Reversing String in Java

This post is about String manipulation. I have provided solutions to some problems/teasers such as how to reverse a String without using the reverse method, how to reverse individual words in a sentence without reversing the sentence and how to reverse a sentence without reversing the words in it. There are many ways to do it and I have presented one or two. It was pure fun doing this exercise.

And remember unless there is a real need to rewrite core functionality don't do it.

import java.util.Stack;
import java.util.StringTokenizer;

public class MyStringTests {

    /**
     * Reverses word and if a sentence is provided, reverses
     * the sentence and all words in the sentence.
     * @param s is the input string
     * @return the modified string
     * eg:
     * HELLOO WORLD 
     * becomes
     * DLROW OOLLEH
     * eg:
     * HELOO becomes
     * OOLEH
     */
    public static String reverseWord(String s) {
        if (s == null || s.length() <= 1) return s;
        char[] a = s.toCharArray();
        int len = a.length - 1;
        char temp;
        for(int i = 0, j = len; i <= j; i++,j--) {
            temp = a[i];
            a[i] = a[j];
            a[j] = temp;
        }
        return new String(a);
    }

    /**
     * Ideal way to reverse a string in Java is to use
     * the reverse method of StringBuffer. Unless there
     * is a real need to rewrite core functionality don't
     * do it.
     * @param s is the input string
     * @return the modified string
     * eg:
     * HELLOO WORLD 
     * becomes
     * DLROW OOLLEH
     * eg:
     * HELOO becomes
     * OOLEH
     */
    public static String reverseWordUsingReverse(String s) {
        if (s == null || s.length() <= 1) return s;
        StringBuffer sb = new StringBuffer(s);
        return sb.reverse().toString();
    }
    
    /**
     * Reverses a sentence without reversing individual 
     * words in it
     * @param s is the input string
     * @return the modified string
     * eg:
     * HELLOO WORLD 
     * becomes
     * WORLD HELLO
     */
    public static String reverseSentence(String s) {
        if (s == null || s.length() <= 1) return s;
        StringTokenizer strtok = new StringTokenizer(s);
        if (strtok.countTokens() <= 1) return s;
        Stack<String> stack = new Stack<String>();
        while (strtok.hasMoreTokens()) {
            stack.push((String)strtok.nextToken());
        }
        StringBuffer sb = new StringBuffer();
        while (!stack.isEmpty()) {
            sb.append(stack.pop());
            sb.append(" ");
        }
        return sb.toString();
    }
    
    /**
     * Reverses a sentence without reversing individual 
     * words in it
     * @param s is the input string
     * @return the modified string
     * eg:
     * HELLOO WORLD 
     * becomes
     * WORLD HELLO
     */
    public static String reverseSentenceUsingSplit(String s) {
        if (s == null || s.length() <= 1) return s;
        String[] sa = s.split(" ");
        if (sa.length <= 1) return s;
        StringBuffer newString = new StringBuffer();
        for(int i = 0, j = (sa.length - 1); i <= j; i++,j--) {
            newString.append(sa[j]).append(" ").append(sa[i]);
        }
        return newString.toString();
    }
    
    /**
     * Reverses each word in a sentence but not the 
     * sentence
     * @param s is the input string
     * @return the modified string
     * eg:
     * HELLOO WORLD 
     * becomes
     * OOLLEH DLROW 
     */
    public static String reverseWordInSentence(String s) {
        if (s == null || s.length() <= 1) return s;
        StringTokenizer strtok = new StringTokenizer(s);
        StringBuffer sb = new StringBuffer();
        while (strtok.hasMoreTokens()) {
            sb.append(reverseWord(
                        (String)strtok.nextToken()));
            sb.append(" ");
        }
        return sb.toString();
    }
    

    public static void main(String[] args) {
        //Test 
        String s = "HELLOO WORLD";
        print(s);
        print(reverseWord(s));
        print(reverseWordUsingReverse(s));
        print(reverseSentence(s));
        print(reverseSentenceUsingSplit(s));
        print(reverseWordInSentence(s));
    }
    
    //Helper print method
    public static <T> void print(T s) {
        System.out.println(s);
    }

    
}

The output is:
HELLOO WORLD
DLROW OOLLEH
DLROW OOLLEH
WORLD HELLOO
WORLD HELLOO
OOLLEH DLROW