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    // When making changes to this class, please also update the copy at
062    // com.google.common.base.AbstractIterator
063    @GwtCompatible
064    public abstract class AbstractIterator<T> extends UnmodifiableIterator<T> {
065      private State state = State.NOT_READY;
066    
067      /** Constructor for use by subclasses. */
068      protected AbstractIterator() {}
069    
070      private enum State {
071        /** We have computed the next element and haven't returned it yet. */
072        READY,
073    
074        /** We haven't yet computed or have already returned the element. */
075        NOT_READY,
076    
077        /** We have reached the end of the data and are finished. */
078        DONE,
079    
080        /** We've suffered an exception and are kaput. */
081        FAILED,
082      }
083    
084      private T next;
085    
086      /**
087       * Returns the next element. <b>Note:</b> the implementation must call {@link
088       * #endOfData()} when there are no elements left in the iteration. Failure to
089       * do so could result in an infinite loop.
090       *
091       * <p>The initial invocation of {@link #hasNext()} or {@link #next()} calls
092       * this method, as does the first invocation of {@code hasNext} or {@code
093       * next} following each successful call to {@code next}. Once the
094       * implementation either invokes {@code endOfData} or throws an exception,
095       * {@code computeNext} is guaranteed to never be called again.
096       *
097       * <p>If this method throws an exception, it will propagate outward to the
098       * {@code hasNext} or {@code next} invocation that invoked this method. Any
099       * further attempts to use the iterator will result in an {@link
100       * IllegalStateException}.
101       *
102       * <p>The implementation of this method may not invoke the {@code hasNext},
103       * {@code next}, or {@link #peek()} methods on this instance; if it does, an
104       * {@code IllegalStateException} will result.
105       *
106       * @return the next element if there was one. If {@code endOfData} was called
107       *     during execution, the return value will be ignored.
108       * @throws RuntimeException if any unrecoverable error happens. This exception
109       *     will propagate outward to the {@code hasNext()}, {@code next()}, or
110       *     {@code peek()} invocation that invoked this method. Any further
111       *     attempts to use the iterator will result in an
112       *     {@link IllegalStateException}.
113       */
114      protected abstract T computeNext();
115    
116      /**
117       * Implementations of {@link #computeNext} <b>must</b> invoke this method when
118       * there are no elements left in the iteration.
119       *
120       * @return {@code null}; a convenience so your {@code computeNext}
121       *     implementation can use the simple statement {@code return endOfData();}
122       */
123      protected final T endOfData() {
124        state = State.DONE;
125        return null;
126      }
127    
128      @Override
129      public final boolean hasNext() {
130        checkState(state != State.FAILED);
131        switch (state) {
132          case DONE:
133            return false;
134          case READY:
135            return true;
136          default:
137        }
138        return tryToComputeNext();
139      }
140    
141      private boolean tryToComputeNext() {
142        state = State.FAILED; // temporary pessimism
143        next = computeNext();
144        if (state != State.DONE) {
145          state = State.READY;
146          return true;
147        }
148        return false;
149      }
150    
151      @Override
152      public final T next() {
153        if (!hasNext()) {
154          throw new NoSuchElementException();
155        }
156        state = State.NOT_READY;
157        return next;
158      }
159    
160      /**
161       * Returns the next element in the iteration without advancing the iteration,
162       * according to the contract of {@link PeekingIterator#peek()}.
163       *
164       * <p>Implementations of {@code AbstractIterator} that wish to expose this
165       * functionality should implement {@code PeekingIterator}.
166       */
167      public final T peek() {
168        if (!hasNext()) {
169          throw new NoSuchElementException();
170        }
171        return next;
172      }
173    }