15. Creating User Interfaces

[Page 451 ( continued )]

13.13. (Optional) Displaying Images

You learned how to create image icons and display them in labels and buttons in §12.10, "Image Icons." For example, the following statements create an image icon and display it in a label:

 ImageIcon icon =   new   ImageIcon(   "image/us.gif"   ); JLabel jlblImage =   new   JLabel(imageIcon); 

An image icon displays a fixed- size image. To display an image in a flexible size, you need to use the java.awt.Image class. An image can be created from an image icon using the getImage() method as follows :

 Image image = imageIcon.getImage(); 

Using a label as an area for displaying images is simple and convenient , but you don't have much control over how the image is displayed. A more flexible way to display images is to use the drawImage method of the Graphics class on a panel. Four versions of the drawImage method are shown in Figure 13.22.

Figure 13.22. You can apply the drawImage method on a Graphics object to display an image in a GUI component.

ImageObserver is an asynchronous update interface that receives notifications of image information as the image is constructed . The Component class implements ImageObserver . Therefore, every GUI component is an instance of ImageObserver . To draw images using the drawImage method in a Swing component, such as JPanel , override the paintComponent method to tell the component how to display the image in the panel.

Note

You can also create an ImageIcon from an Image object using new ImageIcon(image) .

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Listing 13.13 gives the code that displays an image from image/us.gif . The file image/us.gif (line 12) is under the class directory. The Image from the file is created in lines 11 “14. The drawImage method displays the image to fill in the whole panel, as shown in Figure 13.23.

Figure 13.23. An image is displayed in a panel.

Listing 13.13. DisplayImage.java

 1   import   java.awt.*;  2   import   javax.swing.*;  3  4   public class   DisplayImage   extends   JFrame {  5   public   DisplayImage() {  6  add(   new   ImageCanvas());  7    }  8  9   public static void   main(String[] args) { 10      JFrame frame =   new   DisplayImage(); 11      frame.setTitle(   "DisplayImage"   ); 12      frame.setSize(   300   ,   300   ); 13      frame.setLocationRelativeTo(   null   );  // Center the frame  14      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 15      frame.setVisible(   true   ); 16    } 17  } 18 19    class   ImageCanvas   extends   JPanel {  20  ImageIcon imageIcon =   new   ImageIcon(   "image/us.gif"   );  21  Image image = imageIcon.getImage();  22 23  /** Draw image on the panel */  24    public void   paintComponent(Graphics g) {  25   super   .paintComponent(g); 26 27   if   (image !=   null   ) 28  g.drawImage(image,     ,     , getWidth(), getHeight(),   this   );  29    } 30  }