如何在ComboBox上使用ActionListener为变量赋值
鲁品
问题内容：
问题答案：
我正在使用BlueJ，但遇到了问题。我有一个带有3个选项的组合框。每个选项都是一个字符串。这三个选项是日，周和月。我想做的是，如果选择Day，则变量等于30（双精度），依此类推。有人可以帮我买吗，告诉我如何根据组合框中的所选选项为“
emailvalue”赋予值？
编辑：我要添加我的实际程序代码，它是每月的数据计算器。它确实可以编译。
这是代码：
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import java.awt.image.*;
import java.awt.geom.*;
import java.awt.Color;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
/**
* @author (Andrew Napier, Sikander Singh)
* @version (December 18th, 2012)
*/
public class MonthlyData extends JFrame
{
private JLabel title, stitle, l1,l2,l3,l4,l5,l6,l7,l8,l9,l10,l11,l12,l13,l14;
private JLabel l15,l16,l17,l18,l19,l20,l21,l22,l23,l24,l25,l26,l27,l28,l29;
private JLabel Usage,Estimated,solu1,solu2,total2;
private JPanel panel1, panel2, panel3, panel4, panel5, panel6, panel7;
String[] deviceStrings = { "Blackberry", "Other", "Internetstick"};
String[] date = { "Day", "Week", "Month"};
public double email2 = 0;
public double email1;
static final int email50 = 50;
static final int email100 = 100;
static final int email150 = 150;
static final int email200 = 200;
static final int email250 = 250;
static final int email300 = 300;
public JSlider Email = new JSlider(0,300,0);
public JSlider Web = new JSlider(0,500,0);
public JSlider IM = new JSlider(0,500,0);
public JSlider Video = new JSlider(0,300,0);
public JSlider Song = new JSlider(0,200,0);
public JSlider App_G = new JSlider(0,100,0);
public String currentdate;
public double emailvalue;
public MonthlyData ()
{
super ("MonthlyData");
setSize (1000,1100);
////////////////////////////////////////////////////// panel1
panel1 = new JPanel ();
panel1.setLayout (new GridLayout(1,1));
panel1.setVisible(true);
panel1.setBackground (Color.black);
title = new JLabel (" WANNA KNOW HOW MUCH DATA",JLabel.LEFT);
title.setForeground(Color.white);
title.setFont (new Font("Times New Roman",Font.BOLD,25));
l1 = new JLabel ("");
l2 = new JLabel ("");
/////////////////////////////////////////////// panel2
panel2 = new JPanel ();
panel2.setLayout (new GridLayout(1,1));
panel2.setVisible(true);
panel2.setBackground (Color.gray);
stitle = new JLabel (" PICK A PHONE OR INTERNET STICK AND THEN DRAG THE .",JLabel.LEFT);
stitle.setForeground(Color.black);
stitle.setFont (new Font("Times New Roman",Font.BOLD,14));
//////////////////////////////////////////////// panel3
panel3 = new JPanel ();
panel3.setLayout (new FlowLayout());
panel3.setVisible(true);
panel3.setBackground (Color.white);
l3 = new JLabel ("Select Your Device:",JLabel.RIGHT);
l3.setForeground(Color.red);
JComboBox devicelist = new JComboBox (deviceStrings);
l4 = new JLabel ("Tell us how much of what's listed here you think you'll use:");
///////////////////////////////////////////////// panel4
panel4 = new JPanel ();
panel4.setLayout (new GridLayout(6,7));
panel4.setVisible(true);
panel4.setBackground (Color.white);
l5 = new JLabel ("Email");
l6 = new JLabel ("WebPage");
l7 = new JLabel ("Instant Message");
l8 = new JLabel ("Streaming Video");
l9 = new JLabel ("Song");
l10 = new JLabel ("Application or Game");
l11 = new JLabel (""+" Emails");
final JComboBox date1 = new JComboBox (date);
date1.addActionListener(new ActionListener() {//add actionlistner to listen for change
@Override
public void actionPerformed(ActionEvent e) {
String s = (String) date1.getSelectedItem();//get the selected item
switch (s) {//check for a match
case "Day":
emailvalue = 30;
break;
case "Week":
emailvalue = 4;
break;
case "Month":
emailvalue = 1;
System.out.println("Month selected, emailvalue:" + emailvalue);
break;
}
}
});
l12 = new JLabel (" 0.002 MB");
