Hello,everyone,
can anybody help me with this problem?

I find code from:

http://processing.org/hacks/doku.php?id=hacks:filechooser

I modified the code, and integrated it into my sketch:

 import javax.swing.*;
 
   Button rbOpen;    
  File file;  
  PImage Buf ;
 
  void setup(){

      Buf = loadImage("wh.jpg");
      size(950,700);     // setup the size of the window
     
      background(125, 125, 125);   // setup the background color
   
      PFont metaBold;
      metaBold = loadFont("Meta-Bold.vlw.gz");  
      textFont(metaBold, 12);
     
      rbOpen = new Button(500+50,225+35,50,20,"Open");
   
  }
 
  void draw() {
    rbOpen.update();
    image(Buf,0,0,500,500);
  }
  void mouseReleased() {
      if (rbOpen.pressed()) {      
       
      try {
         UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());      
      }
      catch (Exception e) {
       e.printStackTrace();
      }
     final JFileChooser fc = new JFileChooser();
     //In response to a button click:
     int returnVal = fc.showOpenDialog(this);
   
     if (returnVal == JFileChooser.APPROVE_OPTION) {
       file = fc.getSelectedFile();
      // see if it's an image
      // (better to write a function and check for all supported extensions)
        if (file.getName().endsWith("jpg")) {
               PImage img = loadImage(file.getPath());
             if (img != null) {              
               println(file.getPath());
               //println("x");
                //size(950,700);    
               ///println(img.width*img.height);
                //image(img,0,0,500,500);      
    else {
   // just print the contents to the console
   String lines[] = loadStrings(file); // loadStrings can take a Java File Object too
   for (int i = 0; i < lines.length; i++) {
     println(lines[i]);
   }
 }
     }
      else {
        println("Open command cancelled by user.");
       }
      return;
 }
  }  
  class Button {
   
 int x, y;
 int w, h;
 color basecolor, highlightcolor;
 color currentcolor;
 boolean over = false;
 boolean pressed = false;  
 boolean locked = false;
 String btext;

 Button(int _x, int _y, int _w, int _h, String _btext) {
   x = _x;
   y = _y;
   w = _w;
   h = _h;
   basecolor =color(153, 153, 153);;
   highlightcolor = color(128, 0, 0);
   currentcolor = basecolor;
   btext = _btext;
 }
 
boolean pressed()
 {
   if(over) {
     locked = true;
     return true;
   } else {
     locked = false;
     return false;
   }    
 }
 
 void update() {
 
   if (locked==false) {
     over();
     if (over() && mousePressed) {
       currentcolor = highlightcolor;
       }
     else
       currentcolor = basecolor;
   }
   else {
     locked = false;
   }
   
   draw();
 }
 
 boolean overRect(int x, int y, int width, int height) {
   if (mouseX >= x && mouseX <= x+width &&
     mouseY >= y && mouseY <= y+height) {
     return true;
   } else {
     return false;
   }
 }

 boolean over()
 {
   if( overRect(x, y, w, h) ) {
     over = true;
     return true;
   } else {
     over = false;
     return false;
   }
 }
 
 void draw()
 {
   stroke(255);
   fill(currentcolor);
   rect(x, y, w, h);
   fill(255, 255, 255);
   text(btext, x+2, y+h*3/4);
   fill(255);
   noStroke();
 }
}

Fixed the curly brace issue and made the filedialog display.
I've always had problems using the UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());  
so I disabled it.. that's how I got it working now.

Code:

import javax.swing.*;

Button rbOpen;    
File file;    
PImage Buf ;

void setup(){

  Buf = loadImage("wh.jpg");
  size(950,700);     // setup the size of the window

  background(125, 125, 125);   // setup the background color

  PFont metaBold;  
  metaBold = loadFont("Meta-Bold.vlw.gz");  
  textFont(metaBold, 12);  

  rbOpen = new Button(500+50,225+35,50,20,"Open");

}

void draw() {
  rbOpen.update();
  image(Buf,0,0,500,500);
}

void mouseReleased() {
  if (rbOpen.pressed()) {  

    /*try {
	UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());  
    }
    catch (Exception e) {
	e.printStackTrace();  
    }*/
    final JFileChooser fc = new JFileChooser();
    //In response to a button click:
    int returnVal = fc.showOpenDialog(this);

    if (returnVal == JFileChooser.APPROVE_OPTION) {
	file = fc.getSelectedFile();
	// see if it's an image
	// (better to write a function and check for all supported extensions)
	if (file.getName().endsWith("jpg")) {
	  PImage img = loadImage(file.getPath());
	  if (img != null) {	
	    println(file.getPath());
	    //println("x");
	    //size(950,700);    
	    ///println(img.width*img.height);
	    //image(img,0,0,500,500);  
	  }
	}  
	else {
	  // just print the contents to the console
	  String lines[] = loadStrings(file); // loadStrings can take a Java File Object too
	  for (int i = 0; i < lines.length; i++) {
	    println(lines[i]);  
	  }
	}  
    }
    else {
	println("Open command cancelled by user.");
    }
    return;
  }
}    

class Button {

  int x, y;
  int w, h;
  color basecolor, highlightcolor;
  color currentcolor;
  boolean over = false;
  boolean pressed = false;    
  boolean locked = false;
  String btext;

  Button(int _x, int _y, int _w, int _h, String _btext) {
    x = _x;
    y = _y;
    w = _w;
    h = _h;
    basecolor =color(153, 153, 153);
    ;
    highlightcolor = color(128, 0, 0);
    currentcolor = basecolor;
    btext = _btext;
  }

  boolean pressed()  
  {
    if(over) {
	locked = true;
	return true;
    } 
    else {
	locked = false;
	return false;
    }    
  }

  void update() {

    if (locked==false) {
	over();
	if (over() && mousePressed) {
	  currentcolor = highlightcolor;
	}
	else
	  currentcolor = basecolor;
    }
    else {
	locked = false;
    }

    draw();
  }

  boolean overRect(int x, int y, int width, int height) {
    if (mouseX >= x && mouseX <= x+width &&  
	mouseY >= y && mouseY <= y+height) {
	return true;
    } 
    else {
	return false;
    }
  }

  boolean over()  
  {
    if( overRect(x, y, w, h) ) {
	over = true;
	return true;
    } 
    else {
	over = false;
	return false;
    }
  }

  void draw()  
  {
    stroke(255);
    fill(currentcolor);
    rect(x, y, w, h);
    fill(255, 255, 255);
    text(btext, x+2, y+h*3/4);
    fill(255);
    noStroke();
  }
}