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}/../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      */
hasNext()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      */
next()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      */
remove()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      */
forEachRemaining(Consumer<? super E> action)113     default void forEachRemaining(Consumer<? super E> action) {
114         Objects.requireNonNull(action);
115         while (hasNext())
116             action.accept(next());
117     }
118 }
119