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324 lines (305 loc) · 13.3 KB
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/*
* Copyright (c) 2000, 2025, 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;
/**
* <p>Hash table and linked list implementation of the {@code Set} interface,
* with well-defined encounter order. This implementation differs from
* {@code HashSet} in that it maintains a doubly-linked list running through
* all of its entries. This linked list defines the encounter order (iteration
* order), which is the order in which elements were inserted into the set
* (<i>insertion-order</i>). The least recently inserted element (the eldest) is
* first, and the youngest element is last. Note that encounter order is <i>not</i> affected
* if an element is <i>re-inserted</i> into the set with the {@code add} method.
* (An element {@code e} is reinserted into a set {@code s} if {@code s.add(e)} is
* invoked when {@code s.contains(e)} would return {@code true} immediately prior to
* the invocation.) The reverse-ordered view of this set is in the opposite order, with
* the youngest element appearing first and the eldest element appearing last. The encounter
* order of elements already in the set can be changed by using the
* {@link #addFirst addFirst} and {@link #addLast addLast} methods.
*
* <p>This implementation spares its clients from the unspecified, generally
* chaotic ordering provided by {@link HashSet}, without incurring the
* increased cost associated with {@link TreeSet}. It can be used to
* produce a copy of a set that has the same order as the original, regardless
* of the original set's implementation:
* <pre>{@code
* void foo(Set<String> s) {
* Set<String> copy = new LinkedHashSet<>(s);
* ...
* }
* }</pre>
* This technique is particularly useful if a module takes a set on input,
* copies it, and later returns results whose order is determined by that of
* the copy. (Clients generally appreciate having things returned in the same
* order they were presented.)
*
* <p>This class provides all of the optional {@link Set} and {@link SequencedSet}
* operations, and it permits null elements. Like {@code HashSet}, it provides constant-time
* performance for the basic operations ({@code add}, {@code contains} and
* {@code remove}), assuming the hash function disperses elements
* properly among the buckets. Performance is likely to be just slightly
* below that of {@code HashSet}, due to the added expense of maintaining the
* linked list, with one exception: Iteration over a {@code LinkedHashSet}
* requires time proportional to the <i>size</i> of the set, regardless of
* its capacity. Iteration over a {@code HashSet} is likely to be more
* expensive, requiring time proportional to its <i>capacity</i>.
*
* <p>A linked hash set has two parameters that affect its performance:
* <i>initial capacity</i> and <i>load factor</i>. They are defined precisely
* as for {@code HashSet}. Note, however, that the penalty for choosing an
* excessively high value for initial capacity is less severe for this class
* than for {@code HashSet}, as iteration times for this class are unaffected
* by capacity.
*
* <p><strong>Note that this implementation is not synchronized.</strong>
* If multiple threads access a linked hash set concurrently, and at least
* one of the threads modifies the set, it <em>must</em> be synchronized
* externally. This is typically accomplished by synchronizing on some
* object that naturally encapsulates the set.
*
* If no such object exists, the set should be "wrapped" using the
* {@link Collections#synchronizedSet Collections.synchronizedSet}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the set: <pre>
* Set s = Collections.synchronizedSet(new LinkedHashSet(...));</pre>
*
* <p>The iterators returned by this class's {@code iterator} method are
* <em>fail-fast</em>: if the set is modified at any time after the iterator
* is created, in any way except through the iterator's own {@code remove}
* method, the iterator will throw a {@link ConcurrentModificationException}.
* Thus, in the face of concurrent modification, the iterator fails quickly
* and cleanly, rather than risking arbitrary, non-deterministic behavior at
* an undetermined time in the future.
*
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw {@code ConcurrentModificationException} on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
*
* <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 maintained by this set
*
* @author Josh Bloch
* @see Object#hashCode()
* @see Collection
* @see Set
* @see HashSet
* @see TreeSet
* @see Hashtable
* @since 1.4
*/
public class LinkedHashSet<E>
extends HashSet<E>
implements SequencedSet<E>, Cloneable, java.io.Serializable {
@java.io.Serial
private static final long serialVersionUID = -2851667679971038690L;
/**
* Constructs a new, empty linked hash set with the specified initial
* capacity and load factor.
*
* @apiNote
* To create a {@code LinkedHashSet} with an initial capacity that accommodates
* an expected number of elements, use {@link #newLinkedHashSet(int) newLinkedHashSet}.
*
* @param initialCapacity the initial capacity of the linked hash set
* @param loadFactor the load factor of the linked hash set
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
public LinkedHashSet(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor, true);
}
/**
* Constructs a new, empty linked hash set with the specified initial
* capacity and the default load factor (0.75).
*
* @apiNote
* To create a {@code LinkedHashSet} with an initial capacity that accommodates
* an expected number of elements, use {@link #newLinkedHashSet(int) newLinkedHashSet}.
