java中的全局变量与静态变量的区别与联系？有时候为什么专门定义静态变量。(标题党~~)

static代表“每个类一个”而不是“每个对象一个”。即静态变量是类的所有对象共有的。

1 static JFrame f;
 static MyDrawPanel ml;

整个应用程序如下：

定义在public void go(){...}里面的变量的生存空间仅仅是这个方法，出了这个方法，其他的类和方法是没法调用这个·方法里定义的变量的。

 import javax.sound.midi.ControllerEventListener;

 import javax.sound.midi.MidiEvent;
 import javax.sound.midi.MidiSystem;
 import javax.sound.midi.Sequence;
 import javax.sound.midi.Sequencer;
 import javax.sound.midi.ShortMessage;
 import javax.sound.midi.Track;
 import javax.swing.JFrame;
 import javax.swing.*;
 import java.awt.*;

 import java.io.*;

 public class MiniMusicPlayer3 {
        //JFrame frame;
     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 go(){
        //frame=new JFrame("My first music Video");
        //MyDrawPanel myDraw=new MyDrawPanel();
        ml=new MyDrawPanel();
        //f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(ml);
        //f.getContentPane(ml);
        f.setBounds(,,,);
        f.setVisible(true);
        try{
        Sequencer player=MidiSystem.getSequencer();
        player.open();

        int[] eventsIWant={};
        player.addControllerEventListener(ml,eventsIWant);

        Sequence sequence=new Sequence(Sequence.PPQ,);//什么意思，待会查一查。
        Track track=sequence.createTrack();
        int r=;
        for(int i=;i<;i+=){
            r=(int) ((Math.random()*)+);
            track.add(makeEvent(,,r,,i));

            track.add(makeEvent(,,,,i));

            track.add(makeEvent(,,r,,i+));

        }

        player.setSequence(sequence);
        player.setTempoInBPM();
        player.start();
        }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){
            if(msg){
                Graphics2D g2d=(Graphics2D) g;
                int red=(int) (Math.random()*);
                int green=(int) (Math.random()*);
                int blue=(int) (Math.random()*);
                Color randomColor=new Color(red,green,blue);
                g2d.setColor(randomColor);

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

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

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

        }

 }
 }