Generator.java

import java.util.Iterator;

public class Generator<T> implements Iterable<T> {
	
	private final Runnable target;
	
	public Generator(Runnable target) {
		this.target = target;
	}

	@Override
	public Iterator<T> iterator() {
		return new Iter<>(target);
	}
	
	public static <T> void yield(T obj) {
		Iter.yield0(obj);
	}
	
	static class Iter<T> extends Continuation implements Iterator<T> {
		
		private static final ContinuationScope SCOPE = new ContinuationScope("generator");
		
		private boolean hasNext = false;
		Object next = null;
		
		Iter(Runnable r) {
			super(SCOPE, r);
		}
		
		static void yield0(Object obj) {
			Iter<?> iter = (Iter<?>) getCurrentContinuation(SCOPE);
			if (iter == null) {
				throw new IllegalStateException("not inside a generator");
			}
			iter.hasNext = true;
			iter.next = obj;
			Continuation.yield(SCOPE);
		}

		@Override
		public boolean hasNext() {
			if (hasNext) return true;
			if (isDone()) return false;
			run();
			return hasNext;
		}

		@SuppressWarnings("unchecked")
		@Override
		public T next() {
			if (!hasNext()) {
				throw new IllegalStateException("no more elements");
			}
			hasNext = false;
			T result = (T) next;
			next = null;
			return result;
		}
		
	}
	
	public static void main(String[] args) {
		Generator<Integer> gen = new Generator<Integer>(() -> {
			for (int i = 0; i < 10; i++) {
				Generator.yield(i);
			}
		});
		for (var j : gen) {
			System.out.println(j);
		}
	}
	
}