*
* @param initialCapacity the initial capacity of the LinkedHashSet
* @throws IllegalArgumentException if the initial capacity is less
* than zero
*/
public LinkedHashSet(int initialCapacity) {
super(initialCapacity, .75f, true);
}
/**
* Constructs a new, empty linked hash set with the default initial
* capacity (16) and load factor (0.75).
*/
public LinkedHashSet() {
super(16, .75f, true);
}
/**
* Constructs a new linked hash set with the same elements as the
* specified collection. The linked hash set is created with an initial
* capacity sufficient to hold the elements in the specified collection
* and the default load factor (0.75).
*
* @param c the collection whose elements are to be placed into
* this set
* @throws NullPointerException if the specified collection is null
*/
@SuppressWarnings("this-escape")
public LinkedHashSet(Collection<? extends E> c) {
super(HashMap.calculateHashMapCapacity(Math.max(c.size(), 12)), .75f, true);
addAll(c);
}
/**
* Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
* and <em>fail-fast</em> {@code Spliterator} over the elements in this set.
*
* <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
* {@link Spliterator#DISTINCT}, and {@code ORDERED}. Implementations
* should document the reporting of additional characteristic values.
*
* @implNote
* The implementation creates a
* <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
* from the set's {@code Iterator}. The spliterator inherits the
* <em>fail-fast</em> properties of the set's iterator.
* The created {@code Spliterator} additionally reports
* {@link Spliterator#SUBSIZED}.
*
* @return a {@code Spliterator} over the elements in this set
* @since 1.8
*/
@Override
public Spliterator<E> spliterator() {
return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
}
/**
* Creates a new, empty LinkedHashSet suitable for the expected number of elements.
* The returned set uses the default load factor of 0.75, and its initial capacity is
* generally large enough so that the expected number of elements can be added
* without resizing the set.
*
* @param numElements the expected number of elements
* @param <T> the type of elements maintained by the new set
* @return the newly created set
* @throws IllegalArgumentException if numElements is negative
* @since 19
*/
public static <T> LinkedHashSet<T> newLinkedHashSet(int numElements) {
if (numElements < 0) {
throw new IllegalArgumentException("Negative number of elements: " + numElements);
}
return new LinkedHashSet<>(HashMap.calculateHashMapCapacity(numElements));
}
LinkedHashMap<E, Object> map() {
return (LinkedHashMap<E, Object>) map;
}
/**
* {@inheritDoc}
* <p>
* If this set already contains the element, it is relocated if necessary so that it is
* first in encounter order.
*
* @since 21
*/
public void addFirst(E e) {
map().putFirst(e, PRESENT);
}
/**
* {@inheritDoc}
* <p>
* If this set already contains the element, it is relocated if necessary so that it is
* last in encounter order.
*
* @since 21
*/
public void addLast(E e) {
map().putLast(e, PRESENT);
}
/**
* {@inheritDoc}
*
* @throws NoSuchElementException {@inheritDoc}
* @since 21
*/
public E getFirst() {
return map().sequencedKeySet().getFirst();
}
/**
* {@inheritDoc}
*
* @throws NoSuchElementException {@inheritDoc}
* @since 21
*/
public E getLast() {
return map().sequencedKeySet().getLast();
}
/**
* {@inheritDoc}
*
* @throws NoSuchElementException {@inheritDoc}
* @since 21
*/
public E removeFirst() {
return map().sequencedKeySet().removeFirst();
}
/**
* {@inheritDoc}
*
* @throws NoSuchElementException {@inheritDoc}
* @since 21
*/
public E removeLast() {
return map().sequencedKeySet().removeLast();
}
/**
* {@inheritDoc}
* <p>
* Modifications to the reversed view are permitted and will be propagated to this set.
* In addition, modifications to this set will be visible in the reversed view.
*
* @return {@inheritDoc}
* @since 21
*/
public SequencedSet<E> reversed() {
class ReverseLinkedHashSetView extends AbstractSet<E> implements SequencedSet<E> {
public int size() { return LinkedHashSet.this.size(); }
public Iterator<E> iterator() { return map().sequencedKeySet().reversed().iterator(); }
public boolean add(E e) { return LinkedHashSet.this.add(e); }
public void addFirst(E e) { LinkedHashSet.this.addLast(e); }
public void addLast(E e) { LinkedHashSet.this.addFirst(e); }
public E getFirst() { return LinkedHashSet.this.getLast(); }
public E getLast() { return LinkedHashSet.this.getFirst(); }
public E removeFirst() { return LinkedHashSet.this.removeLast(); }
public E removeLast() { return LinkedHashSet.this.removeFirst(); }
public SequencedSet<E> reversed() { return LinkedHashSet.this; }
public Object[] toArray() { return map().keysToArray(new Object[map.size()], true); }
public <T> T[] toArray(T[] a) { return map().keysToArray(map.prepareArray(a), true); }
}
return new ReverseLinkedHashSetView();
}
}