Android错误之 java.lang.IllegalStateException: System services not available to Activities before onC

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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。

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