JComboBox 在项目选择后不更新

Question

我有一些代码错误（我将在下面显示），当用户从 JComboBox 中选择一个项目时，程序没有更新并认为没有选择任何项目。值得一提的是 .setSelectedIndex 确实会更新并让程序知道所选项目已更改，即使它是 .setSelectedIndex 更改的结果这是代码：（我将把所有项目的代码放在这里，因为我不知道在这种情况下什么是最小的可重现示例，因为我不知道是什么导致了问题）

package sample;

import sun.security.mscapi.CPublicKey;

import javax.swing.*;
import java.awt.*;
import java.awt.Button;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Main extends JFrame {
    static final int WIDTH = 600;
    static final int HEIGHT = 400;
    public static final String TITLE = "Schedule Maker";

    public JButton start;

    //Hour array
    private int[] hours = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,};
    private JTextField heading = new JTextField("Hours");
    private JTextField selectedHour = new JTextField(15);
    private JComboBox hourBox = new JComboBox();

    public Main() {
        for (int i = 0; i < hours.length; i++) {
            hourBox.addItem(hours[i]);
        }
        hourBox.setSelectedIndex(-1);

        heading.setEditable(false);
        hourBox.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent event) {
                selectedHour.setText("You selected : " + ((JComboBox) event.getSource()).getSelectedItem());
            }
        });

        int item = hourBox.getSelectedIndex();

        this.setLayout(new FlowLayout());
        this.add(heading);
        this.add(selectedHour);
        this.add(hourBox);
    }

    public void create() {

        start = new JButton();

        JFrame frame = new Main();
        JPanel panel = new JPanel();

        frame.setTitle("Title (I'm great at names i know)");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);
        frame.setVisible(true);
        frame.pack();
        frame.setTitle("TITLE");
        frame.setSize(300, 200);
        frame.setResizable(true);
        frame.setVisible(true);
        frame.add(panel);
        frame.add(start);
        start.setText("Select an hour");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ActionListener click = new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                if (e.getSource() == start) {
                    if (hourBox.getSelectedIndex() == -1) {
                        start.setText("Please select an hour");
                    } else {
                        start.setText("Some text since the rest of the program is not done yet");
                    }
                }
            }

        };

        start.addActionListener(click);

    }

    public static void main(String args[]) {
        Main m = new Main();
        m.create();
    }
}

Answer 1

我发现您的代码有几个问题。让我列出它们，从有问题的错误到样式问题；）

您正在为您的 class 创建 两个实例 。 main 方法中的第一个，create() 方法中的第二个。您在 create() 中引用的 hourBox 与您在 JFrame 中显示的对象 不同！都是很纠结的。这就是导致您出现问题的原因！请允许我整理一下……以下几点是我看到的其他问题，但它们与您的问题无关。我无论如何都会列出它们来解释我所做的更改。
JComboBox 使用的是通用类型，因此您应该指定它：JComboBox<Integer> hourBox = new JComboBox<Integer>();
有很多不必要的代码，例如 setVisible()/setTitle()/setSize()/setDefaultCloseOperation() 的重复调用或 panel 等未使用的对象。
您不需要完整的 int 值数组即可将您的时间输入组合框。看下面的代码！

试图将这些想法应用到您的代码中，我想出了这个：


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

public class Main extends JFrame {

    private static final long serialVersionUID = 8333256750721907342L;
    
    static final int WIDTH = 600;
    static final int HEIGHT = 400;
    
    public static final String TITLE = "Schedule Maker";
    private static final int HOURS = 24;

    
    public Main() {
        
        this.setTitle(TITLE);
        this.setResizable(true);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLayout(new FlowLayout());
        
        JTextField selectedHour = new JTextField(15);
        JComboBox<Integer> hourBox = new JComboBox<Integer>();
        
        for (int i = 1; i <= HOURS; i++) {
            hourBox.addItem(i); // add hour to combo box
        }
        
        hourBox.setSelectedIndex(-1);
        hourBox.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent event) {
                selectedHour.setText(
                        "You selected : " + hourBox.getSelectedItem());
                setTitle("HOUR: " + hourBox.getSelectedItem());
            }
        });

        //int item = hourBox.getSelectedIndex();

        this.add(new JTextField("Hours"));
        this.add(selectedHour);
        this.add(hourBox);
        
        JButton start = new JButton();
        start.setText("Select an hour");
        this.add(start);
        
        start.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (e.getSource() == start) {
                    if (hourBox.getSelectedIndex() == -1) {
                        start.setText("Please select an hour");
                    } else {
                        start.setText("gud boi!");
                    }
                }
            }
        });
        
        this.pack();
        this.setSize(300, 200);
        this.setLocationRelativeTo(null); // center window on screen
        this.setVisible(true);
    }

    
    public static void main(String args[]) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Main();
            }
        });
    }
    
}

...它按预期工作。

JComboBox 在项目选择后不更新

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java

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jcombobox