Quick and dirty implementation of the Java’s InputStream, that can generate a desired number of random characters, without buffering it in memory. May be useful for unit testing.

import org.apache.commons.lang3.RandomStringUtils;
import java.io.IOException;
import java.io.InputStream;

public class RandomCharsInputStream extends InputStream {

    private static final int ALPHABETIC = 1;

    private static final int ALPHANUMERIC = 2;

    private static final int ASCII = 3;

    private static final int NUMERIC = 4;

    private static final int CHARS = 5;

    private int type = ALPHABETIC;

    private long size = 0;

    private long charsRead = 0;

    private char[] chars;

    public static final RandomCharsInputStream newAlphabetic(long size) {
        return new RandomCharsInputStream(size, ALPHABETIC);
    }

    public static final RandomCharsInputStream newAlphanumeric(long size) {
        return new RandomCharsInputStream(size, ALPHANUMERIC);
    }

    public static final RandomCharsInputStream newAscii(long size) {
        return new RandomCharsInputStream(size, ASCII);
    }

    public static final RandomCharsInputStream newNumeric(long size) {
        return new RandomCharsInputStream(size, NUMERIC);
    }

    public static final RandomCharsInputStream newWithChars(long size, char... chars) {
        return new RandomCharsInputStream(size, chars);
    }

    public static final RandomCharsInputStream newWithChars(long size, String chars) {
        return new RandomCharsInputStream(size, chars.toCharArray());
    }

    private RandomCharsInputStream(long size, int type) {
        this.size = size;
        this.type = type;
    }

    private RandomCharsInputStream(long size, char... chars) {
        this.size = size;
        this.type = CHARS;
        this.chars = chars;
    }

    @Override
    public int read() throws IOException {
        if (charsRead >= size)
            return -1;

        char c;
        switch (type) {
            case ALPHABETIC:
                c = RandomStringUtils.randomAlphabetic(1).charAt(0);
                break;

            case ALPHANUMERIC:
                c = RandomStringUtils.randomAlphanumeric(1).charAt(0);
                break;

            case ASCII:
                c = RandomStringUtils.randomAscii(1).charAt(0);
                break;

            case NUMERIC:
                c = RandomStringUtils.randomNumeric(1).charAt(0);
                break;

            case CHARS:
                c = RandomStringUtils.random(1, chars).charAt(0);
                break;

            default:
                throw new IllegalArgumentException("Unknown random type: " + type);
        }

        charsRead++;
        return c;
    }
}    

Usage examples:

RandomCharsInputStream in = RandomCharsInputStream.newAscii(10000);

RandomCharsInputStream in = RandomCharsInputStream.newNumeric(100);

RandomCharsInputStream in = RandomCharsInputStream.newWithChars(30, "xyz");