我正在学习Swing并编写了一个应用程序，允许用户选择一个图像文件并将其显示在JPanel上。它有效，但我想处理
在这些情况下，我想清除JPanel并在文本区域显示错误消息。
我尝试按照以下方式执行此操作。但我不确定这是否是正确执行此操作的方式。我希望您的建议。
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import javax.swing.*;
import javax.swing.filechooser.FileNameExtensionFilter;
class Display extends JFrame {
private MyImagePanel canvas;
private JButton okbutton;
private JTextArea result;
private JFileChooser filechooser;
private static Insets insets = new Insets(0, 0, 0, 0);
public Display(String name) {
super(name);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLayout(new GridBagLayout());
addWidgets();
showGUI();
}
private void showGUI() {
this.pack();
this.setVisible(true);
}
private void addWidgets() {
canvas = new MyImagePanel();
okbutton = new JButton("OK");
filechooser = new JFileChooser("Select imagefile");
FileNameExtensionFilter filter = new FileNameExtensionFilter(
"Images", "jpg", "JPG", "GIF", "gif", "JPEG", "png", "PNG");
filechooser.setFileFilter(filter);
filechooser.setControlButtonsAreShown(false);
result = new JTextArea(10, 10);
addComponent(filechooser, 0, 0, 2, 4, GridBagConstraints.CENTER, GridBagConstraints.NONE);
addComponent(canvas, 2, 0, 2, 2, GridBagConstraints.CENTER, GridBagConstraints.NONE);
addComponent(result, 2, 2, 2, 1, GridBagConstraints.CENTER, GridBagConstraints.NONE);
addComponent(okbutton, 3, 3, 1, 1, GridBagConstraints.CENTER, GridBagConstraints.NONE);
}
public void addOKButtonListener(ActionListener okl) {
okbutton.addActionListener(okl);
}
public void displayResult(String msg) {
result.setText(msg);
}
public void clearResultField() {
result.setText("");
}
public void displayImage(String imagefilename) {
canvas.setImage(imagefilename);
}
public String getSelectedFile() {
java.io.File selectedFile = filechooser.getSelectedFile();
String filePathName = "";
if (selectedFile == null) {
result.setText("select a file");
} else {
filePathName = selectedFile.getPath();
}
return filePathName;
}
public void addComponent(Component component, int gridx, int gridy,
int gridwidth, int gridheight, int anchor, int fill) {
GridBagConstraints gbc = new GridBagConstraints(gridx, gridy,
gridwidth, gridheight, 1.0, 1.0, anchor, fill, insets, 0, 0);
this.add(component, gbc);
}
}//end class Display
class MyImagePanel extends JPanel {
private BufferedImage bi;
public MyImagePanel() {
super();
bi = null;
}
public void setImage(String imagefilename) {
try {
bi = ImageIO.read(new File(imagefilename));
this.setPreferredSize(new Dimension(bi.getWidth(), bi.getHeight()));
} catch (Exception e) {
bi = null;
}
this.revalidate();
this.repaint();
}
@Override
public void paintComponent(Graphics g) {
// clear the background
g.setColor(getBackground());
g.fillRect(0, 0, getWidth(), getHeight());
if (bi != null) {
// draw the image
g.drawImage(bi, 0, 0, null);
}
}
}
class GUIController {
private Display display;
public GUIController(Display d) {
display = d;
display.addOKButtonListener(new OKButtonListener());
}
class OKButtonListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
display.clearResultField();//make result field blank
String fileselection = display.getSelectedFile();
if (fileselection.length() > 0) {
display.displayImage(fileselection);
} else {
display.displayResult("actionPerformed:no file selected");
display.displayImage("");
}
}
}
}
public class ImageDisplay {
public static void main(String[] args) {
Display d = new Display("image demo");
GUIController ctrl = new GUIController(d);
}
}
答案 0 :(得分：5)
如果drawImage()为img，则java.awt.Graphics中的多个null方法“无效”。因此，将图像设置为null就足够了。您明确清除了背景，但super.paintComponent(g)是将面板清除为背景颜色的替代方法。
附录：您可能还想研究文章How to Use File Choosers和Working with Images中的示例。
附录：我使用了不同的布局，并将图像添加到JScrollPane。我还setImage()返回一个结果，让Display知道发生了什么。
附录：这个更新，更简单的revision扩展JFileChooser来直接处理批准和取消。
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.*;
import javax.imageio.ImageIO;
import javax.swing.*;
import javax.swing.filechooser.FileNameExtensionFilter;
/** @see https://stackoverflow.com/questions/4053090 */
public class ImageDisplay extends JFrame {
private static final String title = "Select a file";
private MyImagePanel imagePanel = new MyImagePanel();
private JLabel result = new JLabel(title, JLabel.CENTER);
private MyChooser fileChooser = new MyChooser();
public ImageDisplay(String name) {
super(name);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.addWidgets();
this.pack();
this.setVisible(true);
}
private void addWidgets() {
FileNameExtensionFilter filter = new FileNameExtensionFilter(
"Images", "jpg", "JPG", "GIF", "gif", "JPEG", "png", "PNG");
fileChooser.setFileFilter(filter);
this.add(fileChooser, BorderLayout.WEST);
this.add(new JScrollPane(imagePanel), BorderLayout.CENTER);
this.add(result, BorderLayout.SOUTH);
}
class MyChooser extends JFileChooser {
@Override
public void approveSelection() {
File f = fileChooser.getSelectedFile();
if (imagePanel.setImage(f)) {
result.setText(f.getName());
} else {
result.setText(title);
}
}
@Override
public void cancelSelection() {
imagePanel.setImage(null);
result.setText(title);
}
}
class MyImagePanel extends JPanel {
private BufferedImage bi;
public MyImagePanel() {
this.setPreferredSize(new Dimension(500, 700));
}
/** Return true if read() succeeded. */
public boolean setImage(File f) {
try {
bi = ImageIO.read(f);
} catch (Exception e) {
bi = null;
}
if (bi != null) {
setPreferredSize(new Dimension(bi.getWidth(), bi.getHeight()));
}
this.revalidate();
this.repaint();
return bi != null;
}
@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawImage(bi, 0, 0, null);
}
}
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
new ImageDisplay("Image Demo").setVisible(true);
}
});
}
}