    001    /*
    002     * Copyright (C) 2007 The Guava Authors
    003     *
    004     * Licensed under the Apache License, Version 2.0 (the "License");
    005     * you may not use this file except in compliance with the License.
    006     * You may obtain a copy of the License at
    007     *
    008     * http://www.apache.org/licenses/LICENSE-2.0
    009     *
    010     * Unless required by applicable law or agreed to in writing, software
    011     * distributed under the License is distributed on an "AS IS" BASIS,
    012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    013     * See the License for the specific language governing permissions and
    014     * limitations under the License.
    015     */
    016
    017    package com.google.common.collect;
    018
    019    import static com.google.common.base.Preconditions.checkState;
    020
    021    import com.google.common.annotations.GwtCompatible;
    022
    023    import java.util.NoSuchElementException;
    024
    025    /**
    026     * This class provides a skeletal implementation of the {@code Iterator}
    027     * interface, to make this interface easier to implement for certain types of
    028     * data sources.
    029     *
    030     * <p>{@code Iterator} requires its implementations to support querying the
    031     * end-of-data status without changing the iterator's state, using the {@link
    032     * #hasNext} method. But many data sources, such as {@link
    033     * java.io.Reader#read()}, do not expose this information; the only way to
    034     * discover whether there is any data left is by trying to retrieve it. These
    035     * types of data sources are ordinarily difficult to write iterators for. But
    036     * using this class, one must implement only the {@link #computeNext} method,
    037     * and invoke the {@link #endOfData} method when appropriate.
    038     *
    039     * <p>Another example is an iterator that skips over null elements in a backing
    040     * iterator. This could be implemented as: <pre>   {@code
    041     *
    042     *   public static Iterator<String> skipNulls(final Iterator<String> in) {
    043     *     return new AbstractIterator<String>() {
    044     *       protected String computeNext() {
    045     *         while (in.hasNext()) {
    046     *           String s = in.next();
    047     *           if (s != null) {
    048     *             return s;
    049     *           }
    050     *         }
    051     *         return endOfData();
    052     *       }
    053     *     };
    054     *   }}</pre>
    055     *
    056     * This class supports iterators that include null elements.
    057     *
    058     * @author Kevin Bourrillion
    059     * @since 2.0 (imported from Google Collections Library)
    060     */
    061    @GwtCompatible
    062    public abstract class AbstractIterator<T> extends UnmodifiableIterator<T> {
    063      private State state = State.NOT_READY;
    064
    065      /** Constructor for use by subclasses. */
    066      protected AbstractIterator() {}
    067
    068      private enum State {
    069        /** We have computed the next element and haven't returned it yet. */
    070        READY,
    071
    072        /** We haven't yet computed or have already returned the element. */
    073        NOT_READY,
    074
    075        /** We have reached the end of the data and are finished. */
    076        DONE,
    077
    078        /** We've suffered an exception and are kaput. */
    079        FAILED,
    080      }
    081
    082      private T next;
    083
    084      /**
    085       * Returns the next element. <b>Note:</b> the implementation must call {@link
    086       * #endOfData()} when there are no elements left in the iteration. Failure to
    087       * do so could result in an infinite loop.
    088       *
    089       * <p>The initial invocation of {@link #hasNext()} or {@link #next()} calls
    090       * this method, as does the first invocation of {@code hasNext} or {@code
    091       * next} following each successful call to {@code next}. Once the
    092       * implementation either invokes {@code endOfData} or throws an exception,
    093       * {@code computeNext} is guaranteed to never be called again.
    094       *
    095       * <p>If this method throws an exception, it will propagate outward to the
    096       * {@code hasNext} or {@code next} invocation that invoked this method. Any
    097       * further attempts to use the iterator will result in an {@link
    098       * IllegalStateException}.
    099       *
    100       * <p>The implementation of this method may not invoke the {@code hasNext},
    101       * {@code next}, or {@link #peek()} methods on this instance; if it does, an
    102       * {@code IllegalStateException} will result.
    103       *
    104       * @return the next element if there was one. If {@code endOfData} was called
    105       *     during execution, the return value will be ignored.
    106       * @throws RuntimeException if any unrecoverable error happens. This exception
    107       *     will propagate outward to the {@code hasNext()}, {@code next()}, or
    108       *     {@code peek()} invocation that invoked this method. Any further
    109       *     attempts to use the iterator will result in an
    110       *     {@link IllegalStateException}.
    111       */
    112      protected abstract T computeNext();
    113
    114      /**
    115       * Implementations of {@link #computeNext} <b>must</b> invoke this method when
    116       * there are no elements left in the iteration.
    117       *
    118       * @return {@code null}; a convenience so your {@code computeNext}
    119       *     implementation can use the simple statement {@code return endOfData();}
    120       */
    121      protected final T endOfData() {
    122        state = State.DONE;
    123        return null;
    124      }
    125
    126      @Override
    127      public final boolean hasNext() {
    128        checkState(state != State.FAILED);
    129        switch (state) {
    130          case DONE:
    131            return false;
    132          case READY:
    133            return true;
    134          default:
    135        }
    136        return tryToComputeNext();
    137      }
    138
    139      private boolean tryToComputeNext() {
    140        state = State.FAILED; // temporary pessimism
    141        next = computeNext();
    142        if (state != State.DONE) {
    143          state = State.READY;
    144          return true;
    145        }
    146        return false;
    147      }
    148
    149      @Override
    150      public final T next() {
    151        if (!hasNext()) {
    152          throw new NoSuchElementException();
    153        }
    154        state = State.NOT_READY;
    155        return next;
    156      }
    157
    158      /**
    159       * Returns the next element in the iteration without advancing the iteration,
    160       * according to the contract of {@link PeekingIterator#peek()}.
    161       *
    162       * <p>Implementations of {@code AbstractIterator} that wish to expose this
    163       * functionality should implement {@code PeekingIterator}.
    164       */
    165      public final T peek() {
    166        if (!hasNext()) {
    167          throw new NoSuchElementException();
    168        }
    169        return next;
    170      }
    171    }