关于用for语句打出一个菱形。。。求大神赐教

import javax.swing.JOptionPane;

public class Star
{
    public static void drawStar(int n)
    {
        for(int i=-n;i<n;i++)
        {
            for(int j=-n;j<n;j++)
            {
                if(Math.abs(i+j)<n&&Math.abs(i-j)<n)
                {
                    System.out.print("*");
                }
                else
                {                    
                    System.out.print(" ");
                }
            }  
            System.out.println();
        }   
    }
    public Star()
    {
        try{
            int num=Integer.parseInt(JOptionPane.showInputDialog(null, "请输入阶数:"));
            drawStar(num);            
        }
        catch(NumberFormatException e)
        {
            javax.swing.JOptionPane.showMessageDialog(null,"数据类型转换异常！"+e.getMessage(),"输入异常!",JOptionPane.WARNING_MESSAGE);
        }
    }
    public static void main(String[] args)
    {
        new Star();
    }
   
}