Static and instance variables

[Daniele Barell]

Hi all.
I'm fallowing Head First Java chapter on GUI.
Following the code:

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

public class MiniMusicPlayer3{
	static JFrame f = new JFrame("My First Music Video!");
	static MyDrawPanel dm;
	
	public static void main(String[] args){
		MiniMusicPlayer3 mini = new MiniMusicPlayer3();
		mini.go();
	}
	
	private void setUpGui(){
		dm = new MyDrawPanel();
		f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		f.setContentPane(dm);
		f.setBounds(30,30,300,300);
		f.setVisible(true);
	}
	public void go(){
		setUpGui();
		try{
			Sequencer sequencer = MidiSystem.getSequencer();
			sequencer.open();
			sequencer.addControllerEventListener(dm,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(ShortMessage.NOTE_ON,1,r,100,i));
				track.add(makeEvent(ShortMessage.CONTROL_CHANGE,1,127,0,i));
				track.add(makeEvent(ShortMessage.NOTE_OFF,1,r,100,i + 2));
			}
			
			sequencer.setSequence(seq);
			sequencer.start();
			sequencer.setTempoInBPM(120);
			
		}catch(Exception ex){
			ex.printStackTrace();
		}
	}
	public static MidiEvent makeEvent(int cmd, int chan, int one, int two, int tick){
		MidiEvent event = null;
		try{
			ShortMessage m = new ShortMessage();
			m.setMessage(cmd,chan,one,two);
			event = new MidiEvent(m,tick);
		}catch(Exception ex){
			ex.printStackTrace();
		}
		return event;
	} 
	
	class MyDrawPanel extends JPanel implements ControllerEventListener{
		boolean isMsg = false;
		public void controlChange(ShortMessage event){
			isMsg = true;
			repaint();
		}
		public void paintComponent(Graphics graphic){
			if(isMsg){
				int r = (int)(Math.random()*250);
				int g = (int)(Math.random()*250);
				int b = (int)(Math.random()*250);
				graphic.setColor(new Color(r,g,b));
				int x = (int)(Math.random()*40+10);
				int y = (int)(Math.random()*40+10);
				int w = (int)(Math.random()*120+10);
				int h = (int)(Math.random()*120+10);
				graphic.fillRect(x,y,w,h);
				isMsg = false;
			}
		}
	
	}
}

I cannot understand why the JFrame j and MyDrawPanel dm are static rather than instance variables.

[Stephan van Hulst]

Probably because the application only needs one. I agree that it's poor programming, and you should not emulate this type of code.