加速度計錯誤報告

Question

我對Android編程頗爲陌生。

我試圖寫幾個代碼，但我卡在一個加速度計程序。將程序加載到仿真器後，它響應「Adil_acceleration_3rd已停止」。

我提供你我的文件：

XML文件：

<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:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    tools:context=".MainActivity" > 

    <TextView 
     android:id="@+id/acceleration" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentLeft="true" 
     android:layout_alignParentTop="true" 
     android:text="X: Y: Z:" 
     android:textAppearance="?android:attr/textAppearanceLarge" /> 

</RelativeLayout> 

這是我ActivityMain.java文件：

package com.example.adil_acceleration_3rd; 

import android.hardware.Sensor; 
import android.hardware.SensorEvent; 
import android.hardware.SensorEventListener; 
import android.hardware.SensorManager; 
import android.os.Bundle; 
import android.app.Activity; 
import android.view.Menu; 
import android.widget.TextView; 


public class MainActivity extends Activity implements SensorEventListener{ 

    Sensor accelerometer; 
    SensorManager sm; 
    TextView acceleration; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     sm=(SensorManager)getSystemService(SENSOR_SERVICE); 
     accelerometer=sm.getDefaultSensor(Sensor.TYPE_ACCELEROMETER); 
     sm.registerListener(this, accelerometer, SensorManager.SENSOR_DELAY_NORMAL); 


    } 

    @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; 
    } 

    @Override 
    public void onAccuracyChanged(Sensor sensor, int accuracy) { 
     // TODO Auto-generated method stub 

    } 

    @Override 
    public void onSensorChanged(SensorEvent event) { 
     // TODO Auto-generated method stub 
     acceleration.setText("X:"+event.values[0]+ 
       "\nY:"+event.values[1]+ 
       "\nZ:"+event.values[2]); 
    } 

} 

logcat的消息：

03-16 13:26:41.210: E/Trace(1926): error opening trace file: No such file or directory (2) 
03-16 13:26:41.210: W/Trace(1926): Unexpected value from nativeGetEnabledTags: 0 
03-16 13:26:41.210: W/Trace(1926): Unexpected value from nativeGetEnabledTags: 0 
03-16 13:26:41.210: W/Trace(1926): Unexpected value from nativeGetEnabledTags: 0 
03-16 13:26:41.280: W/Trace(1926): Unexpected value from nativeGetEnabledTags: 0 
03-16 13:26:41.280: W/Trace(1926): Unexpected value from nativeGetEnabledTags: 0 
03-16 13:26:41.871: W/Trace(1926): Unexpected value from nativeGetEnabledTags: 0 
03-16 13:26:41.871: D/AndroidRuntime(1926): Shutting down VM 
03-16 13:26:41.871: W/dalvikvm(1926): threadid=1: thread exiting with uncaught exception (group=0xb4d31908) 
03-16 13:26:41.880: E/AndroidRuntime(1926): FATAL EXCEPTION: main 
03-16 13:26:41.880: E/AndroidRuntime(1926): java.lang.NullPointerException 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at com.example.adil_acceleration_3rd.MainActivity.onSensorChanged(MainActivity.java:46) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at android.hardware.SystemSensorManager$ListenerDelegate$1.handleMessage(SystemSensorManager.java:204) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at android.os.Handler.dispatchMessage(Handler.java:99) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at android.os.Looper.loop(Looper.java:137) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at android.app.ActivityThread.main(ActivityThread.java:5039) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at java.lang.reflect.Method.invoke(Method.java:511) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
03-16 13:26:41.880: E/AndroidRuntime(1926):  at dalvik.system.NativeStart.main(Native Method) 
03-16 13:27:04.801: I/Process(1926): Sending signal. PID: 1926 SIG: 9 

Answer 1

在這些情況下仔細閱讀logcat。大多數情況下，它會給出一個好的提示，或者至少提供問題的位置。

在你的情況下，重要的線在這裏：

03-16 13:26:41.880: E/AndroidRuntime(1926): java.lang.NullPointerException 
03-16 13:26:41.880: E/AndroidRuntime(1926): at com.example.adil_acceleration_3rd.MainActivity.onSensorChanged(MainActivity.java:46) 

這是這一行：

acceleration.setText("X:"+event.values[0]+ 

，它的發生是因爲acceleration宣佈，但從來沒有初始化。

P.S.對於你的下一個問題，請格式化logcat輸出以便於閱讀。