Java小球碰撞并控制数量

package three.day;
 
import java.awt.event.*;
import javax.swing.*;
 
public class Windows extends JFrame{
	DrowJPs jp=new DrowJPs();
	public void init() {
		this.setSize(800,500);
		this.setLocationRelativeTo(rootPane);
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		this.setTitle("天沫丶寒枫");
		this.add(jp);
		this.setVisible(true);
	}
	
	public static void main(String[] args) {
		Windows win=new Windows();
		win.init();
	}
 
}

package three.day;
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;
 
public class DrowJPs extends JPanel implements Runnable{
	int[] x=new int[1000],y=new int[1000],s=new int[1000],xt=new int[1000],yt=new int[1000];
	int[] r=new int[1000],g=new int[1000],b=new int[1000];
	int num=5;
	public DrowJPs() {
		for (int i = 0; i < 1000; i++) {
			x[i]=(int)(Math.random()*450);
			y[i]=(int)(Math.random()*230);
			r[i]=(int)(Math.random()*256);
			g[i]=(int)(Math.random()*256);
			b[i]=(int)(Math.random()*256);
			xt[i]=(int)(Math.random()*4+1);
			yt[i]=(int)(Math.random()*4+1);
			s[i]=(int)(Math.random()*200+20);
		}
		Thread t=new Thread(this);
		Thread t1=new Thread(this);
		t.start();
		t1.start();
	}
	public void paint(Graphics gr) {
		super.paint(gr);
		setBackground(Color.pink);
		for (int i = 0; i < num; i++) {
			gr.setColor(new Color(r[i],g[i],b[i]));
			gr.fillOval(x[i], y[i], s[i], s[i]);
		}
	}
	public void run() {
		while(true) {
			for (int i = 0; i < num; i++) {
				if(x[i]<=0|x[i]>=(790-s[i]))xt[i]*=-1;
				if(y[i]<=0|y[i]>=(465-s[i]))yt[i]*=-1;
				x[i]+=xt[i];y[i]+=yt[i];
				try {
					Thread.sleep(10);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				repaint();
			}
		}
	}
}

        JButton btn = new JButton("增加一个小球");
		JButton btn1 = new JButton("减少一个小球");
		btn.setBounds(0, 0, 400, 600);
		btn1.setBounds(400, 0, 400, 600);
		this.add(btn);
		this.add(btn1);
		btn.addActionListener(new MyListener());
		btn1.addActionListener(new MyListener1());

    class MyListener implements ActionListener{
		public void actionPerformed(ActionEvent e) {
		jp.addnum(0);
		}
	}
	class MyListener1 implements ActionListener{
		public void actionPerformed(ActionEvent e) {
		jp.addnum(1);
		}
	}

public void addnum(int i) {
		if(i==0)num++;
		else num--;
}