How can I add a rectangle in BorderLayout.SOUTH?

Question

I am trying to add a thing like this in my music player application in swing.

I tried to add a rectangle to BorderLayout.SOUTH, but it never appeared on screen. Here is what I did:

public class MyDrawPanel extends JPanel {

    @Override
    public void paintComponent(Graphics g) {
        g.setColor(Color.GREEN);
        g.fillRect(200,200,200,200);

    }


    public static void main(String[] args) {
        JFrame frame = new JFrame();
        MyDrawPanel a = new MyDrawPanel();
        frame.getContentPane().add(BorderLayout.SOUTH,a);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(1000,1000);
        frame.setVisible(true);
    }

}

I just did not try 200,200,200,200, but I tried a lot of values, even with the help of a for loop, but it never appeared on screen. If I used CENTER instead of SOUTH it appeared. I read the documentation to check how fillRect works, but it simply said it added x+width and y+height. The point (0,0) is the top left corner. I checked that by adding a rectangle to CENTER layout. How cam I do it?

I did not share the output, because it was just a blank screen.

Answer 1

I thought this might make a quick little project. Here's the level meter I came up with.

The important parts are the DrawingPanel and the LevelMeterModel. The DrawingPanel takes the information from the LevelMeterModel and paints the bars on a JPanel.

The LevelMeterModel is an int array of levels, a minimum level, and a maximum level. The maximum level could be calculated, but I assumed music has a certain volume and frequency range.

The JFrame holds the DrawingPanel. A Swing Timer varies the levels somewhat randomly. The random numbers are in a small range so the bar heights don't change abruptly.

Here's the complete runnable code.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class LevelMeterGUI implements Runnable {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new LevelMeterGUI());
    }
    
    private final DrawingPanel drawingPanel;
    
    private final LevelMeterModel model;
    
    public LevelMeterGUI() {
        this.model = new LevelMeterModel();
        this.drawingPanel = new DrawingPanel(model);
    }

    @Override
    public void run() {
        JFrame frame = new JFrame("Level Meter GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        frame.add(drawingPanel, BorderLayout.CENTER);
        
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
        
        System.out.println("Frame size: " + frame.getSize());
        
        Timer timer = new Timer(250, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent event) {
                model.setRandomLevels();
                drawingPanel.repaint();
            }
        });
        timer.start();
    }
    
    public class DrawingPanel extends JPanel {

        private static final long serialVersionUID = 1L;
        
        private final int drawingWidth, drawingHeight, margin, rows;
        
        private final Dimension barDimension;
        
        private final LevelMeterModel model;
        
        public DrawingPanel(LevelMeterModel model) {
            this.model = model;
            this.margin = 10;
            this.rows = 20;
            this.barDimension = new Dimension(50, 10);
            
            int columns = model.getLevels().length;
            drawingWidth = columns * barDimension.width + (columns + 1) * margin;
            drawingHeight = rows * barDimension.height + (rows + 1) * margin;
            
            this.setBackground(Color.WHITE);
            this.setPreferredSize(new Dimension(drawingWidth, drawingHeight));
        }
        
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            
            int maximum = model.getMaximumLevel();
            double increment = (double) maximum / rows;
            
            int peak = rows * 75 / 100;
            int x = margin;
            for (int level : model.getLevels()) {
                int steps = (int) Math.round((double) level / increment);
                int y = drawingHeight - margin - barDimension.height;
                for (int index = 0; index < steps; index++) {
                    if (index < peak) {
                        g.setColor(Color.GREEN);
                    } else {
                        g.setColor(Color.RED);
                    }
                    g.fillRect(x, y, barDimension.width, barDimension.height);
                    y = y - margin - barDimension.height;
                }
                x += margin + barDimension.width;
            }
        }
        
    }
    
    public class LevelMeterModel {
        
        private final int minimumLevel, maximumLevel;
        
        private int[] levels;
        
        private final Random random;
        
        public LevelMeterModel() {
            this.minimumLevel = 100;
            this.maximumLevel = 999;
            this.levels = new int[8];
            this.random = new Random();
            setRandomLevels();
        }
        
        public void setRandomLevels() {
            for (int index = 0; index < levels.length; index++) {
                levels[index] = getRandomLevel(levels[index]);
            }
        }
        
        private int getRandomLevel(int level) {
            if (level == 0) {
                return random.nextInt(maximumLevel - minimumLevel) + minimumLevel;
            } else {
                int minimum = Math.max(level * 90 / 100, minimumLevel);
                int maximum = Math.min(level * 110 / 100, maximumLevel);
                return random.nextInt(maximum - minimum) + minimum;
            }
        }

        public int[] getLevels() {
            return levels;
        }

        public int getMinimumLevel() {
            return minimumLevel;
        }

        public int getMaximumLevel() {
            return maximumLevel;
        }
        
    }

}