LinkedHashSet
简介
官方注释
Hash table and linked list implementation of the Set interface,with predictable iteration order. This implementation differs from HashSet 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 (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)
翻译
- 哈希表和链接列表实现Set接口，具有可预知的迭代顺序;
- 此实现与HashSet的不同之处在于，后者维护其所有项目上运行的双向链表;
- 此链接列表定义迭代排序，这是在其中元件被插入到该组（ 插入顺序 ）的顺序。 注意，如果一个元件被重新插入到该组插入顺序不受影响 。
LinkedHashSet是一个有序的HashSet。
源码
构造方法
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public class LinkedHashSet<E>
extends HashSet<E>
implements Set<E>, Cloneable, java.io.Serializable {
private static final long serialVersionUID = -2851667679971038690L;
//指定初始容量和负载因子的构造方法
public LinkedHashSet(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor, true);
}
//指定初始容量的构造方法
public LinkedHashSet(int initialCapacity) {
super(initialCapacity, .75f, true);
}
//无参构造方法
public LinkedHashSet() {
super(16, .75f, true);
}
//将集合c中的所有元素添加到LinkedHashSet中
//HashSet中的计算方式是Math.max((int) (c.size()/.75f) + 1, 16)
public LinkedHashSet(Collection<? extends E> c) {
super(Math.max(2*c.size(), 11), .75f, true);
addAll(c);
}
@Override
public Spliterator<E> spliterator() {
return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
}
}
HashSet的构造方法，使用LinkedHashMap实现，用于LinkedHashSet。
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HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
总结
LinkedHashSet的底层使用LinkedHashMap存储元素；LinkedHashSet是有序的，它是按照插入的顺序排序的；LinkedHashSet不支持按访问顺序对元素排序的（因为accessOrder是false）；
