出于某种原因，每当我运行我的应用时，我都会收到NullPointerException。我认为问题出在newProblem()方法中，但我不知道问题是什么。请帮我解决这个问题，这是我的第二个Android应用程序。
爪哇：
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;
// }
}
日志：
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>
答案 0 :(得分：3)
您定义viewText两次。一旦上课，一次进入onCreate()。
在onCreate()中，替换
TextView viewText = (TextView)findViewById(R.id.textView4);
与
viewText = (TextView)findViewById(R.id.textView4);