我一直在线关注 Android 开发教程，但是当我在模拟器上运行应用程序时，它会因错误而关闭。
我用谷歌搜索了第一个错误无济于事。但是查看日志猫的其余部分，我注意到它从主要活动中抛出了错误。我认为这可能是我的清单文件有问题，但我不确定到底是什么。
我的包名称是 com.example.androidtutorials。
有人可以告诉我我在遵循本教程时做错了什么。我的清单文件如下：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.androidtutorials"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.androidtutorials.MainActivity"
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=".Numbers"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.NUMBERS" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
<activity
android:name=".InternalStorage"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.INTERNALSTORAGE" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
<activity
android:name=".Reading"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.READING" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
<activity
android:name=".Menu"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MENU" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
<activity
android:name=".SaveToSD"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.SAVETOSD" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
</application>
</manifest>
日志猫：
09-20 17:22:58.802: E/Trace(799): error opening trace file: No such file or directory (2)
09-20 17:23:02.662: D/AndroidRuntime(799): Shutting down VM
09-20 17:23:02.662: W/dalvikvm(799): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
09-20 17:23:02.707: E/AndroidRuntime(799): FATAL EXCEPTION: main
09-20 17:23:02.707: E/AndroidRuntime(799): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidtutorials/com.example.androidtutorials.MainActivity}: java.lang.NullPointerException
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.os.Handler.dispatchMessage(Handler.java:99)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.os.Looper.loop(Looper.java:137)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.ActivityThread.main(ActivityThread.java:5041)
09-20 17:23:02.707: E/AndroidRuntime(799): at java.lang.reflect.Method.invokeNative(Native Method)
09-20 17:23:02.707: E/AndroidRuntime(799): at java.lang.reflect.Method.invoke(Method.java:511)
09-20 17:23:02.707: E/AndroidRuntime(799): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
09-20 17:23:02.707: E/AndroidRuntime(799): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
09-20 17:23:02.707: E/AndroidRuntime(799): at dalvik.system.NativeStart.main(Native Method)
09-20 17:23:02.707: E/AndroidRuntime(799): Caused by: java.lang.NullPointerException
09-20 17:23:02.707: E/AndroidRuntime(799): at com.example.androidtutorials.MainActivity.onCreate(MainActivity.java:27)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.Activity.performCreate(Activity.java:5104)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
09-20 17:23:02.707: E/AndroidRuntime(799): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
09-20 17:23:02.707: E/AndroidRuntime(799): ... 11 more
09-20 17:27:43.197: I/Process(799): Sending signal. PID: 799 SIG: 9
09-20 17:27:45.187: D/AndroidRuntime(858): Shutting down VM
09-20 17:27:45.187: W/dalvikvm(858): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
09-20 17:27:45.207: E/AndroidRuntime(858): FATAL EXCEPTION: main
09-20 17:27:45.207: E/AndroidRuntime(858): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidtutorials/com.example.androidtutorials.MainActivity}: java.lang.NullPointerException
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.os.Handler.dispatchMessage(Handler.java:99)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.os.Looper.loop(Looper.java:137)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.ActivityThread.main(ActivityThread.java:5041)
09-20 17:27:45.207: E/AndroidRuntime(858): at java.lang.reflect.Method.invokeNative(Native Method)
09-20 17:27:45.207: E/AndroidRuntime(858): at java.lang.reflect.Method.invoke(Method.java:511)
09-20 17:27:45.207: E/AndroidRuntime(858): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
09-20 17:27:45.207: E/AndroidRuntime(858): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
09-20 17:27:45.207: E/AndroidRuntime(858): at dalvik.system.NativeStart.main(Native Method)
09-20 17:27:45.207: E/AndroidRuntime(858): Caused by: java.lang.NullPointerException
09-20 17:27:45.207: E/AndroidRuntime(858): at com.example.androidtutorials.MainActivity.onCreate(MainActivity.java:27)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.Activity.performCreate(Activity.java:5104)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
09-20 17:27:45.207: E/AndroidRuntime(858): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
09-20 17:27:45.207: E/AndroidRuntime(858): ... 11 more
09-20 17:27:48.847: I/Process(858): Sending signal. PID: 858 SIG: 9
我的主类的 onCreate 方法：
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
width = (Button)findViewById(R.id.button1);
height = (Button)findViewById(R.id.button2);
calc = (Button)findViewById(R.id.button3);
area = (TextView)findViewById(R.id.textView1);
width.setOnClickListener(this);
height.setOnClickListener(this);
calc.setOnClickListener(this);
}