[英]How do I update a JLabel everytime a JComboBox changes?
我有一个带有12个不同选择的JComboBox,并且根据选择的内容,我希望问题(JLabel)更改与选择匹配的内容。 我尝试了一个if语句来查看选择的内容,如果它与应选择的内容相匹配,则问题会相应更改,但是JLabel在任何情况下都不会真正更改。
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.*;
public class Window extends JFrame{
private static final long serialVersionUID = 1L;
public Window(){
super("Area Finder v1.0");
BufferedImage image = null;
try {
image = ImageIO.read(getClass().getClassLoader().getResource("images/areafinder.png"));
} catch (IOException e) {
e.printStackTrace();
}
super.setIconImage(image);
setSize(400, 500);
setLocationRelativeTo(null);
setDefaultCloseOperation(EXIT_ON_CLOSE);
JPanel panel = new JPanel();
JLabel Instr = new JLabel("What would kind of area would you like to find?");
String[] areaChoices = {"Circle", "Square", "Rectangle", "Triangle", "Trapezoid", "Parallelogram", "Hexagon", "Rhombus", "Pentagon", "Polygon", "Ellipse", "Sector"};
final JComboBox<?> choiceBox = new JComboBox(areaChoices);
final Object isSelected = choiceBox.getSelectedItem();
choiceBox.setToolTipText("Select which one you want to find the area of!");
choiceBox.setSelectedIndex(0);
final JLabel q = new JLabel("");
final JTextField inputFromUser = new JTextField("");
JButton findArea = new JButton("Find Area");
/* Question Changer*/
/*End of Question Changer*/
findArea.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent arg0){
if(isSelected == "Circle"){
double radius = Double.parseDouble(inputFromUser.getText());
double area = 3.14 * (radius * radius);
JOptionPane.showMessageDialog(null, "Your Area is " + area);
}else if(isSelected == "Square"){
}
}
});
GridBagConstraints gbc = new GridBagConstraints();
gbc.insets = new Insets(15,15,15,15);
panel.add(Instr);
panel.add(choiceBox);
panel.add(findArea);
panel.add(q);
panel.add(inputFromUser);
add(panel);
}
}
编辑:所以我用System.out.println()做了一些测试; 我发现它可以一次调用所有项目,但是被选中的项目将被首先调用。 例:
choiceBox.addItemListener(new ItemListener(){
@Override
public void itemStateChanged(ItemEvent e) {
if(e.getItem() == "Circle"){
System.out.println("Circle selected");
}else if(e.getItem() == "Square"){
System.out.println("Square selected");
}
}
});
如果选择圆,则输出“选择圆选定的圆”,如果选择“平方”,则打印出“选定圆的选定圆”。
将ItemListener
添加到JComboBox
以在选择更改时做出反应。
另外,当您这样做时:
Object isSelected = choiceBox.getSelectedItem();
...您当时只是获得选定的值; 您不会神奇地绑定isSelected
变量以在组合框更新时进行更新。 如果需要新值,则需要再次调用getSelectedItem()
。
尝试将动作侦听器添加到JComboBox
choiceBox.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
//put the code here
}
});
更改JLabel的文本
label.setText("Changed text");
告诉容器重新布局。
frame.invalidate();
frame.repaint();
这样可以确保重新绘制框架以显示更改后的标签。 要知道组合框何时更改,请添加一个这样的ActionListener
。
combo.addActionListener (new ActionListener ()
{
public void actionPerformed(ActionEvent e)
{
label.setText(combo.getText());
frame.invalidate();
}
});
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