如何在 ActionListener 中使用 JLabel？

Question

I am trying to show a text saying "The background is (color)" whenever I press a button that has the name of the color written on it but the text doesn't show up.

How it is supposed to be:

but mine is like this:

Here is my code:

import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class ThreeBtn extends JFrame {
    private JButton btnRed;
    private JButton btnGreen;
    private JButton btnBlue;
    
    
     public class ButtonListener implements ActionListener {
         
         public void actionPerformed (ActionEvent e) {
             if (e.getSource() == btnRed) {
                 (getContentPane()).setBackground(Color.red);
                 JLabel label = new JLabel("빨간색 배경입니다."); //it means "The background is red" in Korean               label.setFont(new Font("Serif", Font.BOLD, 25));
                 label.setForeground(Color.yellow);
                 (getContentPane()).add(label);
             }
             else if(e.getSource()==btnGreen) {
                 (getContentPane()).setBackground(Color.green);
                 JLabel label = new JLabel("초록색 배경입니다."); //it means "The background is green" in Korean
                 label.setFont(new Font("Serif", Font.BOLD, 25));
                 label.setForeground(Color.yellow);         
                 (getContentPane()).add(label);
             }
             else if(e.getSource()==btnBlue) {
                 (getContentPane()).setBackground(Color.blue);
                 JLabel label = new JLabel("파란색 배경입니다."); //it means "The background is blue" in Korean
                 label.setFont(new Font("Serif", Font.BOLD, 25));
                 label.setForeground(Color.yellow); 
                 (getContentPane()).add(label);
             }
         }
     }
     
     public ThreeBtn() {
         setSize(300, 200);
         setTitle("Three Button Example");
         setDefaultCloseOperation(EXIT_ON_CLOSE);
         
         Container cPane = getContentPane();
         cPane.setLayout(new FlowLayout());
         btnRed = new JButton("RED");
         btnGreen = new JButton("GREEN");
         btnBlue = new JButton("Blue");
         
         ButtonListener listener = new ButtonListener();
         btnRed.addActionListener(listener);
         btnGreen.addActionListener(listener);
         btnBlue.addActionListener(listener);
         
         cPane.add(btnRed);
         cPane.add(btnGreen);
         cPane.add(btnBlue);
         

     }
     
     public static void main(String[] args) {
         (new ThreeBtn()).setVisible(true);
     }
}

I am wondering if I used JLabel in a wrong way in the ActionListener but I can't figure it out.

I would really appreciate your help.

Answer 1

When adding or deleting components to your GUI on runtime, you have to tell Swing that you did so. To do this, you should use revalidate() and repaint() on the container where the modification happened.

However, in your case, you don't actually need to always add a new JLabel on runtime, as you can simply do that when setting up the GUI, and only modifying the label that is already there. By doing it this way, you avoid having to revalidate() and repaint() (and it is also easier and cleaner this way).

I updated your code and did the following modificiations:

• Moved the declaration and initialization of the JLabel out of the actionPerformed() , so you don't have to create a new JLabel each time a button is pressed. Now, only the text is changed. (A side effect of this change is, that the revalidate() and repaint() are actually not needed anymore, as no more component is added during runtime)
• Started the GUI on the Event Dispatch Thread via SwingUtilities.invokeLater() .
• Usually it is also good practice in Swing to not create a subclass of JFrame if this is not specifically needed (like in your case). The better approach is to subclass JPanel , add the necessary components there, override getPreferredSize() and then add this to the JFrame . I have not included this change here, since it might have caused too much confusion.

Updated code:

public class ThreeBtn extends JFrame {

    private JButton btnRed;
    private JButton btnGreen;
    private JButton btnBlue;
    private JLabel label;

    public class ButtonListener implements ActionListener {

        public void actionPerformed(ActionEvent e) {
            Container contentPane = getContentPane();
            if (e.getSource() == btnRed) {
                contentPane.setBackground(Color.red);
                label.setText("red in korean");
            } else if (e.getSource() == btnGreen) {
                contentPane.setBackground(Color.green);
                label.setText("green in korean");
            } else if (e.getSource() == btnBlue) {
                contentPane.setBackground(Color.blue);
                label.setText("blue in korean");
            }
        }
    }

    public ThreeBtn() {
        setSize(300, 200);
        setTitle("Three Button Example");
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        Container cPane = getContentPane();
        cPane.setLayout(new FlowLayout());
        btnRed = new JButton("RED");
        btnGreen = new JButton("GREEN");
        btnBlue = new JButton("BLUE");
        label = new JLabel("");
        label.setFont(new Font("Serif", Font.BOLD, 25));
        label.setForeground(Color.yellow);

        ButtonListener listener = new ButtonListener();
        btnRed.addActionListener(listener);
        btnGreen.addActionListener(listener);
        btnBlue.addActionListener(listener);

        cPane.add(btnRed);
        cPane.add(btnGreen);
        cPane.add(btnBlue);
        cPane.add(label);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> (new ThreeBtn()).setVisible(true));
    }
}

Output:

This now works as per your requirements with the used FlowLayout . However, you may use another layout manager (or combine different ones) if you plan on doing more with it.

Answer 2

The Dimensions of the frame and panel where small so they did not show the text when you pressed the buttons.

Dimension size = label.getPreferredSize(); label.setBounds(150, 100, size.width, size.height);

Answer 3

I dont know if there are any rules for your program, but I would suggest, just limiting your if statements in ActionListener to the point where you set the foreground. Then make your JLabel object a class variable, and add the JLabel in the constructor. Here is the full edited code:

---

import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class ThreeBtn extends JFrame {

    private static final long serialVersionUID = 1L;
    private JButton btnRed;
    private JButton btnGreen;
    private JButton btnBlue;
    private JLabel text = new JLabel();
    
    
     public class ButtonListener implements ActionListener {
         
         public void actionPerformed (ActionEvent e) {
             if (e.getSource() == btnRed) {
                 (getContentPane()).setBackground(Color.red);
                 text.setText("red"); //it means "The background is red" in Korean   
                 text.setFont(new Font("Serif", Font.BOLD, 25));
                 text.setForeground(Color.yellow);
             }
             else if(e.getSource()==btnGreen) {
                 (getContentPane()).setBackground(Color.green);
                 text.setText("green"); //it means "The background is green" in Korean
                 text.setFont(new Font("Serif", Font.BOLD, 25));
                 text.setForeground(Color.yellow);         
             }
             else if(e.getSource()==btnBlue) {
                 (getContentPane()).setBackground(Color.blue);
                 text.setText("blue"); //it means "The background is blue" in Korean
                 text.setFont(new Font("Serif", Font.BOLD, 25));
                 text.setForeground(Color.yellow); 
                 (getContentPane()).add(text);
             }
         }
     }
     
     public ThreeBtn() {
         setSize(300, 200);
         setTitle("Three Button Example");
         setDefaultCloseOperation(EXIT_ON_CLOSE);
         
         Container cPane = getContentPane();
         cPane.setLayout(new FlowLayout());
         btnRed = new JButton("RED");
         btnGreen = new JButton("GREEN");
         btnBlue = new JButton("Blue");
         
         ButtonListener listener = new ButtonListener();
         btnRed.addActionListener(listener);
         btnGreen.addActionListener(listener);
         btnBlue.addActionListener(listener);
         
         cPane.add(btnRed);
         cPane.add(btnGreen);
         cPane.add(btnBlue);
         cPane.add(text);
         

     }
     
     public static void main(String[] args) {
         (new ThreeBtn()).setVisible(true);
     }
}