The full error log is as follows:

 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) 

The error code is as follows:

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);

Sure enough, it’s written on oncreate. Just put it into the oncreate method to run.