The reason to cause non-static variable cannot be referenced from a static context

[Crystal Zeng]

Why can't the static variable named "ml" be assigned a reference value in the first place just like another static variable named "f"?
You can compare the two pictures.
The complete code is as follow.

import javax.sound.midi.*;
import java.io.*;
import javax.swing.*;
import java.awt.*;
public class MiniMusicPlayer
{
	static JFrame f=new JFrame("My First Music Video");
	static MyDrawPanel ml;
	
	public static void main(String[] args)
	{
		MiniMusicPlayer mini=new MiniMusicPlayer();
		mini.go();
	}
	public void setUpGui()
	{
		ml=new MyDrawPanel();
		f.setContentPane(ml);
		f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		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 ex)
		{
		}
		return event;
	}
	class MyDrawPanel extends JPanel implements ControllerEventListener
	{
		boolean msg=false;
		public void controlChange(ShortMessage event)
		{
			msg=true;
			repaint();
		}
		public void paintComponent(Graphics g)
		{
			if(msg)
			{
				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 width=(int)((Math.random()*120)+10);
				int ht=(int)((Math.random()*120)+10);
				
				int x=(int)((Math.random()*40)+10);
				int y=(int)((Math.random()*40)+10);
				g.fillRect(x,y,width,ht);
				msg=false;
				
				
				
			}
		}
	}
}

[Knute Snortum]

I get a different error:

Java Compiler wrote:No enclosing instance of type MiniMusicPlayer is accessible. Must qualify the allocation with an enclosing instance of type MiniMusicPlayer (e.g. x.new A() where x is an instance of MiniMusicPlayer).

This has to do with the class MyDrawPanel being a nonstatic inner class.  As the error says, you need an instance of MiniMusicPlayer to instantiate ml.  So for instance, this works:

static MyDrawPanel ml = new MiniMusicPlayer().new MyDrawPanel();

So why does it work in the method setUpGui()?  Because at that point in the code, you do have an instance of MiniMusicPlayer: it's called this.  These two lines:

ml = new MyDrawPanel();
ml = this.new MyDrawPanel();

... are semantically equivalent.

[Crystal Zeng]

Thank you so much.