Reverse chars of each word in a string
If you don't want to reinvent the wheel, you can implement a very short and simple solution using java.util:

public class Main {
    public static void main(final String[] args) throws IOException {
        final BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        final BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(System.out));

        final String line = bufferedReader.readLine().trim();
        List&lt;String&gt; words = Arrays.asList(line.split("\\s+"));

        String reversedWords = reverseWords(words);

        bufferedWriter.write(reversedWords.toCharArray());

        bufferedReader.close();
        bufferedWriter.close();
    }

    private static String reverseWords(final List&lt;String&gt; words) {
        return words.stream().map(word -&gt; reverseWord(word)).collect(Collectors.joining(" "));
    }

    private static String reverseWord(final String word) {
        return new StringBuilder(word).reverse().toString();
    }
}