Android :: Camera Preview Screen Still Black

I'm trying the camera preview

This is my code and it doesn't throw any error, but the screen still black, any ideas?

this.setContentView(R.layout.camerapreview);
SurfaceView cameraSurface = (SurfaceView)findViewById(R.id.cpPreview);
SurfaceHolder holder = cameraSurface.getHolder();
holder.addCallback(this);
holder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
this.camera = Camera.open();
this.camera.setPreviewDisplay(holder);
this.camera.startPreview();}