PeekingIterator.java

package denominator.common;

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

/**
 * adapted from guava's {@code com.google.common.collect.AbstractIterator}.
 */
public abstract class PeekingIterator<T> implements Iterator<T> {

  private PeekingIterator.State state = State.NOT_READY;
  private T next;

  /**
   * Constructor for use by subclasses.
   */
  protected PeekingIterator() {
  }

  protected abstract T computeNext();

  protected final T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {
    switch (state) {
      case DONE:
        return false;
      case READY:
        return true;
      default:
    }
    return tryToComputeNext();
  }

  private boolean tryToComputeNext() {
    next = computeNext();
    if (state != State.DONE) {
      state = State.READY;
      return true;
    }
    return false;
  }

  @Override
  public final T next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    state = State.NOT_READY;
    return next;
  }

  public T peek() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    return next;
  }

  @Override
  public void remove() {
    throw new UnsupportedOperationException();
  }

  private enum State {
    /**
     * We have computed the next element and haven't returned it yet.
     */
    READY,

    /**
     * We haven't yet computed or have already returned the element.
     */
    NOT_READY,

    /**
     * We have reached the end of the data and are finished.
     */
    DONE,
  }
}