class PrintAsciiLetters {
    public static void main(String[] args) {
        int big = 65, small = 97;
        for (int i = 0; i < 26; ++i)
            System.out.printf("%c %c ", (char) (big + i), (char) (small + i));
        System.out.println();
    }
}

$ javac PrintAsciiLetters.java && java PrintAsciiLetters
A a B b C c D d E e F f G g H h I i J j K k L l M m N n O o P p Q q R r S s T t U u V v W w X x Y y Z z