Class - java.util.LinkedHashSet
Created by : Mr Dk.
2019 / 11 / 22 13:24
Nanjing, Jiangsu, China
Definition
public class LinkedHashSet<E>
extends HashSet<E>
implements Set<E>, Cloneable, java.io.Serializable {
}
与 HashSet 不同，在所有元素之间维护了一个双向链表，定义了迭代顺序：以元素的插入顺序为顺序。重新插入元素不会影响元素在链表中的顺序
该集合提供所有 Set 的基本操作，允许 null 元素。提供常数时间的增、删、查操作，但会比 HashSet 慢一些，因为要额外维护双向链表。但是遍历时间只和 size 有关，与 capacity 无关。与 HashSet 一样，capacity 和 load factor 是影响性能的两个参数，但是选择较大的 capacity 对性能带来的影响没有像对 HashSet 那样大。
该集合的实现不是线程安全的，要么外部同步，要么 Set s = Collections.synchronizedSet(new LinkedHashSet(...));。
/**
* <p>Hash table and linked list implementation of the <tt>Set</tt> interface,
* with predictable iteration order. This implementation differs from
* <tt>HashSet</tt> in that it maintains a doubly-linked list running through
* all of its entries. This linked list defines the iteration ordering,
* which is the order in which elements were inserted into the set
* (<i>insertion-order</i>). Note that insertion order is <i>not</i> affected
* if an element is <i>re-inserted</i> into the set. (An element <tt>e</tt>
* is reinserted into a set <tt>s</tt> if <tt>s.add(e)</tt> is invoked when
* <tt>s.contains(e)</tt> would return <tt>true</tt> immediately prior to
* the invocation.)
*
* <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>
* void foo(Set s) {
* Set 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 <tt>Set</tt> operations, and
* permits null elements. Like <tt>HashSet</tt>, it provides constant-time
* performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
* <tt>remove</tt>), assuming the hash function disperses elements
* properly among the buckets. Performance is likely to be just slightly
* below that of <tt>HashSet</tt>, due to the added expense of maintaining the
* linked list, with one exception: Iteration over a <tt>LinkedHashSet</tt>
* requires time proportional to the <i>size</i> of the set, regardless of
* its capacity. Iteration over a <tt>HashSet</tt> 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 <tt>HashSet</tt>. Note, however, that the penalty for choosing an
* excessively high value for initial capacity is less severe for this class
* than for <tt>HashSet</tt>, 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 <tt>iterator</tt> 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 <tt>remove</tt>
* 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 <tt>ConcurrentModificationException</tt> 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}/../technotes/guides/collections/index.html">
* 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
*/
Constructor
由于 LinkedHashSet 继承自 HashSet，而 HashSet 内部已经维护了 HashMap。所以成员变量中不再需要维护一个 Map。默认的初始 capacity 为 16，load factory 为 0.75f。
/**
* Constructs a new, empty linked hash set with the specified initial
* capacity and load factor.
*
* @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).
*
* @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
*/
public LinkedHashSet(Collection<? extends E> c) {
super(Math.max(2*c.size(), 11), .75f, true);
addAll(c);
}
但是源码到这里就结束了？？？逗我呢？双向链表呢？虽然是一路继承下来了，可能 LinkedHashSet 借用了 LinkedHashMap 中的实现方式。但并没有发现 LinkedHashSet 和 LinkedHashMap 有什么依赖关系。？？？？？？ 😓🥱