How do I update a JLabel everytime a JComboBox cha

问题:

I have a JComboBox with 12 different selections, and depending on what is selected I want the question (JLabel) to change matching the selection. I've tried an if statement to see what is selected and if it matches what should be selected, then the question changes accordingly, but the JLabel never really changes under an circumstance.

Code

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);
    }
}

EDIT: So I did a few tests with System.out.println(); and I figured out it's calling all the items at once, but the one that's selected is being called first. Example:

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");
        }

    }

});

Prints out "Circle Selected Square Selected" if you select circle, but "Square Selected Circle Selected" if you select Square.

回答1:

Add an ItemListener to the JComboBox to react when the selection changes.

Also, when you do this:

Object isSelected = choiceBox.getSelectedItem();

... you are just getting the selected value at that time; you aren't magically binding the isSelected variable to update whenever the combobox updates. You need to call getSelectedItem() again if you want the new value.

回答2:

try adding an action listener to the JComboBox

choiceBox.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
//put the code here
}
});

回答3:

Change the text of the JLabel

label.setText("Changed text");

Tell the container to relayout.

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

This makes sure that the frame is redrawn to show the changed label. To know when the combo box has changed it's selection, add an ActionListener like this.

combo.addActionListener (new ActionListener ()
{
    public void actionPerformed(ActionEvent e)
    {
        label.setText(combo.getText());
        frame.invalidate();
    }
});