guava/src/com/google/common/base/AbstractIterator.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.GwtCompatible;

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

/**
 * Note this class is a copy of
 * {@link com.google.common.collect.AbstractIterator} (for dependency reasons).
 */
@GwtCompatible
abstract class AbstractIterator<T> implements Iterator<T> {
  private State state = State.NOT_READY;

  protected AbstractIterator() {}

  private enum State {
    READY, NOT_READY, DONE, FAILED,
  }

  private T next;

  protected abstract T computeNext();

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

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);
    switch (state) {
      case DONE:
        return false;
      case READY:
        return true;
      default:
    }
    return tryToComputeNext();
  }

  private boolean tryToComputeNext() {
    state = State.FAILED; // temporary pessimism
    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;
    T result = next;
    next = null;
    return result;
  }

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