有人可以告诉我它从哪里拾取图像吗？

Question

package demo;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/*`enter code here`
 * ComboBoxDemo.java uses these additional files:`enter code here`
 *   images/Bird.gif
 *   images/Cat.gif
 *   images/Dog.gif
 *   images/Rabbit.gif
 *   images/Pig.gif
 */
public class Demo extends JPanel
implements ActionListener {
    JLabel picture;

    @SuppressWarnings({ "rawtypes", "unchecked" })
    public Demo() {
        super(new BorderLayout());

        String[] petStrings = { "Bird", "Cat", "Dog", "Rabbit", "Pig" };

        //Create the combo box, select the item at index 4.
        //Indices start at 0, so 4 specifies the pig.
        JComboBox petList = new JComboBox(petStrings);
        petList.setSelectedIndex(4);
        petList.addActionListener(this);

        //Set up the picture.
        picture = new JLabel();
        picture.setFont(picture.getFont().deriveFont(Font.ITALIC));
        picture.setHorizontalAlignment(JLabel.CENTER);
        updateLabel(petStrings[petList.getSelectedIndex()]);
        picture.setBorder(BorderFactory.createEmptyBorder(10,0,0,0));

        //The preferred size is hard-coded to be the width of the
        //widest image and the height of the tallest image + the border.
        //A real program would compute this.
        picture.setPreferredSize(new Dimension(177, 122+10));

        //Lay out the demo.
        add(petList, BorderLayout.PAGE_START);
        add(picture, BorderLayout.PAGE_END);
        setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
    }

    /** Listens to the combo box. */
    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox)e.getSource();
        String petName = (String)cb.getSelectedItem();                                                          
        updateLabel(petName);
    }

    protected void updateLabel(String name) {
        ImageIcon icon = createImageIcon("images/" + name + ".png");
        picture.setIcon(icon);
        picture.setToolTipText("A drawing of a " + name.toLowerCase());
        if (icon != null) {
            picture.setText(null);
        } else {
            picture.setText("Image not found");
        }
    }

    /** Returns an ImageIcon, or null if the path was invalid. */
    protected static ImageIcon createImageIcon(String path) {
        java.net.URL imgURL = Demo.class.getResource(path);
        if (imgURL != null) {
            return new ImageIcon(imgURL);
        } else {
            System.err.println("Couldn't find file: " + path);
            return null;
        }
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("ComboBoxDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


        JComponent newContentPane = new Demo();
        newContentPane.setOpaque(true);
        frame.setContentPane(newContentPane);


        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

This is the code i have got from oracle but i don't know where it would need to pick images from because every time i create a source folder with images, it still wouldn't pick it up. Or if you could tell me which part of the code is picking the image up? I need to use same kinda code for my work.

Answer 1

Create a folder named images in the same folder where your Demo.java is. And place your images there. The image names, according to your code, are "Bird.png", "Cat.png", "Dog.png", "Rabbit.png", "Pig.png".

So your project structure is something like this:

demo
    src
    └───demo
        │   Demo.java
        └───images
                Bird.png
                Cat.png
                Dog.png
                Rabbit.png
                Pig.png

Answer 2

ImageIcon icon = createImageIcon("images/" + name + ".png");

This line is providing image path which is images/name.png and value of name could be among these {"Bird", "Cat", "Dog", "Rabbit", "Pig"}

picture.setIcon(icon);

The above line is setting the icon to JLabel picture .

Create a folder named images in root directory of the project and put all .png images in images folder.

Answer 3

See: http://www.thinkplexx.com/learn/howto/java/system/java-resource-loading-explained-absolute-and-relative-names-difference-between-classloader-and-class-resource-loading

I finally found out where the problem lies, but I cannot understand why they way Oracle made the code doesn't work.

Go back to the original code published at https://www.codetantra.com/java/jdk6.0/tutorial/uiswing/examples/components/ComboBoxDemoProject/src/components/ComboBoxDemo.java

You need to go to the folder where your project is. You will see: bin images src

The images folder may have all the expected image files in it, but the program does not get to them somehow. Copy the gif files into the folder that holds bin, images, and src.

You will now see: bin images src Bird.gif Cat.gif and so forth.

Now go back and change Oracle's code:

// ImageIcon icon = createImageIcon("images/" + name + ".gif"); //This way does not work.

to

    ImageIcon icon = createImageIcon(name + ".gif");

Answer 4

If you want to debug where the program fetches the files, you could use the following creating a File object and requesting it's absolute path.

Ideone

System.out.println(new File("images/Bird.png").getAbsolutePath()); // C:\... or /...