How can I generate a 7 character alphanumeric string?

How can I generate a 7 character alphanumeric string?

I did the stuff like this but not working. the base48Encode method
parameter I have passed the current system time in milli secs
private static final String CHARACTER_SET =
"23456789abcdefghijkmnpqrstuvwxyzABCDEFGHIJKLMNPQRSTUVWXYZ";
public static String base48Encode(double d) {
Double num = Double.valueOf(d);
Integer length = CHARACTER_SET.length();
String encodeString = new String();
while (num > length) {
encodeString = CHARACTER_SET.charAt(num.intValue() % length) +
encodeString;
num = Math.ceil(new Double(num / length) - 1);
}
encodeString = CHARACTER_SET.charAt(num.intValue()) + encodeString;
return encodeString;
}