API showOpenDialog()指向
showDialog()，它表示：“如果父级为null，则对话框不依赖可见窗口，并且放置在与外观相关的位置，例如屏幕的中心。”
下面的示例将选择器放置在
L＆F的屏幕中央。你可能会看到它与你的相比。
package gui;import java.awt.BorderLayout;import java.awt.Dimension;import java.awt.EventQueue;import java.awt.Graphics;import java.awt.Toolkit;import java.awt.event.ActionEvent;import java.awt.event.KeyEvent;import java.awt.image.BufferedImage;import java.io.File;import java.io.IOException;import javax.imageio.ImageIO;import javax.swing.AbstractAction;import javax.swing.Action;import javax.swing.JFileChooser;import javax.swing.Jframe;import javax.swing.JMenu;import javax.swing.JMenuBar;import javax.swing.JMenuItem;import javax.swing.JPanel;import javax.swing.JPopupMenu;import javax.swing.JScrollPane;import javax.swing.KeyStroke;public class ImageApp extends JPanel { private static final int MASK = Toolkit.getDefaultToolkit().getMenuShortcutKeyMask(); private JFileChooser chooser = new JFileChooser(); private Action openAction = new ImageOpenAction("Open"); private Action clearAction = new ClearAction("Clear"); private JPopupMenu popup = new JPopupMenu(); private BufferedImage image; public static void main(String[] args) { EventQueue.invokeLater(new Runnable() { @Override public void run() { new ImageApp().create(); } }); } public void create() { Jframe f = new Jframe(); f.setTitle("Title"); f.add(new JScrollPane(this), BorderLayout.CENTER); JMenuBar menuBar = new JMenuBar(); JMenu menu = new JMenu("File"); menu.setMnemonic('F'); menu.add(new JMenuItem(openAction)); menu.add(new JMenuItem(clearAction)); menuBar.add(menu); f.setJMenuBar(menuBar); f.setDefaultCloseOperation(Jframe.EXIT_ON_CLOSE); f.pack(); f.setSize(new Dimension(640, 480)); f.setLocationRelativeTo(null); f.setVisible(true); } public ImageApp() { this.setComponentPopupMenu(popup); popup.add("Popup Menu"); popup.add(new JMenuItem(openAction)); popup.add(new JMenuItem(clearAction)); } @Override public Dimension getPreferredSize() { if (image == null) { return new Dimension(); } else { return new Dimension(image.getWidth(), image.getHeight()); } } @Override public void paintComponent(Graphics g) { super.paintComponent(g); g.drawImage(image, 0, 0, null); } private class ClearAction extends AbstractAction { public ClearAction(String name) { super(name); this.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_C); this.putValue(Action.ACCELERATOR_KEY, KeyStroke.getKeyStroke(KeyEvent.VK_C, MASK)); } @Override public void actionPerformed(ActionEvent e) { image = null; revalidate(); repaint(); } } private class ImageOpenAction extends AbstractAction { public ImageOpenAction(String name) { super(name); this.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_O); this.putValue(Action.ACCELERATOR_KEY, KeyStroke.getKeyStroke(KeyEvent.VK_O, MASK)); } @Override public void actionPerformed(ActionEvent e) { int returnVal = chooser.showOpenDialog(chooser); if (returnVal == JFileChooser.APPROVE_OPTION) { File f = chooser.getSelectedFile(); try { image = ImageIO.read(f); revalidate(); repaint(); } catch (IOException ex) { ex.printStackTrace(System.err); } } } }}
