选中时JCheckBox不更新

Question

I'm using JCheckBox to find what a costumer wants to buy, if the JCheckBox(hat) is selected then total += hatprice, but it's not updating the total. My code :

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class NgaFillimi extends JFrame implements ActionListener{

    private JCheckBox hat;
    private JLabel thetotal;
    private int total = 0;

    public NgaFillimi() {

        Container c = getContentPane();
        c.setLayout(new FlowLayout());
        hat = new JCheckBox("hat");
        thetotal = new JLabel("The total is " + total);

        hat.addActionListener(this);

        c.add(thetotal);
        c.add(hat);
    }

    @Override
    public void actionPerformed(ActionEvent evt) {
        if (hat.isSelected()) {
            total += 50;
        }
    }

    public static void main(String[] args) {
        NgaFillimi gui = new NgaFillimi();
        gui.setSize(300,200);
        gui.setVisible(true);
        gui.setDefaultCloseOperation(EXIT_ON_CLOSE);
        gui.setTitle("GUI");
    }

}

Answer 1

You're suffering from "magical thinking". Yes, the total variable is being changed, but the JLabel's text is not going to magically change on its own just because total changes. You the coder have to change it yourself by re-calling thetotal.setText(...) within your actionPerformed method once total has been changed.

Why does your code not work? Because all the JLabel knows is that its text is set once to a String and that's it. The String object that it displays never changes (nor can it since Strings are immutable). Again, the only way for the JLabel to change its display is to explicitly call its setText method.

In addition, if you fixed your code so that the JLabel appropriately updates every time the JCheckBox is selected, it won't behave in a good way, since every time the JCheckBox is un-selected and then re-selected, total increments again, which doesn't seem right. Better to remove 50 when the JCheckBox is unselected and then add 50 back when it's selected.

---

I'm trying to what you said now that I fixed it, but it's getting negative, since I have a bunch of other JCheckBoxes

Then don't add/subtract but rather consider giving your gui a method, say called sumAllCheckBoxes() and have all the JCheckBox listeners call this method whether they are checked or unchecked. The method will zero out total -- total = 0; , and then go through all the JCheckBoxes, adding cost if the JCheckBox is checked, eg:

public void sumAllCheckBoxes() {
    total = 0;
    if (someCheckBox.isSelected()) {
        total += someValue;
    }
    if (someOtherCheckBox.isSelected()) {
        total += someOtherValue;
    }

    // .... etc...

    // then set the text for theTotal here

}

At the end it sets the JLabel. The key is to think through what you want your code doing at each step of your program.

Answer 2

You haven't updated the actual JLabel . Rather, you updated the total variable. To update the JLabel , you would need code that says thetotal.setText("The total is " + total) .