l13 = new JLabel (""+email2+" MB");
double web1 = 0;
l14 = new JLabel (""+web1+" WebPages");
JComboBox date2 = new JComboBox (date);
l15 = new JLabel (" 0.07 MB");
double web2 = 0;
l16 = new JLabel (""+web2+" MB");
double IM1 = 0;
l17 = new JLabel (""+IM1+" IMs");
JComboBox date3 = new JComboBox (date);
l18 = new JLabel (" 0.005 MB");
double IM2 = 0;
l19 = new JLabel (""+IM2+" MB");
double video1 = 0;
l20 = new JLabel (""+video1+" Minutes");
JComboBox date4 = new JComboBox (date);
l21 = new JLabel (" 1 MB");
double video2 = 0;
l22 = new JLabel (""+video2+" MB");
double song1 = 0;
l23 = new JLabel (""+song1+" MP3's");
JComboBox date5 = new JComboBox (date);
l24 = new JLabel (" 4 MB");
double song2 = 0;
l25 = new JLabel (""+song2+" MB");
double app1 = 0;
l26 = new JLabel (""+app1+" App/Games");
JComboBox date6 = new JComboBox (date);
l27 = new JLabel (" 0.44 MB");
double app2 = 0;
l28 = new JLabel (""+app2+" MB");
//////////////////////////////////////////////// panel5
panel5 = new JPanel ();
panel5.setLayout (new GridLayout(1,1));
panel5.setVisible(true);
panel5.setBackground (Color.white);
solu1 = new JLabel ("WE THINK YOUR MONTHLY DATA USAGE WILL BE:");
solu1.setForeground (Color.red);
solu1.setFont (new Font("Times New Roman",Font.BOLD,20));
double total = 0;
total2 = new JLabel (""+total+" MB");
total2.setForeground (Color.red);
total2.setFont (new Font("Times New Roman",Font.BOLD,20));
/////////////////////////////////////////// panel6
panel6 = new JPanel ();
panel6.setLayout (new GridLayout (1,1));
panel6.setVisible (true);
panel6.setBackground (Color.white);
Usage = new JLabel ("Usage per unit");
Usage.setForeground (Color.red);
Estimated = new JLabel (" Estimated total");
Estimated.setForeground (Color.red);
////////////////////////////////////////////// panel7
panel7 = new JPanel ();
panel7.setLayout (new GridLayout (1,1));
panel7.setVisible (true);
panel7.setBackground (Color.white);
solu2 = new JLabel ("Data usage varies by device and other factors and the .");
solu2.setForeground (Color.black);
/////////////////////////////////////////////////// Implements
Container container = getContentPane ();
container.setLayout (null); //changed
container.setBackground (Color.white);
container.add (panel1).setBounds (0,0,1000,120); //panel1
panel1.add (title).setSize(500,500);
container.add (panel2).setBounds (0,100,1000,120); //panel2
panel2.add (stitle);
container.add (panel3).setBounds (0,220,1000,100); //panel3
panel3.add (l3);
panel3.add (devicelist);
panel3.add (l4);
container.add (panel4).setBounds (0,340,1000,150); //panel4
Email.addChangeListener(new SliderHandler());
Web.addChangeListener(new SliderHandler());
IM.addChangeListener(new SliderHandler());
Video.addChangeListener(new SliderHandler());
Song.addChangeListener(new SliderHandler());
App_G.addChangeListener(new SliderHandler());
panel4.add (l5);
panel4.add (Email);
panel4.add (l11);
panel4.add (date1);
panel4.add (l12);
panel4.add (l13);
panel4.add (l6);
panel4.add (Web);
panel4.add (l14);
panel4.add (date2);
panel4.add (l15);
panel4.add (l16);
panel4.add (l7);
panel4.add (IM);
panel4.add (l17);
panel4.add (date3);
panel4.add (l18);
panel4.add (l19);
panel4.add (l8);
panel4.add (Video);
panel4.add (l20);
panel4.add (date4);
panel4.add (l21);
panel4.add (l22);
panel4.add (l9);
panel4.add (Song);
panel4.add (l23);
panel4.add (date5);
panel4.add (l24);
panel4.add (l25);
panel4.add (l10);
panel4.add (App_G);
panel4.add (l26);
panel4.add (date6);
panel4.add (l27);
panel4.add (l28);
container.add (panel5).setBounds (210,540,1000,50); // panel5
panel5.add (solu1);
