java - JComboBox getSelectedIndex() 方法

Question

我正在制作一个简单的文本编辑器，您可以在其中设置字体样式、字体大小、全部清除等。为了设置字体大小，我添加了 JComboBox 并实现了 ItemListener。这是我的主窗口类:

import javax.swing.*;

public class MainWindow extends JFrame{
Editor e = new Editor();

public MainWindow(){
    super(".:My Text Editor:.");
    getContentPane().add(e);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    pack();
    setVisible(true);
}

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

}

}

这是我的编辑类:

import javax.swing.*;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

public class Editor extends JPanel{
JPanel optionPanel = new JPanel();
JTextArea editArea = new JTextArea();
JButton boldBtn = new JButton("Bold");
JButton italicBtn = new JButton("Italic");
JButton plainBtn = new JButton("Plain");
JButton clearBtn = new JButton("Clear all");
String [] fontSizes = {"10","11","12","13","14","15","16","17","18","19","20"};
int fontSize;
JComboBox combo = new JComboBox(fontSizes);

public Editor(){
    createUI();
    addEvents();
}

public void createUI(){
    optionPanel.add(boldBtn);
    optionPanel.add(italicBtn);
    optionPanel.add(plainBtn);
    optionPanel.add(combo);
    optionPanel.add(clearBtn);

    setLayout(new BorderLayout());
    add(optionPanel,BorderLayout.NORTH);
    add(new JScrollPane(editArea),BorderLayout.CENTER);
    setPreferredSize(new Dimension(640,480));
}

public void addEvents(){

    boldBtn.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
            editArea.setFont(new Font("Sans Serif",Font.BOLD,fontSize));
        }
    });

    italicBtn.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
            editArea.setFont(new Font("Sans Serif",Font.ITALIC,fontSize));

        }
    });

    plainBtn.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
            editArea.setFont(new Font("Sans Serif",Font.PLAIN,fontSize));
        }
    });

    combo.addItemListener(new ItemListener(){

        public void itemStateChanged(ItemEvent e){
            int ind = combo.getSelectedIndex();
            System.out.println(ind); 
            fontSize = Integer.parseInt(fontSizes[ind]);
            editArea.setFont(new Font("Sans Serif",Font.PLAIN,fontSize));
        }
    });

    clearBtn.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
            editArea.setText("");
        }
    });
}

}

现在，奇怪的是当我把 System.out.println(ind);行只是为了查看 getSelectedIndex() 方法返回给我的索引。根据我点击的项目，它返回给我:

1
1
0
0
2
2
3
3

为什么会这样？不应该只返回 1 0 2 3 吗？提前致谢。

Answer 1

JCombobox 为您使用 ItemEvent.getStateChanged() 区分的 SELECTED 和 DESELECTED 触发 itemStateChanged 两次。因此，将您的代码包装在 if 中，如下所示:

public void itemStateChanged( ItemEvent event ) {
    if( event.getStateChanged() == ItemEvent.SELECTED ) {
        // code here
    }
}