I want to capture a photo manually not by using existing camera apps. So i made this Activity:

public class CameraActivity extends Activity {

    private Camera mCamera;
    private Preview mPreview;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_camera);


         // Create an instance of Camera
       // mCamera = Camera.open(0);

        // Create our Preview view and set it as the content of our activity.
        mPreview = new Preview(this);
        mPreview.safeCameraOpen(0);


        FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
        preview.addView(mPreview);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_camera, menu);
        return true;
    }

}

This is the CameraPreview which i want to use:

public class Preview extends ViewGroup implements SurfaceHolder.Callback {

    SurfaceView mSurfaceView;
    SurfaceHolder mHolder;
    //CAM INSTANCE ================================
    private Camera mCamera;
    List<Size> mSupportedPreviewSizes;
    //=============================================

    /**
     * @param context
     */
    public Preview(Context context) {
        super(context);

        mSurfaceView = new SurfaceView(context);
        addView(mSurfaceView);

        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        mHolder = mSurfaceView.getHolder();
        mHolder.addCallback(this);
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    /**
     * @param context
     * @param attrs
     */
    public Preview(Context context, AttributeSet attrs) {
        super(context, attrs);
        // TODO Auto-generated constructor stub
    }

    /**
     * @param context
     * @param attrs
     * @param defStyle
     */
    public Preview(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        // TODO Auto-generated constructor stub
    }

    /* (non-Javadoc)
     * @see android.view.ViewGroup#onLayout(boolean, int, int, int, int)
     */
    @Override
    protected void onLayout(boolean arg0, int arg1, int arg2, int arg3, int arg4) {
        // TODO Auto-generated method stub

    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width,
            int height) {
        // Now that the size is known, set up the camera parameters and begin
        // the preview.
        Camera.Parameters parameters = mCamera.getParameters();
        parameters.setPreviewSize(width, height);
        requestLayout();
        mCamera.setParameters(parameters);

        /*
          Important: Call startPreview() to start updating the preview surface. Preview must be
          started before you can take a picture.
        */
        mCamera.startPreview();

    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        // TODO Auto-generated method stub

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        // Surface will be destroyed when we return, so stop the preview.
        if (mCamera != null) {
            /*
              Call stopPreview() to stop updating the preview surface.
            */
            mCamera.stopPreview();
        }

    }



    /**
      * When this function returns, mCamera will be null.
      */
    private void stopPreviewAndFreeCamera() {

        if (mCamera != null) {
            /*
              Call stopPreview() to stop updating the preview surface.
            */
            mCamera.stopPreview();

            /*
              Important: Call release() to release the camera for use by other applications. 
              Applications should release the camera immediately in onPause() (and re-open() it in
              onResume()).
            */
            mCamera.release();

            mCamera = null;
        }
    }



    public void setCamera(Camera camera) {
        if (mCamera == camera) { return; }

        stopPreviewAndFreeCamera();

        mCamera = camera;

        if (mCamera != null) {
            List<Size> localSizes = mCamera.getParameters().getSupportedPreviewSizes();
            mSupportedPreviewSizes = localSizes;
            requestLayout();

            try {
                mCamera.setPreviewDisplay(mHolder);
            } catch (IOException e) {
                e.printStackTrace();
            }

            /*
              Important: Call startPreview() to start updating the preview surface. Preview must 
              be started before you can take a picture.
              */
            mCamera.startPreview();
        }
    }


    public boolean safeCameraOpen(int id) {
        boolean qOpened = false;

        try {
            releaseCameraAndPreview();
            mCamera = Camera.open(id);
            mCamera.startPreview();
            qOpened = (mCamera != null);
        } catch (Exception e) {
          //  Log.e(R.string.app_name, "failed to open Camera");
            e.printStackTrace();
        }

        return qOpened;    
    }


    Camera.PictureCallback mPicCallback = new Camera.PictureCallback() {

        @Override
        public void onPictureTaken(byte[] data, Camera camera) {
            // TODO Auto-generated method stub
            Log.d("lala", "pic is taken");
        }
    };

    public void takePic() {
        mCamera.startPreview();
    //  mCamera.takePicture(null, mPicCallback, mPicCallback);

    }

    private void releaseCameraAndPreview() {
        this.setCamera(null);
        if (mCamera != null) {
            mCamera.release();
            mCamera = null;
        }
    }

}

This is the Layout of CameraActivity:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".CameraActivity" >


  <FrameLayout
    android:id="@+id/camera_preview"
    android:layout_width="0dip"
    android:layout_height="fill_parent"
    android:layout_weight="1"
    />

  <Button
    android:id="@+id/button_capture"
    android:text="Capture"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    />


</LinearLayout>

But when i start the CameraActivity, i just see a blank white background and the Capture-Button??? Why i dont see the Camera-Screen?