Java8集合框架——HashSet源码分析

本文的目录结构:

  • 一、HashSet 的 Javadoc 文档注释和简要说明
  • 二、HashSet 的内部实现:内部属性和构造函数
  • 三、HashSet 的 add 操作和扩容
  • 四、HashSet 的 remove 操作

 

一、HashSet 的 Javadoc 文档注释和简要说明

  截个图,然后来观摩 HashSet 的javadoc,照样是几点总结摘抄:

  • Set 接口的实现类,内部使用了一个 HashMap 实例;不保证 set 的迭代顺序(无序);允许存储 null
  • 通常情况下(假如 hash 分布比较均匀),基本操作(add, remove, contains 和 size)可以看成是 O(1) 的;迭代/遍历的时间和 HashSet 的元素数量以及内部 HashMap 实例的的内部数组大小有关,因此对于迭代遍历性能有要求的, HashSet 的初始容量不能设置太大或者负载因子不能太小。
  • HashSet 也是非线程安全的,需要其他的工具类来保证线程安全
  • HashSet 也是 fail-fast;同样也并不保证出现有并发修改就百分百抛出 ConcurrentModificationException,

二、HashSet 的内部实现:内部属性和构造函数

  这个就比较简洁了,只有一个共用的 Object 对象,其实就是内部 HasMap 的 value。

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();

  再来看一看构造函数,都似曾相识,其实基本就是 HashMap 的构造函数调用一遍,还有一个包级私有的构造函数,内部创建了 LinkedHashMap,这个是给 LinkedHashSet 用的,双链表保证有序。

/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* default initial capacity (16) and load factor (0.75).
* 默认初始容量 16 和 负载因子 0.75
*/
public HashSet() {
map = new HashMap<>();
}

/**
* Constructs a new set containing the elements in the specified
* collection. The <tt>HashMap</tt> is created with default load factor
* (0.75) and an initial capacity sufficient to contain the elements in
* the specified collection.
*
* @param c the collection whose elements are to be placed into this set
* @throws NullPointerException if the specified collection is null
*/
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}

/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* the specified initial capacity and the specified load factor.
* 指定 初始容量 和 负载因子
* @param initialCapacity the initial capacity of the hash map
* @param loadFactor the load factor of the hash map
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}

/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* the specified initial capacity and default load factor (0.75).
* 初定 初始容量,默认负载因子 0.75
* @param initialCapacity the initial capacity of the hash table
* @throws IllegalArgumentException if the initial capacity is less
* than zero
*/
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}

/**
* Constructs a new, empty linked hash set. (This package private
* constructor is only used by LinkedHashSet.) The backing
* HashMap instance is a LinkedHashMap with the specified initial
* capacity and the specified load factor.
* 给 LinkedHashSet 用的构造函数,其实和 HashMap 套路类似,只是 LinkedHashMap 有序
* @param initialCapacity the initial capacity of the hash map
* @param loadFactor the load factor of the hash map
* @param dummy ignored (distinguishes this
* constructor from other int, float constructor.)
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

 

三、HashSet 的 add 操作和扩容

  也比较简单直接,就是调用内部 HashMap 实例的 put 方法。当然扩容也是 HashMap 本身的扩容。
  这里还是摘抄下 javadoc,大致就是说 set 中不会有重复的 key,e 和 e2 重复的判断条件是 (e==null ? e2==null : e.equals(e2))。 e 存在的时候该方法返回 false。

  Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

/**
* Adds the specified element to this set if it is not already present.
* More formally, adds the specified element <tt>e</tt> to this set if
* this set contains no element <tt>e2</tt> such that
* <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns <tt>false</tt>.
*
* @param e element to be added to this set
* @return <tt>true</tt> if this set did not already contain the specified
* element
*/
public boolean add(E e) {
return map.put(e, PRESENT)==null;

 

四、HashSet 的 remove 操作

  remove 移除元素,内部判断存在的条件是 o==null ? e==null : o.equals(e)

/**
* Removes the specified element from this set if it is present.
* More formally, removes an element <tt>e</tt> such that
* <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
* if this set contains such an element. Returns <tt>true</tt> if
* this set contained the element (or equivalently, if this set
* changed as a result of the call). (This set will not contain the
* element once the call returns.)
*
* @param o object to be removed from this set, if present
* @return <tt>true</tt> if the set contained the specified element
*/
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
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