全错误日志如下：
FATAL EXCEPTION: main Process: com.example.administrator.followview, PID: 8587 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.administrator.followview/com.example.administrator.followview.MainActivity}: java.lang.IllegalStateException: System services not available to Activities before onCreate() at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2327) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate() at android.app.Activity.getSystemService(Activity.java:5253) at com.example.administrator.followview.MainActivity.<init>(MainActivity.java:18) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1067) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2317) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)错误代码如下：
public class MainActivity extends Activity implements View.OnTouchListener { private Button mButton; private ViewGroup mViewGroup; private int xDelta; private int yDelta; WindowManager wm = (WindowManager) this .getSystemService(Context.WINDOW_SERVICE); int width = wm.getDefaultDisplay().getWidth(); int height = wm.getDefaultDisplay().getHeight(); @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mViewGroup = (ViewGroup) findViewById(R.id.root);果然写到了onCreate的上面，将其放入onCreate方法运行即可。
