为JComboBox显示一个不可选择的默认值

Question

我有一个JComboBox，包含三个项目{"Personel", "Magasinier", "Fournisseur"}。

我想让这个JComboBox显示值"Choisir une option :"，这是一个不可选择的值。

我在initComponents();之后尝试了这个代码:

this.jComboBox1.setSelectedItem("Choisir une option :");

但是它不工作。

Answer 1

您可以覆盖JComboBox模型中的选择代码，使用如下代码:

public class JComboExample {
  private static JFrame frame = new JFrame();
  private static final String NOT_SELECTABLE_OPTION = " - Select an Option - ";
  private static final String NORMAL_OPTION = "Normal Option";
  public static void main(String[] args) throws Exception {
    JComboBox<String> comboBox = new JComboBox<String>();
    comboBox.setModel(new DefaultComboBoxModel<String>() {
      private static final long serialVersionUID = 1L;
      boolean selectionAllowed = true;
      @Override
      public void setSelectedItem(Object anObject) {
        if (!NOT_SELECTABLE_OPTION.equals(anObject)) {
          super.setSelectedItem(anObject);
        } else if (selectionAllowed) {
          // Allow this just once
          selectionAllowed = false;
          super.setSelectedItem(anObject);
        }
      }
    });
    comboBox.addItem(NOT_SELECTABLE_OPTION);
    comboBox.addItem(NORMAL_OPTION);
    frame.add(comboBox);
    frame.pack();
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
        frame.setVisible(true);
      }
    });
  }
}

这将显示一个初始选择为" - Select an Option - "的组合框。一旦用户选择了另一个选项，就不可能再选择原来的选项。

Answer 2

我偶然发现了这个问题，并对邓肯的回答做了一些修改。我的解决方案是这样的:

import javax.swing.DefaultComboBoxModel;
import javax.swing.JComboBox;
public class JEComboBox<T> extends JComboBox<T> {
  public JEComboBox(final T placeHolder){
    setModel(new DefaultComboBoxModel<T>() {
      private static final long serialVersionUID = 1L;
      boolean selectionAllowed = true;
      @Override
      public void setSelectedItem(Object anObject) {
        if (!placeHolder.equals(anObject)) {
          super.setSelectedItem(anObject);
        } else if (selectionAllowed) {
          // Allow this just once
          selectionAllowed = false;
          super.setSelectedItem(anObject);
        }
      }
    });
    addItem(placeHolder);
  }
}

添加占位符时，创建一个匿名对象并覆盖toString方法。实现可以像这样:

public class car{
  String final model;
  public car(String model){
    this.model = model;
  }
}

和创建JEComboBox:

JEComboBox comboBoxWithPlaceHolder = new JEComboBox<Car>(new Car{
  public String toString(){
    return "- Select your car -"
  }
});

优点

• Combobox是通用的

弊

• 你需要实现T和Override toString()方法的匿名子类型，因此不能在final类上工作(如果comboBox持有从接口继承的类，它会变得混乱，因为匿名子类型需要实现接口，因此将有null返回方法)

Answer 3

我认为最简单和最快的方法是使用自定义的ListCellRenderer

public class TestComboBox<T> extends JComboBox<T> {
  public TestComboBox() {
    super();
    setRenderer(new ItemRenderer());
  }
  public TestComboBox(ComboBoxModel<T> aModel) {
    super(aModel);
    setRenderer(new ItemRenderer());
  }
  public TestComboBox(T[] items) {
    super(items);
    setRenderer(new ItemRenderer());
  }
  public TestComboBox(Vector<T> items) {
    super(items);
    setRenderer(new ItemRenderer());
  }
  class ItemRenderer extends DefaultListCellRenderer {
    public Component getListCellRendererComponent(JList<?> list, Object value, int index, boolean isSelected, boolean cellHasFocus){
        super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
        if (getSelectedItem() == null && index < 0){
            setText("placeholder");
        }
        return this;
    }
  }
}