Question

我试图写入一个代码, 将使用 JFileChoooser 选中的图像显示为另一个 JFrame 。我尝试了下面的代码, 但只有以下错误。

Exception in thread "main" java.lang.NullPointerException
at javax.swing.ImageIcon.<init>(ImageIcon.java:228)
at power.<init>(fCGUI.java:53)
at fCGUI.main(fCGUI.java:11)

这是代码:

import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

public class fCGUI
{
    public static void main(String []args)
    {
        power p=new power();
        p.setVisible(true);
    }
}

class power extends JFrame
{
    JFileChooser chooser;
    BufferedImage img;
    JButton button,button2;
    JFrame comp;
    String filename;
    File file ; 

    public power()
    {
        setSize(450,450);
        panel.setLayout(new BorderLayout());

        JPanel panel=new JPanel();
        getContentPane().add(panel);
        button =new JButton("press");

        panel.add(button,BorderLayout.NORTH);

        chooser = new JFileChooser();

        ActionListener action=new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                if (e.getSource()==button)
                {
                    chooser.showOpenDialog(null);
                    file = chooser.getSelectedFile();

                    try
                    {
                        img=ImageIO.read(file);
                    }
                    catch(IOException e1) {}
                }

                if (e.getSource()==button2)
                {
                    comp.setVisible(true);
                }
            }
        };

        ImageIcon icon=new ImageIcon(img);
        JLabel label=new JLabel(icon);

        JPanel secpanel=new JPanel();

        comp=new JFrame();
        comp.setSize(650,500);
        comp.setLayout(new BorderLayout());
        comp.setTitle("View Report");

        JRootPane compPane=comp.getRootPane();
        Container contePane=compPane.getContentPane();
        contePane.add(secpanel);

        secpanel.add(label,BorderLayout.CENTER);

        button2=new JButton("access");
        button2.addActionListener(action);
        button.addActionListener(action);

        panel.add(button2,BorderLayout.SOUTH);
    }
}

Answer 1

img 的值只有在用户单击按钮并选择要显示的文件后才会有真实值。直到此时, img 的值是 unull , 所以当它继续通过你的方法并调用行 ImageIcon 图标= newimageIcon(img); 时, 它试图为 ImageIcon 创建一个 < code> 对象, 用于 < code> null 。

要更正此选项, 您应该仅在用户选择文件时才创建 < code> ImagageIcon 。这里的更改应该更接近于正确工作。 (见以下代码中的评论 < code>/// ADDED 和 < code>// REMOVED , 以查看更改... )

...
class power extends JFrame {
    JFileChooser chooser;
    BufferedImage img;
    JButton button,button2;
    JFrame comp;
    String filename;
    File file ; 
    JLabel label; // ADDED

    public power() {
    ...
            public void actionPerformed(ActionEvent e) {
                if (e.getSource()==button) {
                    chooser.showOpenDialog(null);
                    file = chooser.getSelectedFile();

                    try {
                        img=ImageIO.read(file);
                        ImageIcon icon=new ImageIcon(img); // ADDED
                        label.setIcon(icon); // ADDED

                        Dimension imageSize = new Dimension(icon.getIconWidth(),icon.getIconHeight()); // ADDED
                        label.setPreferredSize(imageSize); // ADDED

                        label.revalidate(); // ADDED
                        label.repaint(); // ADDED
                    }
                    catch(IOException e1) {}
                }

                if (e.getSource()==button2){
                    comp.setVisible(true);
                }
            }
        };

        //ImageIcon icon=new ImageIcon(img); // REMOVED
        //JLabel label=new JLabel(icon); // REMOVED
        label = new JLabel(); // ADDED

        JPanel secpanel=new JPanel();
        ...

为了解释我改变了什么...

The label will now be created as an empty JLabel when you first start the program. It is also stored as a global variable so we can access it later
When the button is clicked, the img is created, as before, and then it is loaded into your label using setIcon();
The label is resized, then revalidate() and repaint() to make sure that the image is drawn after it is set.

Answer 2

ImageIO. read 仅在按下指定 JButton 时才执行。这是因为它位于 ActionListener 内。因此, img 是无效的, 导致 NullPointerException , 当执行 ImageIcon 图标=新图像Icon(img) 时产生 < code> NullPointerexption 。

友情链接