固执的JComboBox [英] The Stubborn JComboBox

问题描述

我在下面的代码中显示了一个JComboBox.程序启动时，它的actionPerformed事件立即触发，导致一些空指针异常，因此我想从不选择任何元素开始.但是，由于某种原因，它不起作用(无论我做什么，它总是从显示"USD/TRY"开始).有人知道吗?

I have a JComboBox shown in the code below. When the program starts its actionPerformed event fires up immediately causing some null pointer exceptions so I want to start with none of the elements selected. However, for some reason it does not work (it always start with displaying "USD/TRY" whatever I do). Anyone has any idea ?

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:构建如下所示的组合框也不会更改任何内容

UPDATE: Building the combo box like below doesnt change anything either

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);
    }
}

推荐答案

您的示例(不完整)调用

Your (incomplete) example invokes

comboBox.setSelectedIndex(0);

在显示之前就取消了，取消了以前的任何设置.在添加侦听器之前，在 之前设置所需的初始索引，不要忽略从 sscce 所示.

right before becoming visible, canceling any previous setting. Set the desired initial index before adding the listener, and don't neglect to start on the EDT, as shown in the sscce below.

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);
    }
}

这篇关于固执的JComboBox的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！