i am implement face detection in opencv android but when i run the open camera code code i am getting null pointer exception

This is the code **mOpenCvCameraView = (JavaCameraView) findViewById(R.id.show_camera_activity_java_surface_view); mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE); mOpenCvCameraView.setCvCameraViewListener(this); setContentView(R.layout.activity_main);** here the line**mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE);** is showing a null pointer exception when ruuning and the app gets crashed..plse help