Rémi Forax's Blog

Use foreach to iterate using an Iterator

When the foreach syntax (for(:)) has been introduced in 1.5, a recurring question was why foreach is not able to iterate using an iterator.

I think i have a trick to do that using the syntax of the closure :

 Iterator<String> scanner=new Scanner(System.in);
 for(String word:() { scanner })
   System.out.println(word);

Why this code works. The foreach statement needs an array or an object that implements java.lang.Iterable. Iterable is an interface that has a single method iterator that returns an iterator thus the closure convertion is applicable.

Rémi

Comments

• You don't need closures to use the foreach with an Iterator. Just create a simple helper class:

  public static final class IteratorIterable<T> implements Iterable<T> {
  private Iteratator<T> iterator;
  public IteratorIterable(Iterator<T> iterator) {
  this.iterator = iterator;
  }
  public Iterator<T> iterator() {
  return iterator;
  }
  }

  and then simply:
  for(String word : new IteratorIterable(scannner)) {
  System.out.println(word);
  }

  Posted by: jessewilson on September 22, 2006 at 09:00 AM

• hi jessewilson,

  that was cool. I've often wondered(but never enough to get such an easy solution) how to do this.

  thank you,

  BR,
  ~A

  Posted by: anjanb2 on September 22, 2006 at 09:47 AM

• Your Iterable closure completely breaks the expectations of the interface. To make an Iterable, you would need to do something like:

  for (String word:() { new Scanner(...)}) {
  System.out.println(word);
  }

  Of course, if you had closures, you could just write:

  new Scanner(...).forEachToken(String word) {
  System.out.println(word);
  }

  Posted by: tackline on September 23, 2006 at 01:45 PM

• An alternative to the IteratorIterable thing is a (probably static) method containing an anonymous class: I'm using [ and ] instead of chevrons because, well, blogs just seem to work better that way.

  
  public static [T] Iterable[T] iterable(final Iterator[T] iterator)
  {
  return new Iterable[T]()
  {
  public Iterator[T] iterator()
  {
  return iterator;
  }
  };
  }

  
  However, this Iterable is broken, because it cannot be used reentrantly. It would be better to wrap the original creation of the Scanner in an Iterable.

  
  public static [T] Iterable[T] newScanner(whateverArgs)
  {
  return new Iterable[T]()
  {
  public Iterator[T] iterator()
  {
  return new Scanner(whateverArgs);
  }
  };
  }

  
  I'd expect that a closure that just prints whatever you give it could be named, e.g.:

  
  interface SideEffect[T]
  {
  void run(T input);
  }

  public static final SideEffect[String] sysOutPrintln=new SideEffect[String]()
  {
  public void run(String input)
  {
  System.out.println(input);
  }
  };

  
  Of course, sysOutPrintln could be changed to be a SideEffect[Object], as there is a println that takes an Object..

  With the closure syntax, perhaps this could be:

  
  public static final SideEffect[Object] sysOutPrintln=(Object o){System.out.println(o);};

  
  Cheers.

  Posted by: ricky_clarkson on September 24, 2006 at 04:56 PM

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