Swing traffic light with exclusive selection

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Locale;

public class Main {

javax.swing supplies buttons, labels, panels and frames. java.awt supplies layouts, colours and fonts. java.awt.event supplies listener/event types. Locale is available where text formatting needs it. Main matches the source filename.

    static JPanel root(String title) {
        for(String key:new String[]{"Label.font","Button.font","RadioButton.font","CheckBox.font","TextField.font","TextArea.font","ComboBox.font"})UIManager.put(key,new Font(Font.SANS_SERIF,Font.PLAIN,18));

root configures the selected UIManager font keys to an 18-point sans-serif font before creating the form’s components. This affects the Java application, not the website’s CSS.

        JPanel panel=new JPanel(new BorderLayout(12,12));panel.setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
        JLabel heading=new JLabel(title);heading.setFont(new Font(Font.SANS_SERIF,Font.BOLD,24));panel.add(heading,BorderLayout.NORTH);return panel;
    }

BorderLayout places the heading in NORTH and leaves CENTER for each experiment’s form. The empty border adds padding; the heading uses a larger bold font. Return the JPanel for further assembly.

    static void show(String title,JPanel panel) {
        JFrame frame=new JFrame(title);frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);frame.setContentPane(panel);frame.pack();frame.setLocationRelativeTo(null);frame.setVisible(true);
    }

show creates a JFrame, installs its content, packs it to component sizes, centres it and makes it visible. EXIT_ON_CLOSE ends this demonstration application when its window closes. The desktop main calls this helper on the event-dispatch thread.

    static class Lights extends JPanel {
        private static final long serialVersionUID=1L;
        String selected="";Lights(){setPreferredSize(new Dimension(320,330));}

Lights extends JPanel, declares a serialization identifier and stores the selected colour name. The constructor sets a preferred drawing size; an empty selected string leaves all lights inactive initially.

        @Override protected void paintComponent(Graphics g){
            super.paintComponent(g);int x=(getWidth()-100)/2;g.setColor(new Color(35,45,58));g.fillRoundRect(x,10,100,300,28,28);

paintComponent overrides Swing’s drawing hook. Call super first, calculate a centred x position and fill a rounded dark rectangle. Swing calls this method when repainting is needed; user code does not call it directly.

            String[] names={"Red","Yellow","Green"};Color[] colors={Color.RED,Color.YELLOW,Color.GREEN};
            for(int i=0;i<3;i++){g.setColor(selected.equals(names[i])?colors[i]:new Color(75,85,95));g.fillOval(x+15,25+i*95,70,70);}
        }
    }

Parallel arrays map Red, Yellow and Green to Color values. Loop through three positions, choose bright colour only for the selected name, and fill each circle. The braces close the paint method and Lights class.

    public static JPanel createPanel(){
        JPanel panel=root("Traffic Light");Lights lights=new Lights();lights.setName("lights");panel.add(lights,BorderLayout.CENTER);
        ButtonGroup group=new ButtonGroup();JPanel controls=new JPanel();JLabel status=new JLabel("No light selected");status.setName("status");

createPanel creates Lights, names it for tests and adds it to CENTER. A ButtonGroup allows only one signal selection. The status label initially says no light is selected.

        for(String name:new String[]{"Red","Yellow","Green"}){JRadioButton button=new JRadioButton(name);group.add(button);controls.add(button);button.addActionListener(e->{lights.selected=name;status.setText(name+" light on");lights.repaint();});}

Each radio-button lambda saves its own colour name into lights.selected, updates the status and calls repaint. repaint requests a later paintComponent call rather than drawing directly inside the action listener.

        controls.add(status);panel.add(controls,BorderLayout.SOUTH);return panel;
    }

Add status to the controls, place them in SOUTH, then return panel. Button events can update the same Lights object after createPanel has returned.

    public static void main(String[] args){SwingUtilities.invokeLater(()->show("Traffic Light",createPanel()));}

}

SwingUtilities.invokeLater schedules the lambda on Swing’s event-dispatch thread. The lambda creates the panel and calls show. This main needs a graphical desktop; the panel and its event handlers are separate from console input.