Combine colliding bouncing balls

BouncingBall.java

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

public class BouncingBall extends JFrame {
    
    public BouncingBall() {
        add(new BallControl());
    }
    
    public static void main(String[] args) {
        JFrame frame = new BouncingBall();
        frame.setSize(600, 500);
        frame.setLocationRelativeTo(null); // Center the frame
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        
    }
}

class BallControl extends JPanel {

    private BallPanel ballPanel = new BallPanel();
    private JButton jbtSuspend = new JButton("Suspend");
    private JButton jbtResume = new JButton("Resume");
    private JButton jbtAdd = new JButton("+1");
    private JButton jbtSubtract = new JButton("-1");
    private JScrollBar jsbDelay = new JScrollBar();

    public BallControl() {
        // Group buttons in a panel
        JPanel panel = new JPanel();
        panel.add(jbtSuspend);
        panel.add(jbtResume);
        panel.add(jbtAdd);
        panel.add(jbtSubtract);

        // Add ball and buttons to the panel
        ballPanel.setBorder(
                new javax.swing.border.LineBorder(Color.red));
        jsbDelay.setOrientation(JScrollBar.HORIZONTAL);
        ballPanel.setDelay(jsbDelay.getMaximum());
        setLayout(new BorderLayout());
        add(jsbDelay, BorderLayout.NORTH);
        add(ballPanel, BorderLayout.CENTER);
        add(panel, BorderLayout.SOUTH);

        // Register listeners
        jbtSuspend.addActionListener(new Listener());
        jbtResume.addActionListener(new Listener());
        jbtAdd.addActionListener(new Listener());
        jbtSubtract.addActionListener(new Listener());
        jsbDelay.addAdjustmentListener(new AdjustmentListener() {
            @Override
            public void adjustmentValueChanged(AdjustmentEvent e) {
                ballPanel.setDelay(jsbDelay.getMaximum() - e.getValue());
            }
        });
    }

    class Listener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == jbtSuspend) {
                ballPanel.suspend();
            } else if (e.getSource() == jbtResume) {
                ballPanel.resume();
            } else if (e.getSource() == jbtAdd) {
                ballPanel.add();
            } else if (e.getSource() == jbtSubtract) {
                ballPanel.subtract();
            }
        }
    }
}

class BallPanel extends JPanel {
    private int delay = 10;
    private ArrayList<Ball> list = new ArrayList<>();
    
    private Timer timer = new Timer(delay, new ActionListener() {
       @Override
        public void actionPerformed(ActionEvent event)  {
           repaint();
       }
    });
    
    public BallPanel() {
        timer.start();
    }
    
    public void add() {
        list.add(new Ball());
    }
    
    public void subtract() {
        list.remove(list.size() - 1);
    }
    
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (int i = 0; i < list.size(); i++) {
            Ball ball1 = list.get(i);
            g.setColor(ball1.color);
            
            if (ball1.x < 0 || ball1.x > getWidth())
                ball1.dx = -ball1.dx;
            
            if (ball1.y < 0 || ball1.y > getHeight())
                ball1.dy = -ball1.dy;
            
            ball1.x += ball1.dx;
            ball1.y += ball1.dy;
            
            if (getcollideIndex(ball1, i) != -1) {
                Ball ball2 = list.get(getcollideIndex(ball1, i));
                list.remove(ball2);
                ball1.radius += ball2.radius;
            }
            
            g.fillOval(ball1.x - ball1.radius, ball1.y - ball1.radius, 
                    ball1.radius * 2, ball1.radius * 2);
        }
    }
    
    public int getcollideIndex(Ball ball1, int index) {
        for (int i = 0; i < list.size(); i++) {
            if (i == index)
                continue;
            
            Ball ball2 = list.get(i);
            int dis = distance(ball1.x + ball1.radius, ball1.y + ball1.radius, 
                    ball2.x + ball2.radius, ball2.y + ball2.radius);
            if (dis <= Math.abs(ball1.radius - ball2.radius) || dis <= (ball1.radius + ball2.radius))
                return i;
        }
        
        return -1;
    }
    
    public static int distance(int x1, int y1, int x2, int y2) {
        return (int) Math.sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
    }
    
    public void suspend() {
        timer.stop();
    }
    
    public void resume() {
        timer.start();
    }
    
    public void setDelay(int delay) {
        this.delay = delay;
        timer.setDelay(delay);
    }
}

class Ball {
    int x = 0;
    int y = 0;
    int dx = 2;
    int dy = 2;
    int radius = 5;
    Color color = new Color((int) (Math.random() * 256), (int) 
            (Math.random() * 256), (int) (Math.random() * 256));
}