(Repaint a partial area) When you repaint the entire viewing area of a panel, sometimes...

(Repaint a partial area) When you repaint the entire viewing area of a panel, sometimes only a tiny portion of the viewing area is changed. You can improve the performance by repainting only the affected area, but do not invoke super.paintComponent(g) when repainting the panel, because this will cause the entire viewing area to be cleared. Use this approach to write an applet to display the temperatures of each hour during the last 24 hours in a histogram. Suppose that temperatures between 50 and 90 degrees Fahrenheit are obtained randomly and are updated every hour. The temperature of the current hour needs to be redisplayed, while the others remain unchanged. Use a unique color to highlight the temperature for the current hour (see Figure 18.19a).

F IGURE 18.19 (a) The histogram displays the average temperature of every hour in the last 24 hours.