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      1 /*
      2  * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
      3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
      4  *
      5  * This code is free software; you can redistribute it and/or modify it
      6  * under the terms of the GNU General Public License version 2 only, as
      7  * published by the Free Software Foundation.  Oracle designates this
      8  * particular file as subject to the "Classpath" exception as provided
      9  * by Oracle in the LICENSE file that accompanied this code.
     10  *
     11  * This code is distributed in the hope that it will be useful, but WITHOUT
     12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
     14  * version 2 for more details (a copy is included in the LICENSE file that
     15  * accompanied this code).
     16  *
     17  * You should have received a copy of the GNU General Public License version
     18  * 2 along with this work; if not, write to the Free Software Foundation,
     19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
     20  *
     21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
     22  * or visit www.oracle.com if you need additional information or have any
     23  * questions.
     24  */
     25 
     26 package java.util;
     27 
     28 import java.util.function.Consumer;
     29 
     30 /**
     31  * An iterator over a collection.  {@code Iterator} takes the place of
     32  * {@link Enumeration} in the Java Collections Framework.  Iterators
     33  * differ from enumerations in two ways:
     34  *
     35  * <ul>
     36  *      <li> Iterators allow the caller to remove elements from the
     37  *           underlying collection during the iteration with well-defined
     38  *           semantics.
     39  *      <li> Method names have been improved.
     40  * </ul>
     41  *
     42  * <p>This interface is a member of the
     43  * <a href="{@docRoot}openjdk-redirect.html?v=8&path=/technotes/guides/collections/index.html">
     44  * Java Collections Framework</a>.
     45  *
     46  * @param <E> the type of elements returned by this iterator
     47  *
     48  * @author  Josh Bloch
     49  * @see Collection
     50  * @see ListIterator
     51  * @see Iterable
     52  * @since 1.2
     53  */
     54 public interface Iterator<E> {
     55     /**
     56      * Returns {@code true} if the iteration has more elements.
     57      * (In other words, returns {@code true} if {@link #next} would
     58      * return an element rather than throwing an exception.)
     59      *
     60      * @return {@code true} if the iteration has more elements
     61      */
     62     boolean hasNext();
     63 
     64     /**
     65      * Returns the next element in the iteration.
     66      *
     67      * @return the next element in the iteration
     68      * @throws NoSuchElementException if the iteration has no more elements
     69      */
     70     E next();
     71 
     72     /**
     73      * Removes from the underlying collection the last element returned
     74      * by this iterator (optional operation).  This method can be called
     75      * only once per call to {@link #next}.  The behavior of an iterator
     76      * is unspecified if the underlying collection is modified while the
     77      * iteration is in progress in any way other than by calling this
     78      * method.
     79      *
     80      * @implSpec
     81      * The default implementation throws an instance of
     82      * {@link UnsupportedOperationException} and performs no other action.
     83      *
     84      * @throws UnsupportedOperationException if the {@code remove}
     85      *         operation is not supported by this iterator
     86      *
     87      * @throws IllegalStateException if the {@code next} method has not
     88      *         yet been called, or the {@code remove} method has already
     89      *         been called after the last call to the {@code next}
     90      *         method
     91      */
     92     default void remove() {
     93         throw new UnsupportedOperationException("remove");
     94     }
     95 
     96     /**
     97      * Performs the given action for each remaining element until all elements
     98      * have been processed or the action throws an exception.  Actions are
     99      * performed in the order of iteration, if that order is specified.
    100      * Exceptions thrown by the action are relayed to the caller.
    101      *
    102      * @implSpec
    103      * <p>The default implementation behaves as if:
    104      * <pre>{@code
    105      *     while (hasNext())
    106      *         action.accept(next());
    107      * }</pre>
    108      *
    109      * @param action The action to be performed for each element
    110      * @throws NullPointerException if the specified action is null
    111      * @since 1.8
    112      */
    113     default void forEachRemaining(Consumer<? super E> action) {
    114         Objects.requireNonNull(action);
    115         while (hasNext())
    116             action.accept(next());
    117     }
    118 }
    119