由於NullPointerException而無法啓動活動

Question

只要我沒有將「Startup」活動作爲第一個活動，我的代碼就可以工作。它適用於我的主要活動是第一個活動，並通過具有所有工作功能的整個應用程序。我必須缺少一些東西才能使這個頁面成爲應用程序開始工作的頁面。

logcat的

10-30 20:30:22.137: D/AndroidRuntime(783): Shutting down VM 
10-30 20:30:22.137: W/dalvikvm(783): threadid=1: thread exiting with uncaught exception (group=0x41465700) 
10-30 20:30:22.147: E/AndroidRuntime(783): FATAL EXCEPTION: main 
10-30 20:30:22.147: E/AndroidRuntime(783): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidassignment2/com.example.androidassignment2.Startup}: java.lang.NullPointerException 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.ActivityThread.access$600(ActivityThread.java:141) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.os.Handler.dispatchMessage(Handler.java:99) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.os.Looper.loop(Looper.java:137) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.ActivityThread.main(ActivityThread.java:5103) 
10-30 20:30:22.147: E/AndroidRuntime(783): at java.lang.reflect.Method.invokeNative(Native Method) 
10-30 20:30:22.147: E/AndroidRuntime(783): at java.lang.reflect.Method.invoke(Method.java:525) 
10-30 20:30:22.147: E/AndroidRuntime(783): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 
10-30 20:30:22.147: E/AndroidRuntime(783): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
10-30 20:30:22.147: E/AndroidRuntime(783): at dalvik.system.NativeStart.main(Native Method) 
10-30 20:30:22.147: E/AndroidRuntime(783): Caused by: java.lang.NullPointerException 
10-30 20:30:22.147: E/AndroidRuntime(783): at com.example.androidassignment2.Startup.onCreate(Startup.java:19) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.Activity.performCreate(Activity.java:5133) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
10-30 20:30:22.147: E/AndroidRuntime(783): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175) 
10-30 20:30:22.147: E/AndroidRuntime(783): ... 11 more 

佈局

<LinearLayout 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:orientation="vertical" > 
    <TextView android:id="@+id/Hello" 
     android:layout_weight="1" 
     android:layout_width="wrap_content" 
     android:layout_height="0dip" 
     android:text="@string/Hello" /> 
    <Button android:id="@+id/QuitButton" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="@string/button_quit" /> 
</LinearLayout> 

代碼

package com.example.androidassignment2; 

import android.app.Activity; 
import android.content.Intent; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.View; 
import android.widget.Button; 

public class Startup extends Activity { 

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


     Button next = (Button) findViewById(R.id.QButton); 
     next.setOnClickListener(new View.OnClickListener() { 
      public void onClick(View view) { 
       Intent myIntent = new Intent(view.getContext(), MainActivity.class); 
       startActivityForResult(myIntent, 0); 
      } 
     }); 

    } 



    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 
} 

，最後的manifest.xml文件。在哪裏懷疑是一個問題可能。

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
      package="com.example.androidassignment2" 
      android:versionCode="1" 
      android:versionName="1.0"> 

    <uses-sdk 
     android:minSdkVersion="8" 
     android:targetSdkVersion="17"/> 

    <application 
     android:allowBackup="true" 
     android:icon="@drawable/ic_launcher" 
     android:label="@string/app_name" 
     android:theme="@style/AppTheme"> 
     <activity 
      android:name="com.example.androidassignment2.Startup" 
      android:label="@string/app_name"> 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN"/> 

       <category android:name="android.intent.category.LAUNCHER"/> 
      </intent-filter> 
     </activity> 

    <activity 
     android:name="com.example.androidassignment2.MainActivity" 
     android:label="@string/title_activity_android_assignment2_1" > 

    </activity> 


     <activity 
      android:name="com.example.androidassignment2.AndroidAssignment2_1" 
      android:label="@string/title_activity_android_assignment2_1"> 
     </activity> 

     <activity 
      android:name="com.example.androidassignment2.AndroidAssignment2_2" 
      android:label="@string/title_activity_android_assignment2_1"> 
     </activity> 


     <activity 
      android:name="com.example.androidassignment2.AndroidAssignment2_3" 
      android:label="@string/title_activity_android_assignment2_1"> 
     </activity> 


     <activity 
      android:name="com.example.androidassignment2.AndroidAssignment2_4" 
      android:label="@string/title_activity_android_assignment2_1"> 
     </activity> 

     <activity 
      android:name="com.example.androidassignment2.AndroidAssignment2_5" 
      android:label="@string/title_activity_android_assignment2_1"> 
     </activity> 

     <activity 
      android:name="com.example.androidassignment2.Goodbye" 
      android:label="@string/title_activity_android_assignment2_1"> 
     </activity> 
    </application> 
</manifest> 

Answer 1

您的按鈕ID是錯誤的。 QButton未在您的xml中定義。

而不是

Button next = (Button) findViewById(R.id.QButton); 

使用

Button next = (Button) findViewById(R.id.QuitButton);