下面是使用 MIDI 文件和 Swing 组件创建 BeatBox 的代码。选择某些复选框会创建轨道。当我们按下“开始”按钮时，音序器正在播放。除了 buildTrackAndStart 方法的两行之外，代码中的所有内容都很清楚：
track.add (makeEvent (176, 1, 127, 0, 16));
track.add (makeEvent (192, 9, 1, 0, 15));
据我所知，消息类型 176 更改了控制器。在 Midi 协议的控制器类型手册中，编号为 127 的类型使“Poly Mode On (+ mono off, + all notes off)”。在上一章中，我们使用这种消息组合来监听事件。
消息类型 192 表示程序更改 - 乐器编号为 1 到通道编号为 9。在书中，这部分代码描述为：我们总是希望确保在 16 拍有一个事件（从 0 到 15 ）。否则，BeatBox 在重新开始之前可能无法完成完整的 16 个节拍。
没有：
track.add (makeEvent (176, 1, 127, 0, 16));
或没有：
track.add (makeEvent (192, 9, 1, 0, 15));
代码工作正常，音乐正常播放。
如果没有这两者，当第一行的复选框被选中时，音乐的节奏会连续运行。为什么在没有解释的情况下让代码更重？这两行代码实际上是做什么的？希望我错过了什么……
import com.sun.jdi.ShortType;
import java.awt.*;
import javax.swing.*;
import javax.sound.midi.*;
import java.util.*;
import java.awt.event.*;
public class BeatBox {
JPanel mainPanel;
ArrayList<JCheckBox> checkboxList;
Sequencer sequencer;
Sequence sequence;
Track track;
JFrame theFrame;
String[] instrumentNames = {"Bass Drum", "Closet Hit-Hat", "Open Hit-Hat", "Acoustic Snare", "Crash Cymbal", "Hand Clap", "High Tom", "Hi Bongo", "Maracas", "Whistle", "Low Conga", "Cowbell", "Vibraslap", "Low-mid Tom", "High Agogo", "Open Hi Conga"};
int[] instruments = {35,42,46,38,49,39,50,60,70,72,64,56,58,47,67,63};
public static void main(String[] args) {
new BeatBox().buildGUI();
}
public void buildGUI() {
theFrame = new JFrame("Cyber BeatBox");
theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
BorderLayout layout = new BorderLayout();
JPanel background = new JPanel(layout);
background.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
checkboxList = new ArrayList<JCheckBox>();
Box buttonBox = new Box(BoxLayout.Y_AXIS);
JButton start = new JButton("Start");
start.addActionListener(new MyStartListener());
buttonBox.add(start);
JButton stop = new JButton("Stop");
start.addActionListener(new MyStopListener());
buttonBox.add(stop);
JButton upTempo = new JButton("Tempo up");
start.addActionListener(new MyUpTempoListener());
buttonBox.add(upTempo);
JButton downTempo = new JButton("Tempo down");
start.addActionListener(new MyDownTempoListener());
buttonBox.add(downTempo);
Box nameBox = new Box(BoxLayout.Y_AXIS);
for (int i = 0; i < 16; i++) {
nameBox.add(new Label(instrumentNames[i]));
}
background.add(BorderLayout.EAST, buttonBox);
background.add(BorderLayout.WEST, nameBox);
theFrame.getContentPane().add(background);
GridLayout grid = new GridLayout(16,16);
grid.setVgap(1);
grid.setHgap(2);
mainPanel = new JPanel(grid);
background.add(BorderLayout.CENTER, mainPanel);
for (int i = 0; i < 256; i++) {
JCheckBox c = new JCheckBox();
c.setSelected(false);
checkboxList.add(c);
mainPanel.add(c);
} // End of cycle
setUpMidi();
theFrame.setBounds(50,50,300,300);
theFrame.pack();
theFrame.setVisible(true);
} // Closing method
public void setUpMidi() {
try {
sequencer = MidiSystem.getSequencer();
sequencer.open();
sequence = new Sequence(Sequence.PPQ,4);
track = sequence.createTrack();
sequencer.setTempoInBPM(120);
} catch(Exception e) {e.printStackTrace();}
} // Closing method
public void buildTrackAndStart() {
int[] trackList = null;
sequence.deleteTrack(track);
track = sequence.createTrack();
for (int i = 0; i < 16; i++) {
trackList = new int[16];
int key = instruments[i];
for (int j = 0; j < 16; j++) {
JCheckBox jc = (JCheckBox) checkboxList.get(j + (16 * i));
if (jc.isSelected()) {
trackList[j] = key;
} else {
trackList[j] = 0;
}
} // Closing inner cycle
makeTracks(trackList);
track.add(makeEvent(176, 1, 127, 0, 16));
} // Closing outer cycle
track.add(makeEvent(192, 9, 1, 0, 15));
try {
sequencer.setSequence(sequence);
sequencer.setLoopCount(sequencer.LOOP_CONTINUOUSLY);
sequencer.start();
sequencer.setTempoInBPM(120);
} catch (Exception e) {
e.printStackTrace();}
} // Closing method buildTrackAndStart
public class MyStartListener implements ActionListener {
public void actionPerformed(ActionEvent a) {
buildTrackAndStart();
}
} // Closing inner class
public class MyStopListener implements ActionListener {
public void actionPerformed(ActionEvent a) {
sequencer.stop();
}
} // Closing inner class
public class MyUpTempoListener implements ActionListener {
public void actionPerformed(ActionEvent a) {
float tempoFactor = sequencer.getTempoFactor();
sequencer.setTempoFactor((float) (tempoFactor * 1.03));
}
} // Closing inner class
public class MyDownTempoListener implements ActionListener {
public void actionPerformed(ActionEvent a) {
float tempoFactor = sequencer.getTempoFactor();
sequencer.setTempoFactor((float) (tempoFactor * .97));
}
} // Closing inner class
public void makeTracks(int[] list) {
for (int i = 0; i < 16; i++) {
int key = list[i];
if (key != 0) {
track.add(makeEvent(144,9,key,100,i));
track.add(makeEvent(128,9,key,100,i+1));
}
}
}
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) {e.printStackTrace();}
return event;
}
} // Closing class