public static void main(String[] args) {
        String word_chain = "Ala ma kota. bolek ma psa. fajne naleśniki";
        char[] chars = word_chain.toCharArray();
        for(int i=2;i<chars.length;i++){
            if(chars[i-2] == '.' && chars[i-1] == ' '){
                chars[i] = Character.toUpperCase(chars[i]);
            }
        }
        System.out.print(chars);
    }