import javax.swing.UIManager;
import java.awt.*;

public class helloapp {
public helloapp()
{
helloFrame frame = new helloFrame () ;
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Dimension frameSize = frame.getSize();
frame.setLocation((screenSize.width-frameSize.width)/2,(screenSize.height-frameSize.height)/2);
frame.setVisible(true);
}
public static void main ( String [] args )
{
try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

}
catch(Exception e){
e.printStackTrace();
}
new helloapp();}
}

import javax.swing.UIManager;
import javax.swing.*;
import java.awt.*;

public class Helloapp {
public Helloapp()
{
JFrame frame = new JFrame () ;
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Dimension frameSize = frame.getSize();
frame.setLocation((screenSize.width-frameSize.width)/2,(screenSize.height-frameSize.height)/2);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public static void main ( String [] args )
{
try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

}
catch(Exception e){
e.printStackTrace();
}
new Helloapp();
}
}

谢谢啊，我去试下