Java 集合框架(Set 接口)

Set 接口简介

对于 Set 接口，Java的官方文档这样提到：

A Set that cannot contain duplicate elements. It models the mathematical set abstraction. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited. Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set instances to be compared meaningfully even if their implementation types differ. Two Set instances are equal if they contain the same elements.

可以简单理解为：

• Set 表示不能包含重复元素的集合；是数学集合的抽象建模
• Set 只包含了从 Collection 接口继承的方法(即，Set 没有提供其他特殊的方法)
• Set 增强了 equals 和 hashCode 方法，允许更有意义的比较 Set 实例(即，如果两个 Set 实例包含相同的元素，那么它们是相等的)

然后讲了下关于 Set 的实现：

The Java platform contains three general-purpose Set implementations: HashSet, TreeSet, and LinkedHashSet. HashSet, which stores its elements in a hash table, is the best-performing implementation; however it makes no guarantees concerning the order of iteration. TreeSet, which stores its elements in a red-black tree, orders its elements based on their values; it is substantially slower than HashSet. LinkedHashSet, which is implemented as a hash table with a linked list running through it, orders its elements based on the order in which they were inserted into the set (insertion-order). LinkedHashSet spares its clients from the unspecified, generally chaotic ordering provided by HashSet at a cost that is only slightly higher.

简单的来说一下：

• Set 接口包含了3个通用实现(general-purpose implements)：
  HashSet、TreeSet、LinkedHashSet
• HashSet：哈希表实现，性能最好，不能保证迭代的顺序
• TreeSet：红黑树实现，性能最差，根据元素进行自然排序
• LinkedHashSet：哈希表 + 链表实现，性能介于两者之间，保证元素插入顺序

性能比较：HashSet > LinkedHashSet > TreeSet
(虽然文档中提到，LinkedHashSet 的性能是接近于 HashSet 的)

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Set 接口相关操作

前面也已经提到，Set 接口的方法都是继承自 Collection 接口的，故其操作和 Collection 接口一致。这些操作包括：
    基础操作、批量操作、数组操作、以及 JDK1.8 后提供的聚合操作

继承自 Collection 接口的操作:

• 基础操作(Basic Operations)

int size() // 返回集合中元素的个数
boolean isEmpty() // 判断集合是否存在元素
boolean contains(Object element) // 判断集合是否包含
boolean add(E element) // 添加元素
boolean remove(Object element) // 删除元素
Iterator<E> iterator() // 获取该集合的迭代器

• 批量操作(Bulk Operations)

boolean containsAll(Collection<?> c) // 包含
boolean addAll(Collection<? extends E> c) //并集
boolean removeAll(Collection<?> c) // 差集
boolean retainAll(Collection<?> c) // 交集
void clear() // 清空

• 数组操作(Array Operations)

Object[] toArray() // 集合转换为数组
<T> T[] toArray(T[] a)  // 集合转换为指定类型的数组

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更多参考：The Set Interface (The Java™ Tutorials > Collections > Interfaces)