8266571: Sequenced Collections

Reviewed-by: alanb

src/java.base/share/classes/java/util/SequencedSet.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.util;
+
+/**
+ * A collection that is both a {@link SequencedCollection} and a {@link Set}. As such,
+ * it can be thought of either as a {@code Set} that also has a well-defined
+ * <a href="SequencedCollection.html#encounter">encounter order</a>, or as a
+ * {@code SequencedCollection} that also has unique elements.
+ * <p>
+ * This interface has the same requirements on the {@code equals} and {@code hashCode}
+ * methods as defined by {@link Set#equals Set.equals} and {@link Set#hashCode Set.hashCode}.
+ * Thus, a {@code Set} and a {@code SequencedSet} will compare equals if and only
+ * if they have equal elements, irrespective of ordering.
+ * <p>
+ * {@code SequencedSet} defines the {@link #reversed} method, which provides a
+ * reverse-ordered <a href="Collection.html#view">view</a> of this set. The only difference
+ * from the {@link SequencedCollection#reversed SequencedCollection.reversed} method is
+ * that the return type of {@code SequencedSet.reversed} is {@code SequencedSet}.
+ * <p>
+ * This class is a member of the
+ * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
+ * Java Collections Framework</a>.
+ *
+ * @param <E> the type of elements in this sequenced set
+ * @since 21
+ */
+public interface SequencedSet<E> extends SequencedCollection<E>, Set<E> {
+    /**
+     * {@inheritDoc}
+     *
+     * @return a reverse-ordered view of this collection, as a {@code SequencedSet}
+     */
+    SequencedSet<E> reversed();
+}