Java编程小错误_2

import javax.sound.midi.*;
import java.io.*;
import javax.swing.*;
import java.awt.*;

public class MiniMusicPlayer3 {

    static JFrame f = new JFrame("My First Music Video");
    static MyDrawPanel ml;

    public static void main(String[] args) {
        MiniMusicPlayer3 mini = new MiniMusicPlayer3();
        mini.go();
    }

    public void setUpGui() {
        ml = new MyDrawPanel();
        f.setContentPane(ml);
        f.setBounds(30,30,300,300);
        f.setVisible(true);
    }
    public void go() {
        setUpGui();
        try {
            Sequencer sequencer = MidiSystem.getSequencer();
            sequencer.open();
            sequencer.addControllerEventListener(ml,new int[] {127});

            Sequence seq = new Sequence(Sequence.PPQ,4);
            Track track = seq.createTrack();

            int r = 0;
            for(int i = 0; i < 60;i+= 4) {
                r = (int) ((Math.random() * 50) + 1);
                track.add(makeEvent(144,1,r,100,i));

                track.add(makeEvent(176,1,127,0,i));

                track.add(makeEvent(128,1,r,100,i+2));
            }
            sequencer.setSequence(seq);
            sequencer.start();
            sequencer.setTempoInBPM(120);
        } catch (Exception ex) {ex.printStackTrace();}
    }

    public MidiEvent makeEvent(int comd,int chan,int one,int two,int tick) {
        MidiEvent event = null;
        try {
            ShortMessage a = new ShortMessage();
            a.setMessage(comd,chan,one,two);
            event = new MidiEvent(a,tick);
        } catch(Exception e) {}
        return event;
    }

    class MyDrawPanel extends JPanel implements ControllerEventListener {
        

        boolean msg = false;

        public void controlChange(ShortMessage event) {
            msg = true;
            repaint();
        }

        public void paintcomponent(Graphics g) { //应为paintComponent
            if (msg) {
                Graphics2D g2 = (Graphics2D) g;

                int r = (int) (Math.random() * 250);
                int gr = (int) (Math.random() * 250);
                int b = (int) (Math.random() * 250);

                g.setColor(new Color(r,gr,b));

                int ht = (int) ((Math.random() * 120) + 10);
                int width = (int) ((Math.random() * 120) + 10);

                int x = (int) ((Math.random() * 40) + 10);
                int y = (int) ((Math.random() * 40) + 10);

                g.fillRect(x,y,ht,width);
                msg = false;
            }
        }
    }
}