Java:使 jcombobox 的一项不可选择(如子标题)并编辑该项目的字体

Question

如何使组合框中的一个项目不可选择，因为我需要用子主题分隔组合框中的项目。

是否可以单独修改该特定项目的字体？

        jComboBox_btech_course.setFont(new java.awt.Font("Tahoma", 0, 14));
        jComboBox_btech_course.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Select Course" }));
        jComboBox_btech_course.setName("");

private class theHandler implements ActionListener
{
    public void actionPerformed(ActionEvent evt) 
    {
        //BTech courses

        if(jComboBox_mtech_dept.getSelectedItem().equals("Civil Engineering"))
        {
            jComboBox_btech_course.removeAllItems();
            jComboBox_btech_course.addItem("Building Construction");
            jComboBox_btech_course.addItem("Principle And Practice");
            jComboBox_btech_course.addItem("Surveying");
            jComboBox_btech_course.addItem("Engineering Geology");
            jComboBox_btech_course.addItem("Structural Analysis");
            jComboBox_btech_course.addItem("Hydraulic Engineering");
            jComboBox_btech_course.addItem("Environmental Engineering");
            jComboBox_btech_course.addItem("Structural Design");
            jComboBox_btech_course.addItem("Geotechnical Engineering");
            /*This item has to be unselectable*/
            jComboBox_btech_course.addItem("***Sub-topic***");
            jComboBox_btech_course.addItem("Transportation Engineering");
            jComboBox_btech_course.addItem("Foundation Engineering");
            jComboBox_btech_course.addItem("Estimation & Valuation");
            jComboBox_btech_course.addItem("Hydrology & Flood Control");
            jComboBox_btech_course.addItem("System Analysis, Project Planning And Construction Management");
            jComboBox_btech_course.addItem("Irrigation Engineering");
            jComboBox_btech_course.addItem("Computer Application in Civil Engineering");
            jComboBox_btech_course.addItem("Planning, Design & Detailing");
        }
    }
}

Answer 1

前言:在建议的解决方案中，我假设您要禁用以"**" 开头的项目。您可以将此逻辑更改为您想要的任何内容。在一个改进的版本中，MyComboModel 类(见下文)甚至可以存储哪些项目被禁用，允许任意项目被标记为禁用。

您的问题的解决方案涉及两件事:

1。禁止选择您想要禁用的项目

为此，您可以使用自定义 ComboBoxModel ，并覆盖它的 setSelectedItem()如果要选择的项目是禁用的，则不执行任何操作的方法:

class MyComboModel extends DefaultComboBoxModel<String> {
    public MyComboModel() {}
    public MyComboModel(Vector<String> items) {
        super(items);
    }
    @Override
    public void setSelectedItem(Object item) {
        if (item.toString().startsWith("**"))
            return;
        super.setSelectedItem(item);
    };
}

您可以通过将它的实例传递给 JComboBox 构造函数来设置这个新模型:

JComboBox<String> cb = new JComboBox<>(new MyComboModel());

2。用不同的字体显示禁用的项目

为此你必须使用自定义 ListCellRenderer在getListCellRendererComponent()您可以为禁用和启用项目配置不同视觉外观的方法:

Font f1 = cb.getFont();
Font f2 = new Font("Tahoma", 0, 14);

cb.setRenderer(new DefaultListCellRenderer() {
    @Override
    public Component getListCellRendererComponent(JList<?> list, Object value,
            int index, boolean isSelected, boolean cellHasFocus) {
        if (value instanceof JComponent)
            return (JComponent) value;

        boolean itemEnabled = !value.toString().startsWith("**"); 

        super.getListCellRendererComponent(list, value, index,
                isSelected && itemEnabled, cellHasFocus);

        // Render item as disabled and with different font:
        setEnabled(itemEnabled);
        setFont(itemEnabled ? f1 : f2);

        return this;
    }
});