Nsryjdtyk

Recently a friend of mine showed me her code seeking my advice on why it wouldn't work. Her original code was this:

public static void printStem(String word) ...



public static void main(String args)

{



  Scanner keyboard = new Scanner(System.in);

  System.out.println("Please enter the words: "); 

  String word = keyboard.next(); 

  printStem(word);



  while (keyboard.hasNext())

  {

    printStem(word);

    word = keybord.next();

  }

}

This will yield really weird results. It will ask the user twice, then executes printStem twice (which might be expected), and after that goes ahead and always prints only the first entered corpus (word).

Eventually I figured out that it would work as expected when removing the keyboard.next() from outside the loop like so

public static void printStem(String word) ...



public static void main(String args)

{

  Scanner keyboard = new Scanner(System.in);

  System.out.println("Please enter the words: "); 



  while (keyboard.hasNext())

  {

    String word = keybord.next();

    printStem(word);

  }

}

When asked why this would be I had no plausible explanation, as this should behave identical. My best guess is that something must be smelly with hasNext() but I couldn't figure out why exactly. So. What is going on here? Any explanation is appreciated :)

Some explanation about hasNext():

Returns true if this scanner has another token in its input. 

This method may block while waiting for input to scan. 

The scanner does not advance past any input.

In your first piece of code

1. you scan for a word: String word = keyboard.next();




2. You print it: printStem(word);




3. You enter into a while loop which waits until you give some input: keyboard.hasNext()




4. In step 3 you take the input but never store it in String word and you print it. Naturally previous value of word will be printed.




5. Then you do a next read by next().

Explanation for next():

Finds and returns the next complete token from this scanner. A complete token is preceded and followed by input that matches the delimiter pattern. This method may block while waiting for input to scan, even if a previous invocation of hasNext() returned true.

Hence you get a weird behavior.

This will yield really weird results

Yeah, because the logic is wrong.

You get the input

String word = keyboard.next(); 

print it

printStem(word);

then print it again, and ask for another word:

while (keyboard.hasNext())

{

    printStem(word);

    word = keybord.next();

}

So every time you loop you print the word they entered last time, rather than the word they entered this time. You just need to swap the two lines in the while-loop, which then makes the keyboard.next() and printStem(word) outside of the loop body redundant.

as this should behave identical

No it shouldn't. You reversed the order of operations in the while-loop body.