String chars = "abcdefghijklmnopqrstuvwxyz";
System.out.println(chars.charAt((int)(Math.random() * 26)));

------解决方案--------------------
Java code

ArrayList list = new ArrayList();
        for (char c = 'a'; c <= 'z'; c++) {
            list.add(c);
        }
        String str = "";
        for (int i = 0; i < 5; i++) {
            int num = (int) (Math.random() * 26);
            str = str + list.get(num);
        }
        System.out.println(str);