关于Java中引用的困惑

Question

I am confused about this piece of code:

    public void setGraphicsColor(Colors colors) { 
      Graphics gc = bufferImage.getGraphics();
      switch(colors) {
      case BLACK:
          gc.setColor(Color.black);
          break;
      case DARK_GRAY:
          gc.setColor(Color.darkGray);
          break;
      case WHITE:
          gc.setColor(Color.white);
          break;
      default:
          break;
      };
}

Would gc refer to the same Graphics object that bufferImage has? So when gc is setting the color attributes, is it changing a local copy or changing the Graphics object in bufferImage . According to what I read online, it should refer to the actual object instead of a local clone. However, when I debugged this code, gc.setColor() does not seem to be doing anything to bufferImage . Sorry if this is a dumb question.

EDIT: More context

The graphics wrapper class MazePanel:

     public class MazePanel extends Panel  {
        /* Panel operates a double buffer see
         * http://www.codeproject.com/Articles/2136/Double-buffer-in-standard-Java-AWT
         * for details
         */
        Image bufferImage ;

        Color color;

        public enum Colors {
            BLACK, DARK_GRAY, WHITE
        };

        public MazePanel() {
            super() ;
            this.setFocusable(false) ;
        }
        @Override
        public void update(Graphics g) {
            paint(g) ;
        }
        @Override
        public void paint(Graphics g) {
            g.drawImage(bufferImage,0,0,null) ;
        }
        /*
        public void setBufferImage(Image buffer) {
            bufferImage = buffer ;
        }
        */
        public void initBufferImage() {
            bufferImage = createImage(Constants.VIEW_WIDTH, Constants.VIEW_HEIGHT);
            if (null == bufferImage)
            {
                System.out.println("Error: creation of buffered image failed, presumedly container not displayable");
            }
        }

        public Image getBufferImage() {
            return this.bufferImage;
        }

        public void setBufferImage(Image im) {
            this.bufferImage = im;
        }

        public Graphics getBufferImageGraphics() {
            if (null == bufferImage)
                initBufferImage() ;
            return bufferImage.getGraphics() ;
        }

        public void setGraphicsColor(Colors colors) { 
            Graphics gc = bufferImage.getGraphics();
            switch(colors) {
            case BLACK:
                gc.setColor(Color.black);
                break;
            case DARK_GRAY:
                gc.setColor(Color.darkGray);
                break;
            case WHITE:
                gc.setColor(Color.white);
                break;
            default:
                break;
            };
        }

        public void setGraphicsColor(Color color) {
            Graphics gc = bufferImage.getGraphics();
            gc.setColor(color);
        }

        public void setGraphicsColor(int r, int g, int b) {
            Graphics gc = bufferImage.getGraphics();
            Color color = new Color(r, g, b);
            gc.setColor(color);
        }

        public void fillGraphicsRect(int x, int y, int width, int height) {
            Graphics gc = bufferImage.getGraphics();
            gc.fillRect(x, y, width, height);
        }
        public void update() {
            paint(getGraphics()) ;
        }
}

Here the wrapper is being used : (The commented out code works)

 public void redraw(MazePanel mp, int state, int px, int py, int view_dx, int view_dy, int walk_step, int view_offset, RangeSet rset, int ang) {

        if (state != Constants.STATE_PLAY)
            return ;
        // new adjustment
//      this.gc = mp.getBufferImageGraphics() ;
        this.mazePanel.setBufferImage(mp.getBufferImage());
        this.rset = rset ;
        this.view_dx = view_dx ;
        this.view_dy = view_dy ;
        this.angle = ang ;

        // calculate view
        viewx = (px*map_unit+map_unit/2) + viewd_unscale(view_dx*(step_size*walk_step-view_offset));
        viewy = (py*map_unit+map_unit/2) + viewd_unscale(view_dy*(step_size*walk_step-view_offset));
        // update graphics
        // draw black background on lower half
//      gc.setColor(Color.black);
        mazePanel.setGraphicsColor(Colors.BLACK);
//      gc.fillRect(0, 0, view_width, view_height/2);
        mazePanel.fillGraphicsRect(0, 0, view_width, view_height/2);

Answer 1

I FIXED IT!!!

I realized that calling Graphics gc = bufferedImage.getGraphics() is returning a different graphics object each time!.

Thus, I set the graphics when I set the bufferedImage and then use that instance!!!

Answer 2

I'm not 100% sure of your question, but gc is a Graphics object that is obtained from the BufferedImage. If you call gc.setColor(...) it will by itself do nothing to the BufferedImage, but if you then use the gc object to say draw a line, the line will be drawn on the BufferedImage, and in the color that gc was set to.