HJÆLP til UML-klassen Drums.java i Dr.Java.

Kode fra Dr. Java, klassen Drums.java:
import javax.sound.midi.*;
import java.awt.event.*;
import javax.swing.*;

/**
* This program the MIDI percussion channel with a Swing window. It moniters keystrokes and mouse motion
* in the window and uses them to create music.
* Keystrokes between 35 and 81, inclusive, generate different percussive sounds.
* See the VK_constants in java.awt.events.KeyEvents, or just eksperiment.
* Mouse position controls volume: move the mouse to the right of the window to increase the volume.
*/

public class Drums extends JFrame {
  MidiChannel channel; //The channel we play on: 10 is for percussion
  int velocity = 64; //Default volume is 50%
   
    public static void main(String[ ] args) throws MidiUnavailableException
  {
    //We don't need Sequenser in this example, since we send MIDI
    //events directly to Synthesizer instead.
   
  Synthesizer synthesizer = MidiSystem.getSynthesizer();
  synthesizer.open();
  JFrame frame = new Drums(synthesizer);
 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(50, 128); //We use the window witdh as volume control
    frame.setVisible(true);
  }
   
  public Drums(Synthesizer synth) {
    super ("Drums");
 
  // Channel 10 is the GeneralMidi percussion channel.In Java code we number channels from 0 and use channel 9 instead. to the Synthesizer instead.

  channel = synth.getChannels()[9];
 
  addKeyListener (new KeyAdapter(){
   
    public void keyPressed (KeyEvent e) {
      int key = e.getKeyCode();
      if (key>= 35 && key<= 81){
        channel.noteOn(key, velocity);
      }
    }
    public void keyReleased(KeyEvent e) {
      int key = e.getKeyCode();
      if (key >= 35 && key <= 81) channel.noteOff(key);
    }
  });
 
  addMouseMotionListener(new MouseMotionAdapter(){
    public void mouseMoved(MouseEvent e) {
      velocity = e.getX();
    }
  });
  } 
}

Spørgsmål:
Hvad er klassen Synthesizer og hvor er den defineret?
Hvad er klassen MidiChannel og hvor er den defineret?
Hvad vil der ske, når man trykker tasten 'a' på tastaturet?

Et eller flere svar, vil være en fantastisk hjælp...