7
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I have implemented Python-style generators within Java. I might be reinventing the wheel here, but I feel that the ability to define a Generator with an anonymous class is the most flexible approach. Here's the relevant code:

generator/Generator.java

public abstract class Generator<T> implements Iterable<T>, Iterator<T>
{
    private Lock lock = null;
    private Lock lock2 = null;
    private Semaphore semaphore = null;
    private T curr;
    private Thread execution;
    private Runnable onTermination = null;
    private Consumer<? super T> forEachAction = null;

    public Generator(Object... params)
    {
        execution = new Thread(() ->
        {
            try
            {
                T t = get(params);
                onTermination = ThrowingRunnable.of(execution::join);
                yield(t);
                getPermit();
            }
            catch(Exception unexpected)
            {
                onTermination = ThrowingRunnable.of(() ->
                {
                    Exception e = new NoSuchElementException("Failed to retrieve element!");
                    e.initCause(unexpected);
                    throw e;
                });
                semaphore.release();
            }
        });
        execution.setDaemon(true);
    }

    @Override
    public final Iterator<T> iterator()
    {
        return this;
    }

    @Override
    public final boolean hasNext()
    {
        return onTermination == null;
    }

    @Override
    public final T next()
    {
        if(!hasNext())
            throw new NoSuchElementException(
                "There are no more elements to be generated by this Generator!");
        if(semaphore == null && lock == null)
        {
            lock = new ReentrantLock();
            lock2 = new ReentrantLock();
            lock.lock();
            semaphore = new Semaphore(0);
            execution.start();
            getPermit();
            return curr;
        }
        lock2.lock();
        lock.unlock();
        getPermit();
        lock.lock();
        lock2.unlock();
        getPermit();
        if(onTermination != null)
        {
            lock.unlock();
            onTermination.run();
        }
        return curr;
    }

    protected final void yield(T t)
    {
        if(forEachAction != null)
        {
            forEachAction.accept(t);
            return;
        }
        curr = t;
        semaphore.release();
        lock.lock();
        lock.unlock();
        semaphore.release();
        lock2.lock();
        lock2.unlock();
    }

    private final void getPermit()
    {
        try
        {
            if(semaphore != null)
                semaphore.acquire();
        }
        catch(InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    /**
     * Consumes all remaining elements possible. Obviously, don't use on
     * infinite Generators.
     */
    @Override
    public void forEach(Consumer<? super T> action)
    {
        Objects.requireNonNull(action);
        if(!hasNext())
            throw new IllegalStateException("Exhausted elements before calling forEach!");
        forEachAction = action;
        if(execution.isAlive())
        {
            lock.unlock();
        }
        else
        {
            try
            {
                execution.start();
            }
            catch(IllegalThreadStateException itse)
            {
                itse.initCause(// double checking
                    new IllegalStateException("Can't exhaust elements and then call forEach!"));
                throw itse;
            }
        }
        ThrowingRunnable.of(execution::join).run();
        onTermination.run();
    }

    protected abstract T get(Object... objs);
}

This is the code that I use for ignoring compile-time exceptions from lambdas (which should be thrown at runtime, with the default handler).

throwing/ThrowingRunnable.java

@FunctionalInterface
public interface ThrowingRunnable extends Runnable, ExceptionFlowController
{
    public abstract void run_() throws Exception;

    @Override
    default void run()
    {
        try
        {
            run_();
        }
        catch (Exception e)
        {
            handle(e);
        }
    }

    static Runnable of(ThrowingRunnable tr, Consumer<Exception> h)
    {
        return new ThrowingRunnable()
        {
            public void run_() throws Exception
            {
                tr.run_();
            }

            public void handle(Exception e)
            {
                h.accept(e);
            }
        };
    }

    static Runnable of(ThrowingRunnable tr)
    {
        return tr;
    }
}

throwing/ExceptionFlowController.java

/**
 * Controls exception flow by piping it into a handler.
 */
public interface ExceptionFlowController
{
    public default void handle(Exception e)
    {
        ThrowingUtil.raise(e);
    }
}

throwing/ThrowingUtil.java

public class ThrowingUtil 
{
    @SuppressWarnings("unchecked")
    static <E extends Exception> void raise(Exception e) throws E
    {
        throw (E) e;// sneakyThrow if you google it, restricted to exceptions only
    }
}

Here's an example of using a Generator to print the first 92 Fibonacci numbers (until 64 bits is no longer enough):

Main.java

public class Main 
{
  public static void main(String[] args) 
  {
    Generator<Number> g = new Generator<>() 
    {
      public Number get(Object[] o)
      {
        return get(0, 1);
      }

      private Number get(long fib0, long fib1)
      {
        yield(fib0);
        fib0 += fib1;
        yield(fib1);
        fib1 += fib0;
        if(fib0 < 0 || fib1 < 0)
            return null;
        return get(fib0, fib1);
      }
    };
    StreamSupport.stream(g.spliterator(),false)
    .takeWhile(Objects::nonNull)
    .forEach(System.out::println);
  }
}

Output:

0
1
1
2
...
4660046610375530309

I was a bit disappointed with the amount of concurrent primitive vomit that I had to use in order to ensure Generators were synchronized properly. Keeping this in mind, here's what I'd like to know:

  1. Any generic code quality suggestions/opinions/revisions.
  2. How can I cut down on the number of Locks and Semaphores/usages of Locks and Semaphores (maybe using well-named condition variables)?

