Question

这是我的代码

public class ComboBoxDemo extends JFrame {
ArrayList<Common.DescriptionPanel> cartoon = new ArrayList<Common.DescriptionPanel>();
ArrayList<ImageIcon> image = new ArrayList<ImageIcon>();
ArrayList<String> title = new ArrayList<String>();
ArrayList<String> description = new ArrayList<String>();
JComboBox combo = new JComboBox();

Common.DescriptionPanel panel = new Common.DescriptionPanel();


public static void main(String[] args) {
    new Common.SetFrame(new ComboBoxDemo(), "Combo Box");
}


public ComboBoxDemo() {
    addCartoon(new ImageIcon("c.jpg"), "Mario", "This is Mario");
    addCartoon(new ImageIcon("d.jpg"), "Sonic", "This is Sonic");
    addCartoon(new ImageIcon("e.jpg"), "Astro Boy", "This is Astro Boy");

    for (int i = 0; i < cartoon.size(); i++) {
        cartoon.get(i).setImage(image.get(i));
        cartoon.get(i).setTitle(title.get(i));
        cartoon.get(i).setDescription(description.get(i));
        combo.addItem(title.get(i));
    }

    combo.setBackground(Color.white);
    combo.setForeground(Color.blue);
    combo.setSelectedItem(cartoon.get(0));

    panel = cartoon.get(0);

    add(combo, BorderLayout.NORTH);
    add(panel, BorderLayout.CENTER);

    combo.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            panel = cartoon.get(combo.getSelectedIndex());

            pack();
            System.out.println(panel.textArea.getText());
        }
    });
}

void addCartoon(ImageIcon image, String title, String description) {
    cartoon.add(new Common.DescriptionPanel());
    this.image.add(image);
    this.title.add(title);
    this.description.add(description);

}

}

，DescriptionPanel的代码是

public class DescriptionPanel扩展了JPanel {

private JLabel imageTitle = new JLabel();
public JTextArea textArea = new JTextArea();

public DescriptionPanel() {
    imageTitle.setHorizontalAlignment(JLabel.CENTER);
    imageTitle.setHorizontalTextPosition(JLabel.CENTER);
    imageTitle.setVerticalTextPosition(JLabel.BOTTOM);
    imageTitle.setFont(Common.SetFont.boldFont);


    textArea.setLineWrap(true);   //when one line doesn't fit, it will jump to next line automatically
    /*
     * The wrapStyleWord property is set to true (line 23) so that the line is wrapped 
     * on words rather than characters. 
     */
    textArea.setWrapStyleWord(true);
    textArea.setEditable(false);
    textArea.setFont(Common.SetFont.boldFont);
    textArea.setForeground(Color.blue);

    JScrollPane scrollpane = new JScrollPane(textArea);
    setLayout(new GridLayout(1, 2)); 

    add(imageTitle);
    add(scrollpane);


}

public void setImage(ImageIcon image) {
    imageTitle.setIcon(image);
}

public void setTitle(String title) {
    imageTitle.setText(title);
}

public void setDescription(String description) {
    textArea.setText(description);
}

}

当我重新选择组合框时，JFrame根本不会改变，所以我替换代码

                panel = cartoon.get(combo.getSelectedIndex());

代码

panel.setTitle(title.get(combo.getSelectedIndex()));
            panel.setDescription(description.get(combo.getSelectedIndex()));
            panel.setImage(image.get(combo.getSelectedIndex()));

它有效。

那么这两个代码的区别是什么？在第一个代码中，面板显然会发生变化，因为当我打印出textarea时，它与初始面板不同，但JFrame不会改变。

为什么第二个代码可以工作？

Answer 1

panel = cartoon.get(combo.getSelectedIndex());

这只是将panel（它在内存中指向的内容）的引用更改为当前组合框位置中存储的内容。它不会影响panel曾引用的内容。

panel.setTitle(title.get(combo.getSelectedIndex()));
panel.setDescription(description.get(combo.getSelectedIndex()));
panel.setImage(image.get(combo.getSelectedIndex()));

更改变量panel引用的当前对象的属性。

由于之前已向框架panel添加add(panel, BorderLayout.CENTER);，因此会影响屏幕上的组件。

您永远不应该在此类组件中维护基于组件的数据。相反，你的组合框应该填充数据，它可以用来根据UI的当前需求渲染所需的结果，并且可以用来实现其他视图。这是Model-View-Controller范例的基本概念。

JFrame如何刷新自己？

1 个答案: