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    <p>The linked implementation appears to have been written for Java 4 and can be simplified a little using guava and <code>java.util.concurrent</code>:</p> <pre><code>import java.util.Iterator; import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.BlockingQueue; import java.util.concurrent.ExecutorService; import java.util.concurrent.atomic.AtomicReference; import com.google.common.base.Throwables; import com.google.common.collect.AbstractIterator; import com.google.common.util.concurrent.Executors; public abstract class Iterators2 { public static &lt;E&gt; Iterator&lt;E&gt; buffer(final Iterator&lt;E&gt; source, int capacity) { return buffer(source, capacity, defaultExecutor); } public static &lt;E&gt; Iterator&lt;E&gt; buffer(final Iterator&lt;E&gt; source, int capacity, final ExecutorService exec) { if (capacity &lt;= 0) return source; final BlockingQueue&lt;E&gt; queue = new ArrayBlockingQueue&lt;E&gt;(capacity); // Temporary storage for an element we fetched but could not fit in the queue final AtomicReference&lt;E&gt; overflow = new AtomicReference&lt;E&gt;(); final Runnable inserter = new Runnable() { @SuppressWarnings("unchecked") public void run() { E next = (E) END_MARKER; if (source.hasNext()) { next = source.next(); // ArrayBlockingQueue does not allow nulls if (next == null) next = (E) NULL_MARKER; } if (queue.offer(next)) { // Keep buffering elements as long as we can if (next != END_MARKER) exec.submit(this); } else { // Save the element. This also signals to the // iterator that the inserter thread is blocked. overflow.lazySet(next); } } }; // Fetch the first element. // The inserter will resubmit itself as necessary to fetch more elements. exec.submit(inserter); Iterator&lt;E&gt; iterator = new AbstractIterator&lt;E&gt;() { protected E computeNext() { try { E next = queue.take(); E overflowElem = overflow.getAndSet(null); if (overflowElem != null) { // There is now a space in the queue queue.put(overflowElem); // Awaken the inserter thread exec.submit(inserter); } if (next == END_MARKER) { return endOfData(); } else if (next == NULL_MARKER) { return null; } else { return next; } } catch (InterruptedException ex) { Thread.currentThread().interrupt(); return endOfData(); } } }; return iterator; } protected Iterators2() { throw Throwables.propagate(new InstantiationException(Iterators2.class + " is a static class and cannot be instantiated")); } private static ExecutorService defaultExecutor = java.util.concurrent.Executors.newCachedThreadPool(Executors.daemonThreadFactory()); private static final Object END_MARKER = new Object(); private static final Object NULL_MARKER = new Object(); } </code></pre> <hr> <p>Note: the above implementation makes no attempt to handle exceptions in the source iterator (if one is thrown, the inserter task will terminate abruptly, leaving the calling thread deadlocked.)</p>
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    1. COWhy would you accept an ExecutorService, when would you want something other than a daemon thread factory like you've done by default?
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    2. COYou might want to allocate threads from a fixed-size pool. Or you might want to modify the default priority. Or you might want to keep track of all the threads in case you need to kill them when a database connection goes down. It's simpler to use the existing interfaces (`ExecutorService` and `ThreadFactory`) rather than add a bunch of overloads for all the different options.
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    3. CONote, this implementation does not appropriately handle exceptions from the source Iterator.
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