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;
2 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。

  1 import javax.sound.midi.ControllerEventListener;
  2 
  3 import javax.sound.midi.MidiEvent;
  4 import javax.sound.midi.MidiSystem;
  5 import javax.sound.midi.Sequence;
  6 import javax.sound.midi.Sequencer;
  7 import javax.sound.midi.ShortMessage;
  8 import javax.sound.midi.Track;
  9 import javax.swing.JFrame;
 10 import javax.swing.*;
 11 import java.awt.*;
 12 
 13 import java.io.*;
 14 
 15 public class MiniMusicPlayer3 {
 16        //JFrame frame;
 17     static JFrame f=new JFrame("My first music Video");
 18     static MyDrawPanel ml;
 19    public static void main(String[] args){
 20        MiniMusicPlayer3 mini=new MiniMusicPlayer3();
 21        mini.go();
 22    }
 23    public void go(){
 24        //frame=new JFrame("My first music Video");
 25        //MyDrawPanel myDraw=new MyDrawPanel();
 26        ml=new MyDrawPanel();
 27        //f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 28        f.getContentPane().add(ml);
 29        //f.getContentPane(ml);
 30        f.setBounds(30,30,400,400);
 31        f.setVisible(true);
 32        try{
 33        Sequencer player=MidiSystem.getSequencer();
 34        player.open();
 35        
 36        int[] eventsIWant={127};
 37        player.addControllerEventListener(ml,eventsIWant);
 38        
 39        Sequence sequence=new Sequence(Sequence.PPQ,4);//什么意思，待会查一查。
 40        Track track=sequence.createTrack();
 41        int r=0;
 42        for(int i=5;i<60;i+=4){
 43            r=(int) ((Math.random()*50)+1);
 44            track.add(makeEvent(144,1,r,100,i));
 45            
 46            track.add(makeEvent(176,1,127,0,i));
 47             
 48            track.add(makeEvent(128,1,r,100,i+2));
 49             
 50        }
 51        
 52        player.setSequence(sequence);
 53        player.setTempoInBPM(120);
 54        player.start();
 55        }catch(Exception ex){ex.printStackTrace();}
 56        
 57        
 58        
 59    }
 60    
 61    public MidiEvent makeEvent (int comd,int chan,int one,int two,int tick){
 62         MidiEvent event=null;
 63         try{
 64             ShortMessage a=new ShortMessage();
 65             a.setMessage(comd,chan,one,two);
 66             event=new MidiEvent(a,tick);
 67         }catch(Exception e) {} 
 68         return event;
 69     }
 70 
 71 
 72    class MyDrawPanel extends JPanel implements ControllerEventListener{
 73        boolean msg=false;
 74        
 75        public void controlChange(ShortMessage event){
 76            msg=true;
 77            repaint();
 78            
 79        }
 80        
 81        public void paintComponent(Graphics g){
 82            if(msg){
 83                Graphics2D g2d=(Graphics2D) g;
 84                int red=(int) (Math.random()*255);
 85                int green=(int) (Math.random()*255);
 86                int blue=(int) (Math.random()*255);
 87                Color randomColor=new Color(red,green,blue);
 88                g2d.setColor(randomColor);
 89                
 90                int ht=(int) ((Math.random()*120)+10);
 91                int width=(int) ((Math.random()*120)+10);
 92                
 93                int x=(int) ((Math.random()*40)+10);
 94                int y=(int) ((Math.random()*40)+10);
 95                
 96                g.fillRect(x, y, width, ht);
 97                msg=false;
 98            }
 99            
100            
101        }
102    
103 }
104 }