Reverse a string word by word
Stack seems more or less deprecated. Javadoc says the following:


  A more complete and consistent set of LIFO stack 
  operations is provided by the Deque interface and its
  implementations, which should be used in preference to this class.


I guess an ArrayDeque is a good choice here.
It's seems that StringTokenizer is also deprecated. Javadoc says the following:


  StringTokenizer is a legacy class that is retained for 
  compatibility reasons although its use is discouraged in new code. 
  It is recommended that anyone seeking this functionality use the 
  split method of String or the java.util.regex package instead. 

I'd rename 


buff to result 
stack to words
str to word


to describe their purpose. It would be easier to read and help understanding code.
You could move the declaration of buff right before the second while loop.

StringBuilder buff = new StringBuilder();
while (!stack.isEmpty()) {
    buff.append(stack.pop());
    if (!stack.isEmpty())
        buff.append(" ");
}


Try to minimize the scope of local variables. It's not necessary to declare them at the beginning of the method, declare them where they are first used. (Effective Java, Second Edition, Item 45: Minimize the scope of local variables)

if(!str.equals(""))



You could use String.isEmpty() here. It's easier to read since it says in a similar language to English what does this condition do.

if(!str.equals("")) stack.push(str);



According to the Code Conventions for the Java Programming Language,


  if statements always use braces {}.


Omitting them is error-prone. I've found the one-liner above hard to read because if you scan the code line by line it's easy to miss that at the end of the line there is a statement (push).
It's good to know that there is a similar function in Apache Commons Lang: StringUtils.reverseDelimited. You can check the implementation online for other ideas.

(See also: Effective Java, 2nd edition, Item 47: Know and use the libraries The author mentions only the JDK's built-in libraries but I think the reasoning could be true for other libraries too.)




Here is the modified code with the Deque and other suggestions:

public static String reverseWords(String input) {
    Deque&lt;String&gt; words = new ArrayDeque&lt;&gt;();
    for (String word: input.split(" ")) {
        if (!word.isEmpty()) {
            words.addFirst(word);
        }
    }
    StringBuilder result = new StringBuilder();
    while (!words.isEmpty()) {
        result.append(words.removeFirst());
        if (!words.isEmpty()) {
            result.append(" ");
        }
    }
    return result.toString();
}