java.lang.RuntimeException: Unable to start activity ComponentInfo{包名/类名}

写这篇博文，我顶着很大的压力，贴出来会引来网友的一片鄙视，不贴我又觉得对不起Android SDK研发团队。本着对全世界Android无产者负责的态度，今天不得不指出Android编译时隐藏的很深的一个Bug.
以下为我的TestActivity类： public class TestActivity extends Activity {     private static final String TAG = "TestActivity";     private Button button;     @Override     protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);
       button = (Button)findViewById(R.id.btn);        button.setOnClickListener(new OnClickListener() {           @Override           public void onClick(View v) {                Log.d(TAG, "onClick. ");           }      });     } }
该类在AndroidMenifest.xml中是这样设置的：                   android:label="@string/app_name">
布局文件为res/layout/main.xml：     android:orientation="vertical" android:layout_width="fill_parent"     android:layout_height="fill_parent">
     android:name="@+id/btn" android:layout_width="fill_parent"          android:layout_height="wrap_content" android:text="start"/>
很简单吧，当我编译时，也完全正常.但是当在Eclipse中点击运行时，抛出如下异常： java.lang.RuntimeException: Unable to start activity ComponentInfo{com.orange.test/com.orange.test.TestActivity}: java.lang.NullPointerException      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)      at android.app.ActivityThread.access$2300(ActivityThread.java:125)      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)      at android.os.Handler.dispatchMessage(Handler.java:99)      at android.os.Looper.loop(Looper.java:123)      at android.app.ActivityThread.main(ActivityThread.java:4627)      at java.lang.reflect.Method.invokeNative(Native Method)      at java.lang.reflect.Method.invoke(Method.java:521)      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)      at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.NullPointerException      at com.orange.test.TestActivity.onCreate(TestActivity.java:24)      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)      ... 11 more
很是匪夷所思，这么一个简单的问题，button=(Button)findViewById(R.id.btn); button对象值竟然为null，我又去查看gen/目录下的com.orange.test.R.java文件：内容如下 public final class R {     public static final class attr {     }     public static final class drawable {         public static final int icon=0x7f020000;     }      public static final class id {         public static final int btn=0x7f050000;     }     public static final class layout {         public static final int main=0x7f030000;     }     public static final class string {         public static final int app_name=0x7f040001;         public static final int hello=0x7f040000;     } }
一切都正常啊，R.id.btn这不是正常地生成了吗！！！！，找来大拿，不顶用，同样没有发现。最后无意识地发现， android:name="@+id/btn" 应该是android:id="@+id/btn" . 悲剧的是，Android编译时，竟然没有发现这里的错误。 最为可气的是，竟然还在R.java文件中生成了 public static final class id {         public static final int btn=0x7f050000;     } 这得害了多少无辜的程序员。

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