为啥官方的Android重力感应示例代码会出错

为什么官方的Android重力感应示例代码会出错？
在谷歌的官网上有一段重力感应示例代码，但我复制下来运行时却出现了错误，源代码如下：

Java code

package com.sensor.test2;import android.app.Activity;import android.os.Bundle;import android.hardware.SensorManager;import android.hardware.Sensor;import android.hardware.SensorEventListener;import android.hardware.SensorEvent;public class SensorTest extends Activity implements SensorEventListener {    private final SensorManager mSensorManager;    private final Sensor mAccelerometer;    public SensorTest() {        mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);        mAccelerometer = mSensorManager                .getDefaultSensor(Sensor.TYPE_ACCELEROMETER);    }    protected void onResume() {        super.onResume();        mSensorManager.registerListener(this, mAccelerometer,                SensorManager.SENSOR_DELAY_NORMAL);    }    protected void onPause() {        super.onPause();        mSensorManager.unregisterListener(this);    }    public void onAccuracyChanged(Sensor sensor, int accuracy) {    }    public void onSensorChanged(SensorEvent event) {    }}

出错的提示是这样的：
07-05 12:15:28.235: W/dalvikvm(11836): threadid=3: thread exiting with uncaught exception (group=0x40026268)
07-05 12:15:28.245: E/AndroidRuntime(11836): Uncaught handler: thread main exiting due to uncaught exception
07-05 12:15:28.265: E/AndroidRuntime(11836): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.sensor.test2/com.sensor.test2.SensorTest}: java.lang.IllegalStateException: System services not available to Activities before onCreate()
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2417)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.ActivityThread.access$2200(ActivityThread.java:119)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.os.Handler.dispatchMessage(Handler.java:99)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.os.Looper.loop(Looper.java:123)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.ActivityThread.main(ActivityThread.java:4363)
07-05 12:15:28.265: E/AndroidRuntime(11836): at java.lang.reflect.Method.invokeNative(Native Method)
07-05 12:15:28.265: E/AndroidRuntime(11836): at java.lang.reflect.Method.invoke(Method.java:521)
07-05 12:15:28.265: E/AndroidRuntime(11836): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
07-05 12:15:28.265: E/AndroidRuntime(11836): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
07-05 12:15:28.265: E/AndroidRuntime(11836): at dalvik.system.NativeStart.main(Native Method)
07-05 12:15:28.265: E/AndroidRuntime(11836): Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate()
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.Activity.getSystemService(Activity.java:3468)
07-05 12:15:28.265: E/AndroidRuntime(11836): at com.sensor.test2.SensorTest.<init>(SensorTest.java:15)
07-05 12:15:28.265: E/AndroidRuntime(11836): at java.lang.Class.newInstanceImpl(Native Method)
07-05 12:15:28.265: E/AndroidRuntime(11836): at java.lang.Class.newInstance(Class.java:1479)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
07-05 12:15:28.265: E/AndroidRuntime(11836): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2409)
07-05 12:15:28.265: E/AndroidRuntime(11836): ... 11 more

请问这是怎么回事的呢？不仅是官网给出的代码，我还试了CSDN和其他社区中给出的相关代码，都是同样的错误~

最近刚开始学Android开发，有些东西还不太明白，求详解，谢谢！

[解决办法]
Activity是有自己的生命周期的，你的SensorTest()构造方法明显不对，把SensorTest()里面的代码放在onCreate()里面就可以了。
[解决办法]
OnCreate 之后才能getSystemService

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