MultiCollection.java

import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;

public class MultiCollection<E> implements Iterable<E> {
  private final Collection<E>[] sources;
  
  public MultiCollection(Collection<E>... sources) {
      this.sources = sources;
  }
  
  @Override
  public Iterator<E> iterator() {
    return new MultiCollectionIterator();
  }
  
  private class MultiCollectionIterator implements Iterator<E> {
    private int iteratorIndex;
    private Iterator<E> currentIterator;

    @Override
    public boolean hasNext() {
      while ((currentIterator == null || !currentIterator.hasNext()) && iteratorIndex < sources.length) {
        currentIterator = sources[iteratorIndex].iterator();
        iteratorIndex++;
      }

      return currentIterator != null && currentIterator.hasNext();
    }

    @Override
    public E next() {
      if (!hasNext()) throw new NoSuchElementException();

      return currentIterator.next();
    }

    @Override
    public void remove() {
      if (currentIterator == null) throw new IllegalStateException();

      currentIterator.remove();
    }
  }
}