
// 
// 
//  int number1 = (int)(Math.random()*12 + 1); 
//  int number2 = (int)(Math.random()*12 + 1); 
//  int value = number1 * number2; 
// 
//  String totalText = number1 + " * " + number2; 
// 
//  TextView viewText = (TextView)findViewById(R.id.textView4); 
//  viewText.setText(totalText); 
// 
//  EditText inputText = (EditText)findViewById(R.id.numberInputed); 
//  String s = inputText.getText().toString(); 
//  int value2 = Integer.parseInt(s); 
// 
//  if(value == value2) 
//   return true; 
//  else 
//   return false; 
// } 
} 

登录：

09-18 17:40:39.338  740-740/com.anuraagy.quiz E/Trace﹕ error opening trace file: No such file or directory (2) 
09-18 17:40:40.298  740-740/com.anuraagy.quiz D/gralloc_goldfish﹕ Emulator without GPU emulation detected. 
09-18 17:40:42.618  740-740/com.anuraagy.quiz D/AndroidRuntime﹕ Shutting down VM 
09-18 17:40:42.618  740-740/com.anuraagy.quiz W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x40a13300) 
09-18 17:40:42.637  740-740/com.anuraagy.quiz E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.anuraagy.quiz/com.anuraagy.quiz.Quiz}: java.lang.NullPointerException 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
      at android.app.ActivityThread.access$600(ActivityThread.java:130) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
      at android.os.Handler.dispatchMessage(Handler.java:99) 
      at android.os.Looper.loop(Looper.java:137) 
      at android.app.ActivityThread.main(ActivityThread.java:4745) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:511) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
      at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
      at com.anuraagy.quiz.Quiz.newProblem(Quiz.java:84) 
      at com.anuraagy.quiz.Quiz.onCreate(Quiz.java:40) 
      at android.app.Activity.performCreate(Activity.java:5008) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
            at android.app.ActivityThread.access$600(ActivityThread.java:130) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
            at android.os.Handler.dispatchMessage(Handler.java:99) 
            at android.os.Looper.loop(Looper.java:137) 
            at android.app.ActivityThread.main(ActivityThread.java:4745) 
            at java.lang.reflect.Method.invokeNative(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:511) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
            at dalvik.system.NativeStart.main(Native Method) 
09-18 17:45:42.698  740-740/com.anuraagy.quiz I/Process﹕ Sending signal. PID: 740 SIG: 9 

acitivty_quiz.xml：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    tools:context="com.anuraagy.quiz.Quiz"> 


    <Button 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="Submit Answer" 
     android:id="@+id/button" 
     android:layout_alignParentBottom="true" 
     android:layout_centerHorizontal="true" 
     android:layout_marginBottom="57dp" /> 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:textAppearance="?android:attr/textAppearanceLarge" 
     android:text="Question 1" 
     android:id="@+id/textView" 
     android:layout_alignParentTop="true" 
     android:layout_centerHorizontal="true" /> 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:textAppearance="?android:attr/textAppearanceLarge" 
     android:text="My numbers to display" 
     android:id="@+id/textView4" 
     android:layout_below="@+id/textView" 
     android:layout_centerHorizontal="true" 
     android:layout_marginTop="72dp" /> 

    <EditText 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:inputType="number" 
     android:ems="10" 
     android:id="@+id/numberInputed" 
     android:layout_below="@+id/textView4" 
     android:layout_alignLeft="@+id/textView4" 
     android:layout_alignStart="@+id/textView4" /> 
</RelativeLayout> 

Answer 1

你定义viewText两次。一旦上课，一次在onCreate()。

在onCreate()，更换

TextView viewText = (TextView)findViewById(R.id.textView4); 

与

viewText = (TextView)findViewById(R.id.textView4);