public static String reverse(String word) {

    if (word == null || "".equals(word) || word.length() == 1) {
        throw new IllegalArgumentException("Invalid Entry");
    }

    StringBuilder result = new StringBuilder();

    for (int i=word.length() - 1; i >= 0; i--) {
        result.append(word.charAt(i));
    }

    return result.toString();
 }

public static String reverse ( String s ) {
    int length = s.length(), last = length - 1;
    char[] chars = s.toCharArray();
    for ( int i = 0; i < length/2; i++ ) {
        char c = chars[i];
        chars[i] = chars[last - i];
        chars[last - i] = c;
    }
    return new String(chars);
}

public void reverse(char[] chars) {
    for (int i = 0, j = chars.length - 1; i < j; i++, j--) {
        char temp = chars[i];
        chars[i] = chars[j];
        chars[j] = temp;
    }
}

class ReversedString {

    private String string;

    public ReversedString(String string) {
        this.string = string;
    }

    public char charAt(int index) {
        return string.charAt(string.length() - 1 - index);
    }

}