Simple password generator

Recently I've been working on a few projects so I hope to publish some outputs like Open Flash Chart library that allows publishing the charts using Java classes and custom JSON serializer.

Today I have something simple. The easy stuff I couldn't find out of the box so easily - password generator. There is my simple solution to this subject:

public class PasswordGenerator
{
 private static char[] characters;
 
 private static char[] special;
 
 static
 {
  String chars = "abcdefghijklmnopqrstuvxyz";
  characters = new char[chars.length()];
  for( int i = 0; i < chars.length(); i++)
  {
   characters[i] = chars.charAt( i);
  }  
  chars = "_@&*$-^#"; 
  special = new char[chars.length()];
  for( int i = 0; i < chars.length(); i++)
  {
   special[i] = chars.charAt( i);
  }   
 }

 public static String generate( int size)
 {
  StringBuilder builder = new StringBuilder();
  for( int i = 0; i < size; i++)
  {
   double rand = Math.random();
   if( rand < 0.6)
   {
    int charNum = (int)(Math.random() * (double)(characters.length - 1) * 2.0);
    
    if( charNum >= characters.length)
    {
     builder.append( Character.toUpperCase( characters[charNum - characters.length]));
    }
    else
    {
     builder.append( characters[charNum]);
    }
   }
   else if( rand < 0.8)
   {
    int number = (int)(Math.random() * 10.0);
    builder.append( number);
   }
   else
   {
    int charNum = (int)(Math.random() * (double)(special.length - 1));
    
    builder.append( special[charNum]);
   }
  }
  return builder.toString();
 }
}