java - 顽固的 JComboBox

Question

我有一个 JComboBox，如下面的代码所示。当程序启动时，它的 actionPerformed 事件会立即触发，导致一些空指针异常，所以我想从没有选择任何元素开始。但是，由于某种原因它不起作用(无论我做什么，它总是以显示“美元/土耳其里拉”开始)。有人知道吗？

JComboBox comboBox = new JComboBox(new String[]{"USD/TRY", "EUR/TRY", "GBP/TRY"});

comboBox.setSelectedIndex(-1); // doesnt change anything
comboBox.setSelectedIndex(2); // doesnt change anything     
comboBox.setSelectedItem(null); // doesnt change anything

更新:像下面这样构建组合框也不会改变任何东西

JComboBox comboBox = new JComboBox(); 

comboBox.addItem("USD/TRY"); 
comboBox.addItem("EUR/TRY"); 
comboBox.addItem("GBP/TRY"); 

这是 SSCCE:

public class MainFrame {

    private final JTextArea textArea = new JTextArea();
    private IExchangeSource s;

    public MainFrame(final IExchangeSource s) {
        //build gui
        final JComboBox comboBox = new JComboBox();

        comboBox.addItem("USD/TRY");
        comboBox.addItem("EUR/TRY");
        comboBox.addItem("GBP/TRY");

        comboBox.setSelectedIndex(-1); // doesnt change anything
        //comboBox.setSelectedIndex(2); // doesnt change anything


        JFrame f = new JFrame("Currency Converter");
        JPanel p = new JPanel(new BorderLayout());
        textArea.setName("textarea");
        textArea.setWrapStyleWord(true);
        textArea.setLineWrap(true);
        this.s = s;

        comboBox.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent e) {
                String exchange = (String) comboBox.getSelectedItem();

                s.getData(exchange);
            }
        });

        p.add(comboBox, BorderLayout.NORTH);
        p.add(textArea, BorderLayout.CENTER);
        f.setPreferredSize(new Dimension(300, 300));
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.add(p);
        comboBox.setSelectedIndex(0);
        f.setVisible(true);
    }
}

Answer 1

您的(不完整的)示例调用

comboBox.setSelectedIndex(0);

就在变得可见之前，取消之前的任何设置。 在 添加监听器之前设置所需的初始索引，并且不要忘记从 EDT 开始, 如 sscce 所示下面。

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextArea;

public class MainFrame {

    private final JTextArea textArea = new JTextArea();


    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new MainFrame();
            }
        });
    }
    public MainFrame() {
        //build gui
        final JComboBox comboBox = new JComboBox();

        comboBox.addItem("USD/TRY");
        comboBox.addItem("EUR/TRY");
        comboBox.addItem("GBP/TRY");

        JFrame f = new JFrame("Currency Converter");
        JPanel p = new JPanel(new BorderLayout());
        textArea.setName("textarea");
        textArea.setWrapStyleWord(true);
        textArea.setLineWrap(true);

        comboBox.setSelectedIndex(-1);
        comboBox.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                System.out.println(comboBox.getSelectedItem() + ": " + e);
            }
        });

        p.add(comboBox, BorderLayout.NORTH);
        p.add(textArea, BorderLayout.CENTER);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.setSize(new Dimension(300, 300));
        f.add(p);
        f.setVisible(true);
    }
}