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(30,30,400,400);
       f.setVisible(true);
       try{
       Sequencer player=MidiSystem.getSequencer();
       player.open();
       
       int[] eventsIWant={127};
       player.addControllerEventListener(ml,eventsIWant);
       
       Sequence sequence=new Sequence(Sequence.PPQ,4);//什么意思，待会查一查。
       Track track=sequence.createTrack();
       int r=0;
       for(int i=5;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));
            
       }
       
       player.setSequence(sequence);
       player.setTempoInBPM(120);
       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()*255);
               int green=(int) (Math.random()*255);
               int blue=(int) (Math.random()*255);
               Color randomColor=new Color(red,green,blue);
               g2d.setColor(randomColor);
               
               int ht=(int) ((Math.random()*120)+10);
               int width=(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;
           }
           
           
       }
   
}
}