JAVA开发的多线程下载程序 SWING界面

				import java.util.*;
				import java.awt.*;
				import javax.swing.*;
				public class Center{
				  Dimension screenSize,frameSize;
				  JFrame frame;
				  public Center(JFrame frame){
				    this.frame=frame;
				    screenSize=Toolkit.getDefaultToolkit().getScreenSize();
				    frameSize=frame.getSize();
				  }
				  public void center(){
				    if(frameSize.height > screenSize.height){
				      frameSize.height=screenSize.height;
				    }
				    if(frameSize.width >screenSize.width){
				      frameSize.width=screenSize.width;
				    }
				    frame.setLocation((screenSize.width - frameSize.width)/2,
				           (screenSize.height - frameSize.height)/2);
				  }
				}