Edit:
Here's an example where using a Generator is massively convenient compared to creating a stateful Supplier/Iterator to do the same thing:

public static void main(String[] args) 
{
  //       0
  //   1       2
  // _   6   3   4
  //_ _ 8 9 5 _ 7 _
  Node n0 = new Node(), n1 = new Node(), n2 = new Node(),
  n3 = new Node(), n4 = new Node(), n5 = new Node(),
  n6 = new Node(), n7 = new Node(), n8 = new Node(),
  n9 = new Node();
  n0.left = n1;
  n0.right = n2;
  n2.left = n3;
  n2.right = n4;
  n3.left = n5;
  n1.right = n6;
  n4.left = n7;
  n6.left = n8;
  n6.right = n9;
  Generator<Node> g = new Generator<>() 
  {
    public Node get(Object[] o)
    {
      return get(n0);
    }

    private Node get(Node n)
    {
      if(n.left != null)
        get(n.left);
      yield(n);
      if(n.right != null)
        get(n.right);
      return null;
    }
  };
  Generator<Node> rightMost7Nodes = new Generator<Node>()
  {
    int count = 0;
    int target = 7;
    public Node get(Object[] o)
    {
      return get(n0);
    }

    private Node get(Node n)
    {
      if(n.right != null)
        get(n.right);
      if(count++ >= target)
        return null;
      yield(n);
      if(n.left != null)
        get(n.left);
      return null;
    }
  };
  System.out.println("Nodes in-order:");
  StreamSupport.stream(g.spliterator(),false)
  .takeWhile(o -> g.hasNext()) //ignore last element
  .forEach(System.out::println);
  System.out.println("Right-most 7 nodes in reverse order:");
  StreamSupport.stream(rightMost7Nodes.spliterator(),false)
  .takeWhile(o -> g.hasNext()) //ignore last element
  .forEach(System.out::println);
}

Output:

Nodes in-order:
Node(1)
Node(8)
Node(6)
...
Node(2)
Node(7)
Node(4)
Right-most 7 nodes in reverse order:
Node(4)
Node(7)
Node(2)
Node(3)
Node(5)
Node(0)
Node(9)

The flexibility provided by being able to yield mid-logic makes it extremely simple for the programmer to modify the ordering and stored state during stream creation. If the same were to be done with a Supplier/Iterator, the need to correctly modify stored state during the traversal could be unnecessarily complex (using Morris Traversal or a stack-based approach is fine for full iteration, but can get complicated when you stop midway). Furthermore, the code would be inflexible to modify for other types of traversals (pre-order/post-order). For this reason, I plan to use my Generator implementation relatively frequently - which is why I'd like for it to be reviewed as per questions 1 and 2 :)

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  • 5
    \$\begingroup\$ I don't know about Python generators but I find the use of threading here to be pretty dubious. Can you explain why you think a separate thread is needed? \$\endgroup\$ – markspace Aug 28 at 23:30
  • 1
    \$\begingroup\$ Also c.f. Stream.generate() which seems a good deal simpler than your implementation: docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/… \$\endgroup\$ – markspace Aug 28 at 23:34
  • \$\begingroup\$ @markspace I need threading to support Python generator behavior, as yield statements will pause a function, save all state (context), and continue from the last yield on successive calls to next(). Stream::generate is insufficiently powerful for my desired use cases, as it provides only an infinite, unordered stream of elements, whereas my Generator intends to provide (in/)finite, ordered, stateful stream of elements. According to Stream's Javadoc: behavioral parameters "in most cases must be stateless (their result should not depend on any state that might change ...)". \$\endgroup\$ – Avi Aug 29 at 0:59
  • \$\begingroup\$ Furthermore, I need control flow to go from the thread calling next() to switch to the Generator's get logic until such time as the Generator yields or returns, at which point control flow should resume in the thread that called next() with the correct return value. This sounds extremely similar to a context switch to me, which is why I used threads. \$\endgroup\$ – Avi Aug 29 at 1:05
2
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Implementing both Iterable and Iterator at the same time is a bit weird choice and while the API documentation for Iterator makes no claims about the implementation, I think most people would assume that subsequent calls to iterator() return a different object each time and those objects, if used for reading only, do not interfere with each other.