panel5.add (total2);
container.add (panel6).setBounds (670,220,200,220); // panel6
panel6.add (Usage);
panel6.add (Estimated);
container.add (panel7).setBounds (210,580,530,50); // panel7
panel7.add (solu2);
Email.setOpaque(false);
Web.setOpaque(false);
IM.setOpaque(false);
Video.setOpaque(false);
Song.setOpaque(false);
App_G.setOpaque(false);
setVisible (true);
}
public static void main (String[] args)
{
MonthlyData application = new MonthlyData ();
}
public class SliderHandler implements ChangeListener
{
public void stateChanged(ChangeEvent e)
{
if (e.getSource().equals(Email))
{
l11.setText(""+Email.getValue());
email2= (Email.getValue() * 0.002);
l13.setText(email2 + "MB");
}
if (e.getSource().equals(Web))
{
l14.setText(""+Web.getValue());
}
if (e.getSource().equals(IM))
{
l17.setText(""+IM.getValue());
}
if (e.getSource().equals(Video))
{
l20.setText(""+Video.getValue());
}
if (e.getSource().equals(Song))
{
l23.setText(""+Song.getValue());
}
if (e.getSource().equals(App_G))
{
l26.setText(""+App_G.getValue());
}
}
}
public void paint (Graphics g)
{
super.paint(g);
}
}
问题答案：
1）首先，您需要ActionListener添加一个JComboBox以侦听选定项的更改。
2）在方法中actionPerformed(...)，ActionListener通过返回getSelectedItem()来获取选定的项目并转换String为Object。
3）使用switch语句检查所选项目匹配的哪一项，即日，月，周，然后相应地分配一个值。
例如：
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
public class MonthlyData {
public double emailvalue;
private String[] date = {"Day", "Week", "Month"};
private JFrame frame;
public MonthlyData() {
frame = new JFrame();//use an instance rather than extending the class
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
final JComboBox date1 = new JComboBox(date);
ActionListener cbActionListener = new ActionListener() {//add actionlistner to listen for change
@Override
public void actionPerformed(ActionEvent e) {
String s = (String) date1.getSelectedItem();//get the selected item
switch (s) {//check for a match
case "Day":
emailvalue = 1.1;
System.out.println("Day selected, emailvalue:" + emailvalue);
break;
case "Week":
emailvalue = 2.2;
System.out.println("Week selected, emailvalue:" + emailvalue);
break;
case "Month":
emailvalue = 3.3;
System.out.println("Month selected, emailvalue:" + emailvalue);
break;
default:
emailvalue = 4.4;
System.out.println("No match selected, emailvalue:" + emailvalue);
break;
}
}
};
date1.addActionListener(cbActionListener);
frame.add(date1);
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
new MonthlyData();
}
});
}
}
更新：
添加SSCCE并评论后：
如果转到SliderHandler类，请输入email2 =（Email.getValue（） 0.002） emailvalue;
如果您这样做，标签将不会实时更新。但是，如果您将emailvalue替换为5，它将更新
问题是
在启动时emailvalue尚未设置。仅在第一次更改所选项目/索引时设置。
为了解决这个问题，我们必须简单地通过setSelectedItem(..)在它可见之前调用它来为它提供默认的选定值：
final JComboBox date1 = new JComboBox(date);
date1.addActionListener(new ActionListener() {//add actionlistner to listen for change
@Override
public void actionPerformed(ActionEvent e) {
String s = (String) date1.getSelectedItem();//get the selected item
switch (s) {//check for a match
case "Day":
emailvalue = 30;
break;
case "Week":
emailvalue = 4;
break;
case "Month":
emailvalue = 1;
System.out.println("Month selected, emailvalue:" + emailvalue);
break;
}
}
});
date1.setSelectedItem(date[0]);//set Day as default selected item/emailvalue
关于您代码的建议：
-
Swing组件应
EDT通过viaSwingUtilities.invokeLater(Runnable r)块创建和操纵。 -
不要覆盖
paint(..)的JFrame，而增加JPanel和覆盖paintComponent(Graphics g) -
不要调用
setBounds或setSize而使用适当的LayoutManager和/或覆盖getPreferredSize()的JPanel，并返回Dimension其适合其内容比通话S9pack()上JFrame设置它可见之前。 -
不要
JFrame不必要地扩展 -
ActionListener除非将类用作一个类，或者必须将其方法公开给其他类，否则请勿在该类上实现
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