JComboBox返回值 [英] JComboBox returning values
问题描述
使用哪种方法返回用户选择的选择?
What method is used to return the selection chosen by the user?
JPanel ageSelection = new JPanel();
JLabel age = new JLabel("Age:");
ArrayList<Integer> ageList = new ArrayList<Integer>();
for (int i = 1; i <= 100; ++i) {
ageList.add(i);
}
DefaultComboBoxModel<Integer> modelAge = new DefaultComboBoxModel<Integer>();
for (Integer i : ageList) {
modelAge.addElement(i);
}
JComboBox<Integer> ageEntries = new JComboBox<Integer>();
ageEntries.setModel(modelAge);
ageEntries.addActionListener(new putInTextListener());
ageSelection.add(age);
ageSelection.add(ageEntries);
class putInTextListener implements ActionListener {
public void actionPerformed (ActionEvent event) {
ageEntries.getSelectedItem();
}
}
添加最后一行(ageEntries.getSelectedItem();
)时,出现错误:
When the last line is added (ageEntries.getSelectedItem();
), I get an error:
线程"AWT-EventQueue-0"中的异常java.lang.NullPointerException
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
有什么想法吗?
修改后的代码:
class putInAgeListener implements ItemListener {
public void itemStateChanged(ItemEvent e) {
Object myAge = ageEntries.getSelectedItem();
String myAgeData = myAge.toString();
int i = Integer.parseInt(myAgeData);
System.out.print(i);
}
}
推荐答案
1)该语句为空,您可能想从当前选择的Item
1) this statement is empty and probably you want to get Integer / Object / String
value from currently selected Item
Integer / Object / String myWhatever = ageEntries.getSelectedItem();
2)最好将 ItemListener 用于 JComboBox ,而不是
2) better would be use ItemListener for JComboBox, rather than ActionListener, notice ItemListener
fired events SELECTED/DESELECTED
, always twice
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ComboBoxListeners {
private JFrame f;
private JComboBox flyFromCombo;
private JComboBox flyToCombo;
private JLabel tripLabel = new JLabel();
private Object[] itemsFrom;
private Object[] itemsTo;
public ComboBoxListeners() {
itemsFrom = new Object[]{"-", "First - From", "Second - From", "Third - From"};
itemsTo = new Object[]{"-", "First - To", "Second - To", "Third - To"};
//flyFromCombo.setPrototypeDisplayValue("################################################");
flyFromCombo = new JComboBox(itemsFrom);
flyFromCombo.addItemListener(new ItemListener() {
@Override
public void itemStateChanged(ItemEvent e) {
if ((e.getStateChange() == ItemEvent.SELECTED)) {
String str = flyFromCombo.getSelectedItem().toString();
String str1 = flyToCombo.getSelectedItem().toString();
setLabelText(str, str1);
}
}
});
flyToCombo = new JComboBox(itemsTo);
flyToCombo.addItemListener(new ItemListener() {
@Override
public void itemStateChanged(ItemEvent e) {
if ((e.getStateChange() == ItemEvent.SELECTED)) {
String str = flyFromCombo.getSelectedItem().toString();
String str1 = flyToCombo.getSelectedItem().toString();
setLabelText(str, str1);
}
}
});
tripLabel.setPreferredSize(new Dimension(400, 30));
f = new JFrame("ComboBox ItemListeners");
f.setLayout(new GridLayout(0, 1, 15, 15));
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.add(flyFromCombo);
f.add(flyToCombo);
f.add(tripLabel);
f.setLocation(150, 150);
f.pack();
f.setVisible(true);
}
private void setLabelText(String str1, String str2) {
String textForLabel = "";
String helpStringFirst = str1.trim();
if (helpStringFirst != null && helpStringFirst.length() > 0) {
if (!helpStringFirst.equals("-")) {
textForLabel = "Flight No57. from : " + helpStringFirst;
} else {
textForLabel = "Flight from Un-Know : ";
}
}
String helpStringSecond = str2.trim();
if (helpStringSecond != null && helpStringSecond.length() > 0) {
if (!helpStringSecond.equals("-")) {
textForLabel = textForLabel + " --> to : " + helpStringSecond;
} else {
textForLabel += " to : Un-Know ";
}
}
final String pushTextForLabel = textForLabel;
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
tripLabel.setText(pushTextForLabel);
}
});
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
ComboBoxListeners comboBoxListeners = new ComboBoxListeners();
}
});
}
}
编辑
我还没有(也不想)JDK7,
I haven't (and don't want too) JDK7,
import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;
import javax.swing.*;
public class ComboBoxListeners {
private JFrame f;
private JComboBox flyFromCombo;
private JLabel tripLabel = new JLabel();
public ComboBoxListeners() {
ArrayList<Integer> ageList = new ArrayList<Integer>();
for (int i = 1; i <= 100; ++i) {
ageList.add(i);
}
DefaultComboBoxModel modelAge = new DefaultComboBoxModel();
for (Integer i : ageList) {
modelAge.addElement(i);
}
flyFromCombo = new JComboBox(modelAge);
flyFromCombo.addItemListener(new ItemListener() {
@Override
public void itemStateChanged(ItemEvent e) {
if ((e.getStateChange() == ItemEvent.SELECTED)) {
String str = flyFromCombo.getSelectedItem().toString();
tripLabel.setText("Selected Age From JComboBox is : " + str);
}
}
});
tripLabel.setPreferredSize(new Dimension(400, 30));
f = new JFrame("ComboBox ItemListeners");
f.setLayout(new GridLayout(0, 1, 15, 15));
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.add(flyFromCombo);
f.add(tripLabel);
f.setLocation(150, 150);
f.pack();
f.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
ComboBoxListeners comboBoxListeners = new ComboBoxListeners();
}
});
}
}
这篇关于JComboBox返回值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!