JAVA程序中，隨機產生壹個6位的驗證碼 (是由0-9a-ZA-Z)組成。請高手幫助，謝謝

public String get(){String str="";for (int i=0;i<6;i++) { int itmp = random.nextInt(26) + 65; char ctmp = (char)itmp; str+=ctmp; } return str;}