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<p>One useful Groovy iterator is the SyncIterator, based on Ruby's SyncEnumerator which can iterate across several data structures at the same time. It makes use of DefaultTypeTransformation.asCollection() to perform the same coercion that Groovy uses by default to implement methods such as each(), collect(), and the like...</p>
<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
import org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation;

public class SyncIterator implements Iterator, Iterable{
   private theobjects

SyncIterator(Object[] objects){
      theobjects=objects.collect{
         if (it instanceof Iterator) return /*from closure*/ it
         else return /*from closure*/ DefaultTypeTransformation.asCollection(it).iterator()
      }
   }

boolean hasNext(){
      return theobjects.any{it.hasNext()}
   }
   Object next(){
      if (!hasNext()) throw new java.util.NoSuchElementException()
      return theobjects.collect{
         try{
            return /*from closure*/ it.next()
         }catch(NoSuchElementException e){
            return /*from closure*/ null
         }
      }
   }

Iterator iterator(){
      return this;
   }

void remove(){
      throw new UnsupportedOperationException("remove() not supported")
   }

}
</pre></td></tr></table>
<p>Another useful iterator is the Generator, which takes a closure and offers up the stuff it yields as an iterator, without having to keep all of the generated data around in memory. (It's based loosely on Ruby 1.9's Generator, which was also the challenge in <a href="http://www.rubyquiz.com/quiz66.html">Ruby Quiz #66</a>)</p>
<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
//JAVA CODE
import java.util.concurrent.*;
import java.lang.ref.*;
import groovy.lang.Closure;
import java.util.*;

public class Generator&lt;T> implements Iterator&lt;T>, Iterable&lt;T>{
   Semaphore availSemaphore=new Semaphore(0);
   Semaphore emptySemaphore=new Semaphore(1);

//the thread can push one value at at time into pushedValue
   T pushedValue=null;

//pull value moves it from pushedValue to pulledValue
   //until it is released by next()
   T pulledValue=null;
   boolean hasPulledValue=false;

Thread internalThread;

Generator(Closure closure){
      internalThread=new GeneratorThread&lt;T>(this,closure);
      internalThread.setDaemon(true);
      internalThread.start();
   }

private void pullValue(){
      availSemaphore.acquireUninterruptibly();
      pulledValue=pushedValue;
      pushedValue=null;
      hasPulledValue=true;
      emptySemaphore.release();
   }

public boolean hasNext(){
      if (!hasPulledValue)
	 pullValue();
      return emptySemaphore.availablePermits() != 2;
   }

public T next(){
      if (!hasNext())
	 throw new NoSuchElementException("Closure has no more values");
      T retval=pulledValue;
      hasPulledValue=false;
      return retval;
   }

public void remove(){
      throw new UnsupportedOperationException(
	 "Remove is not supported on generators");
   }

public Iterator&lt;T> iterator(){
      return this;
   }

public void finalize(){
      internalThread.interrupt();
   }

static class GeneratorThread&lt;T> extends Thread{
      WeakReference&lt;Generator&lt;T>> generatorRef;
      Closure closure;
      public GeneratorThread(Generator&lt;T> generator, Closure cl){
	 generatorRef=new WeakReference&lt;Generator&lt;T>>(generator);
	 closure=cl;
      }

public void run(){
	 closure.call(new SaveClosure&lt;T>(this));
	 Generator generator=generatorRef.get();

//NOTE: when the closure completes, pullValue() will block forever
	 //waiting for more available data. This release() allows it to
	 //get in one last time, and read a variable indicating that the
	 //thread has died and isn't producing any more data. one final
	 //pullValue() run will have emptySemaphore==1 and
	 //availSemaphore==1, and it will make emptySemaphore==2 thus
	 //indicating that the thread has died
	 if (generator!=null){
	    generator.availSemaphore.release();
	 }
	 //NOTE: if the generator has been garbage collected, we don't care
	 //about letting the generator pull a termination condition.
      }
   }

static class SaveClosure&lt;T> extends Closure{
      WeakReference&lt;Generator&lt;T>&gt; generatorRef;
      Semaphore emptySemaphore;
      Semaphore availSemaphore;
      public SaveClosure(GeneratorThread&lt;T> gt){
	 super(gt,null);
	 generatorRef=gt.generatorRef;
	 Generator&lt;T> generator=generatorRef.get();
	 if (generator!=null){
	    emptySemaphore=generator.emptySemaphore;
	    availSemaphore=generator.availSemaphore;
	 }else{
	    throw new GeneratorDisposedException();
	 }
      }

public void doCall(T value){
	 try{
	    emptySemaphore.acquire();
	 }catch(InterruptedException e){
	    throw new GeneratorDisposedException();
	 }
	 Generator&lt;T> generator=generatorRef.get();
	 if (generator!=null){
	    generator.pushedValue=value;
	 }else{
	    throw new GeneratorDisposedException();
	 }
	 availSemaphore.release();
      }
   }

/**
    * A GeneratorDisposedException is used to terminate the thread
    * that was generating values, once the Generator has been garbage
    * collected.
    */
   static public class GeneratorDisposedException extends RuntimeException{
   }
}
</pre></td></tr></table>
<p> Note that Groovy doesn't have this concept built in. Its MethodClosure to Iterator conversion loads everything into an array all at once, which isn't a particularly good idea you may be running find() on it, and only need to generate the first item to find what you're looking for.</p>

<p> For example, consider the following use of a Generator:</p>
<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
//generates an infinite sequence of Fibonacci numbers
def fibonacci(Closure yield){
    def a=0
    def b=1
    def temp
    while(true){
        yield(b)
        temp=a
        a=b
        b=a+temp
    }
}

//find the first Fibonacci number that's evenly divisible by 20
println(new Generator(this.&amp;fibonacci).find{ it % 20 == 0})

//BROKEN: the groovy runtime wants to run fibonacci to termination loading values into an array.
//this generates an out of memory error.
 this.&amp;fibonacci.find{it % 20 == 0}
</pre></td></tr></table>
<p>As the Groovy runtime implements this now, you would exhaust all available ram when converting the MethodClosure to an iterator, before find() was ever called. With a Generator, values are only generated on demand.</p>

<p>Disadvantages:</p>
<ul>
	<li>because of the use of threads, the generator may generate one more value than is actually needed before the garbage collector disposes of the generator.</li>
	<li>Exceptions thrown from the generator method are not propagated back to the caller (again, because the function is called on a different thread)</li>
</ul>