1 /* 2 * Copyright (c) 1997, 2018, 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}/java.base/java/util/package-summary.html#CollectionsFramework"> 44 * Java Collections Framework</a>. 45 * 46 * @apiNote 47 * An {@link Enumeration} can be converted into an {@code Iterator} by 48 * using the {@link Enumeration#asIterator} method. 49 * 50 * @param <E> the type of elements returned by this iterator 51 * 52 * @author Josh Bloch 53 * @see Collection 54 * @see ListIterator 55 * @see Iterable 56 * @since 1.2 57 */ 58 public interface Iterator<E> { 59 /** 60 * Returns {@code true} if the iteration has more elements. 61 * (In other words, returns {@code true} if {@link #next} would 62 * return an element rather than throwing an exception.) 63 * 64 * @return {@code true} if the iteration has more elements 65 */ hasNext()66 boolean hasNext(); 67 68 /** 69 * Returns the next element in the iteration. 70 * 71 * @return the next element in the iteration 72 * @throws NoSuchElementException if the iteration has no more elements 73 */ next()74 E next(); 75 76 /** 77 * Removes from the underlying collection the last element returned 78 * by this iterator (optional operation). This method can be called 79 * only once per call to {@link #next}. 80 * <p> 81 * The behavior of an iterator is unspecified if the underlying collection 82 * is modified while the iteration is in progress in any way other than by 83 * calling this method, unless an overriding class has specified a 84 * concurrent modification policy. 85 * <p> 86 * The behavior of an iterator is unspecified if this method is called 87 * after a call to the {@link #forEachRemaining forEachRemaining} method. 88 * 89 * @implSpec 90 * The default implementation throws an instance of 91 * {@link UnsupportedOperationException} and performs no other action. 92 * 93 * @throws UnsupportedOperationException if the {@code remove} 94 * operation is not supported by this iterator 95 * 96 * @throws IllegalStateException if the {@code next} method has not 97 * yet been called, or the {@code remove} method has already 98 * been called after the last call to the {@code next} 99 * method 100 */ remove()101 default void remove() { 102 throw new UnsupportedOperationException("remove"); 103 } 104 105 /** 106 * Performs the given action for each remaining element until all elements 107 * have been processed or the action throws an exception. Actions are 108 * performed in the order of iteration, if that order is specified. 109 * Exceptions thrown by the action are relayed to the caller. 110 * <p> 111 * The behavior of an iterator is unspecified if the action modifies the 112 * collection in any way (even by calling the {@link #remove remove} method 113 * or other mutator methods of {@code Iterator} subtypes), 114 * unless an overriding class has specified a concurrent modification policy. 115 * <p> 116 * Subsequent behavior of an iterator is unspecified if the action throws an 117 * exception. 118 * 119 * @implSpec 120 * <p>The default implementation behaves as if: 121 * <pre>{@code 122 * while (hasNext()) 123 * action.accept(next()); 124 * }</pre> 125 * 126 * @param action The action to be performed for each element 127 * @throws NullPointerException if the specified action is null 128 * @since 1.8 129 */ forEachRemaining(Consumer<? super E> action)130 default void forEachRemaining(Consumer<? super E> action) { 131 Objects.requireNonNull(action); 132 while (hasNext()) 133 action.accept(next()); 134 } 135 } 136