图形界面加载中字体设置问题？

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

public class GetSystemFont extends JFrame{

    private JComboBox box;
    private Container container;
    private String str[];
    private GraphicsEnvironment environment; //该类中又获取系统字体的方法；
    private JLabel tipLabel;
    
    public GetSystemFont(){
        super("获取系统字体");
        container = getContentPane();
        tipLabel = new JLabel("系统字体有：");
        container.add(tipLabel);
        
        environment = GraphicsEnvironment.getLocalGraphicsEnvironment();//GraphicsEnvironment是一个抽象类，不能实例化，只能用其中的静态方法获取一个实例
        str = environment.getAvailableFontFamilyNames();//获取系统字体
        
        box = new JComboBox(str);
        
        //为box注册ItemListener监听器
        box.addItemListener(     
            new ItemListener(){
                public void itemStateChanged(ItemEvent event){
                    if(event.getStateChange() == event.SELECTED){
                        int style = tipLabel.getFont().getStyle(); //获取当前字体的类型
                        int size = tipLabel.getFont().getSize();//获取当前字体的大小
                        tipLabel.setFont(new Font(str[box.getSelectedIndex()],style,size));//改变tipLabel的字体，但不改变大小和类型
                    }
                }
            }
        );

        container.setLayout(new FlowLayout());
        container.add(box);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        pack();
        setResizable(false);
        
        //是窗口屏幕居中
        Toolkit kit = Toolkit.getDefaultToolkit();//抽象类，通过静态方法获取实例
        Dimension frameSize = new Dimension(),screenSize = kit.getScreenSize(); //获取屏幕的大小
        getSize(frameSize); //获取窗口大小
        setLocation((screenSize.width-frameSize.width)/2,(screenSize.height-frameSize.height)/2);
    }
    public static void main(String[] args)
    {
        GetSystemFont test = new GetSystemFont();
    }
}