use a string that contains your alphabet and generate a random number that access the index of the string.

import java.util.Random;

public class RandomAlphabet {

      public String getRandomString(int length) {
        final StringBuilder randomString = new StringBuilder();
        final String alphabetPool = "abcdefghijklmnoABCDEFGHIJKLMNO";
        final int endRange = alphabetPool.length()-1;
        final Random rand = new Random();

        for (int i = 0; i < length; i++) {
            randomString.append(alphabetPool.charAt(rand.nextInt(endRange)));
        }

        return randomString.toString();
    }
}

something along the line of this?

or do you mean real mathematical alphabets ? within the unicode range?

import java.io.UnsupportedEncodingException;
import java.util.Random;

public class RandomAlphabet {

    public static void main(String[] args) throws UnsupportedEncodingException {
        System.out.println(RandomAlphabet.getRandomString(10));
        System.out.println(RandomAlphabet.getRandomAlphabet(10));
    }

    private static String getRandomString(int length) {
        final StringBuilder randomString = new StringBuilder();
        final String alphabetPool = "abcdefghijklmnoABCDEFGHIJKLMNO";
        final int endRange = alphabetPool.length()-1;
        final Random rand = new Random();

        for (int i = 0; i < length; i++) {
            randomString.append(alphabetPool.charAt(rand.nextInt(endRange)));
        }

        return randomString.toString();
    }

    private static String getRandomAlphabet(int length) throws UnsupportedEncodingException {
        final StringBuilder randomAlphabet = new StringBuilder();
        final Random rand = new Random();

        for (int i = 0; i < length; i++) {
            int randomInt = rand.nextInt(65535);
            String hexString = "\\u"+Integer.toHexString(randomInt);
            randomAlphabet.append(hexString);
        }

        return new String(randomAlphabet.toString().getBytes("UTF8"), "UTF-8");
    }
}

since the highest unicode value is FFFF which is 65535 you should be able to use random integer representations between 0 - 65535 to real get a random alphabet.

these are just experimental thoughts though.