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  • \$\begingroup\$ Thanks! You're right that it seems weird. I'll probably end up removing the Iterable functionality (iterator() method) and rename forEach to forEachRemaining as per the Iterator API (which makes more sense per my comment "Consumes all remaining elements possible"). \$\endgroup\$ – Avi Aug 29 at 16:16
2
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Ok, so, per @markspace, you've made this a lot more complicated than it has to be. In Python, yield is being used to save the state of a function. In Java, you'd just use a stateful object that executes the desired function. I'm pretty sure you could either create a FibonacciIterator or a FibonacciSupplier and meet your requirements.

In either case, every time you call get()/next(), the code runs until it hits a return (yield). Then state is preserved and control flow returns to the calling code. The next time get()/next() is called, execution continues from the preserved state. Both classes provide an infinite, ordered, stateful stream of elements. Supplier can be plugged into Stream::generate, while Iterable can be iterated over.

It is my (limited) understanding that a Python Generator function is just syntactic sugar that creates a Python Iterator which tracks execution state. This is a convenience so you can work with a function instead of an object. In your Java code, you're already tracking state yourself in your client method - by creating an infinitely deep call stack recursing on get with the new arguments.

If you think I'm mistaken, can you please provide a specific case that the classes below do not solve?

public final class FibonnaciSupplier implements Supplier<Integer> {

    private int currentNumber = 0;
    private int nextNumber = 1;

    @Override
    public Integer get() {
        final int result = this.currentNumber;
        final int sum = this.currentNumber + this.nextNumber;

        this.currentNumber = this.nextNumber;
        this.nextNumber = sum;
        return Integer.valueOf(result);
    }

}

public final class FibonnaciIterator implements Iterator<Integer> {

    private int currentNumber = 0;
    private int nextNumber = 1;

    @Override
    public boolean hasNext() {
        return true;
    }

    @Override
    public Integer next() {
        final int result = this.currentNumber;
        final int sum = this.currentNumber + this.nextNumber;

        this.currentNumber = this.nextNumber;
        this.nextNumber = sum;
        return Integer.valueOf(result);
    }

}

To see an example of the stack overflow issue, try the following generator.

    Generator<Number> g = new Generator<Number>() {
        public Number get(Object[] o) {
            return get(0);
        }

        private Number get(long currentNumber) {
            yield(currentNumber);
            currentNumber += 1;
            if (currentNumber < 0)
                return null;
            return get(currentNumber);
        }
    };

You can also put a breakpoint on the line yield(currentNumber), run your debugger through a few calls to get(), and look at the call stack. It'll look something like:

Daemon Thread [Thread-0] (Suspended (breakpoint at line 18 in Main$1))
Main$1.get(long) line: 18
Main$1.get(long) line: 22
Main$1.get(long) line: 22
Main$1.get(long) line: 22
Main$1.get(Object[]) line: 14
Main$1.get(Object[]) line: 1
Main$1(Generator).lambda$0(Object[]) line: 25
232824863.run() line: not available Thread.run() line: 745

Those repeated get() calls on line 22 are you stepping into a new stack frame every time get() is invoked recursively.

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  • \$\begingroup\$ I now realize that using Supplier and Stream::generate is the best way to go about simple element stream generation (possibly in conjunction with limit(n) to make the stream finite). However, I would argue that persisting state explicitly would be increasingly difficult for increasingly complex states (unlike Fibonacci). For example, providing an reverse in-order stream of elements from a binary tree (e.g. right-most n elements in a tree) would be simple using a recursive generator, whereas an iterative state-based approach would be far more complicated. \$\endgroup\$ – Avi Aug 29 at 16:26
  • \$\begingroup\$ I'm not saying don't use recursion. I'm saying don't wrap a recursive method call in an unboundedly-increasing call stack with totally unnecessary concurrency checking. The Supplier/Iterator implementation can use recursion as necessary, and still remember state as part of the object. \$\endgroup\$ – Eric Stein Aug 29 at 16:47
  • \$\begingroup\$ @Avi Would you mind adding a new generator implementation to your question which shows the issue you're talking about, and how you think your generator solves a problem that a simple iterator/supplier cannot? \$\endgroup\$ – Eric Stein Aug 29 at 16:54
  • \$\begingroup\$ I'm saying that the Supplier/Iterator implementation makes recursion unnecessarily complicated. The Fibonacci generator could have easily contained this: for(int i = 0; i < 46; i++) { yield(fib0); fib0 += fib1; yield(fib1); fib1 += fib0; } return null; in its get(int, int) method and produced the same results. Iterative implementations with state are relatively easy in all Supplier/Iterator/Generator approachs. w.r.t your latest request, I'll post a generator for a binary tree once I have it complete. \$\endgroup\$ – Avi Aug 29 at 16:56

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