1

我有一个带有 12 种不同选择的 JComboBox，根据选择的内容，我希望问题 (JLabel) 更改匹配选择。我尝试了一个 if 语句来查看选择的内容，如果它与应该选择的内容匹配，那么问题会相应地改变，但 JLabel 在某种情况下永远不会真正改变。

代码

import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.*;

public class Window extends JFrame{
    private static final long serialVersionUID = 1L;
    public Window(){
        super("Area Finder v1.0");
        BufferedImage image = null;

        try {
            image = ImageIO.read(getClass().getClassLoader().getResource("images/areafinder.png"));
        } catch (IOException e) {
            e.printStackTrace();
        }
        super.setIconImage(image);
        setSize(400, 500);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        JPanel panel = new JPanel();
        JLabel Instr = new JLabel("What would kind of area would you like to find?");
        String[] areaChoices = {"Circle", "Square", "Rectangle", "Triangle", "Trapezoid", "Parallelogram", "Hexagon", "Rhombus", "Pentagon", "Polygon", "Ellipse", "Sector"};
        final JComboBox<?> choiceBox = new JComboBox(areaChoices);
        final Object isSelected = choiceBox.getSelectedItem();
        choiceBox.setToolTipText("Select which one you want to find the area of!");
        choiceBox.setSelectedIndex(0);
        final JLabel q = new JLabel("");
        final JTextField inputFromUser = new JTextField("");
        JButton findArea = new JButton("Find Area");
        /* Question Changer*/

        /*End of Question Changer*/
        findArea.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent arg0){
                if(isSelected == "Circle"){
                    double radius = Double.parseDouble(inputFromUser.getText());
                    double area = 3.14 * (radius * radius);
                    JOptionPane.showMessageDialog(null, "Your Area is " + area);
                }else if(isSelected == "Square"){
                    
                }
                
            }
        });
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.insets = new Insets(15,15,15,15);
        panel.add(Instr);
        panel.add(choiceBox);
        panel.add(findArea);
        panel.add(q);
        panel.add(inputFromUser);
        add(panel);
    }
}

编辑：所以我用 System.out.println(); 做了一些测试。我发现它会同时调用所有项目，但首先调用的是被选中的项目。例子：

choiceBox.addItemListener(new ItemListener(){
    @Override
    public void itemStateChanged(ItemEvent e) {
        if(e.getItem() == "Circle"){
        System.out.println("Circle selected");
        }else if(e.getItem() == "Square"){
            System.out.println("Square selected");
        }
            
    }
    
});

如果您选择圆形，则打印出“Circle Selected Square Selected”，但如果您选择 Square，则打印“Square Selected Circle Selected”。

4

3 回答 3

5

添加一个ItemListener以JComboBox在选择更改时做出反应。

此外，当你这样做时：

Object isSelected = choiceBox.getSelectedItem();

...您当时只是获得选定的值；每当组合框更新时，您都不会神奇地将isSelected变量绑定到更新。getSelectedItem()如果您想要新值，则需要再次调用。

于 2013-06-08T02:09:02.457 回答
4

尝试将动作侦听器添加到 JComboBox

choiceBox.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
//put the code here
}
});
于 2013-06-08T02:13:30.353 回答
0

更改 JLabel 的文本

label.setText("Changed text");

告诉容器重新布局。

frame.invalidate();
frame.repaint();

这可以确保重新绘制框架以显示更改的标签。要知道组合框何时更改了它的选择，添加ActionListener这样的。

combo.addActionListener (new ActionListener ()
{
    public void actionPerformed(ActionEvent e)
    {
        label.setText(combo.getText());
        frame.invalidate();
    }
});
于 2013-06-08T02:12:34.583 回答