在TextView使用setText发生错误

Question

public class MainActivity extends Activity {
private TextView tx1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    tx1= (TextView) findViewById(R.id.txtexample);
    try{
    tx1.setText("Hello testing");
    Log.i("Correct","No error");
    }
    catch(Exception e){

        Log.i("Error","Error here");
        e.printStackTrace();
    }
}

不知道为什么这段代码会出错？
堆栈跟踪：

08-21 06:19:43.713: E/Trace(3883): error opening trace file: No such file or directory (2)
08-21 06:19:43.892: I/Error(3883): Error here
08-21 06:19:43.902: W/System.err(3883): java.lang.NullPointerException
08-21 06:19:43.902: W/System.err(3883):     at com.example.textview.MainActivity.onCreate(MainActivity.java:18)
08-21 06:19:43.902: W/System.err(3883):     at android.app.Activity.performCreate(Activity.java:5104)
08-21 06:19:43.902: W/System.err(3883):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
08-21 06:19:43.902: W/System.err(3883):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
08-21 06:19:43.913: W/System.err(3883):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
08-21 06:19:43.913: W/System.err(3883):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-21 06:19:43.913: W/System.err(3883):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
08-21 06:19:43.923: W/System.err(3883):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-21 06:19:43.923: W/System.err(3883):     at android.os.Looper.loop(Looper.java:137)
08-21 06:19:43.923: W/System.err(3883):     at android.app.ActivityThread.main(ActivityThread.java:5041)
08-21 06:19:43.923: W/System.err(3883):     at java.lang.reflect.Method.invokeNative(Native Method)
08-21 06:19:43.923: W/System.err(3883):     at java.lang.reflect.Method.invoke(Method.java:511)
08-21 06:19:43.933: W/System.err(3883):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-21 06:19:43.933: W/System.err(3883):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-21 06:19:43.943: W/System.err(3883):     at dalvik.system.NativeStart.main(Native Method)
08-21 06:19:44.103: D/(3883): HostConnection::get() New Host Connection established 0x2a15efa0, tid 3883

activity_main.xml

<TextView android:name="@+id/txtexample" 
 android:layout_width="wrap_content" 
  android:layout_height="wrap_content" />

Answer 1

android:name="@+id/txtexample" 

改成

android:id="@+id/txtexample"