HashSet的实现原理浅析

孟君

发布于 2020-04-22 16:33:46

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发布于 2020-04-22 16:33:46

文章被收录于专栏：孟君的编程札记孟君的编程札记

本文简单分析一下JDK1.7的HashSet源码，看一下其内部的结构以及典型方法的实现

HashSet的内部结构

HashSet类继承AbstractSet，实现Set接口、实现了Cloneable接口以及序列化Serializable接口~如：

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable

所以，其具备Set的基本特性之外，也支持克隆以及序列化相关能力。

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    private transient HashMap<E,Object> map;

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

从上述代码可以看出，HashSet内部使用了HashMap，还有一个static final定义的对象 PRESENT~ 具体map和PRESENT是如何使用的？我们在接下来的代码实现分析中介绍

构造函数的实现

默认构造函数源代码

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

HashSet默认构造函数将构建一个容量为16，加载因子为0.75的HashMap。如果想对HashMap的实现有更多的了解，可以参考之前的博文HashMap的实现原理浅析

包含容量和加载因子的构造函数

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and the specified load factor.
     *
     * @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);
    }

包含集合参数的构造函数

    /**
     * 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);
    }

方法iterator的实现

源代码

    /**
     * Returns an iterator over the elements in this set.  The elements
     * are returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

从构造函数可以看出，HashSet内部封装使用了HashMap，其iterator、size、isEmpty的方法都是通过调用HashMap来实现的。

就像上述的iterator方法~，其就是使用了map.keySet()的iterator方法

方法size的实现

源代码

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    public int size() {
        return map.size();
    }

同样，个数size()方法也是调用map.size()方法

方法isEmpty的实现

源代码

    /**
     * Returns <tt>true</tt> if this set contains no elements.
     *
     * @return <tt>true</tt> if this set contains no elements
     */
    public boolean isEmpty() {
        return map.isEmpty();
    }

HashSet的isEmpty（）方法也是直接调用了 map.isEmpty()实现

方法contains的实现

源代码


    /**
     * Returns <tt>true</tt> if this set contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this set
     * contains an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
     */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

HashSet的contains方法直接调用了map的containsKey(o)方法实现

方法add的实现

源代码

    /**
     * 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;
    }

从上述代码可以看出，HashSet的add方法，其实也是调用了map的put方法，只是，这里map使用的值是static final定义的Object对象，也即PRESENT

import java.util.HashSet;
import java.util.Set;

public class HashSetExample {

    public static void main(String[] args) {

        Set<String> nums = new HashSet<>();
        nums.add("hello");
        nums.add("java");
        nums.add("Eric");
        nums.add("John");
        nums.add("LiLei");
        nums.add("Wang");
        nums.add("Zhang");
        
        System.out.println(nums);
        
    }
}

方法remove的实现

源代码

    /**
     * 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;
    }

HashSet的remove(o)也是调用了map的remove(o)方法。

一起再来看下map的remove（o）方法的内容：

    /**
     * Removes the mapping for the specified key from this map if present.
     *
     * @param  key key whose mapping is to be removed from the map
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V remove(Object key) {
        Entry<K,V> e = removeEntryForKey(key);
        return (e == null ? null : e.value);
    }

可以看到如果找到元素，则删除后返回元素的值。因为HashSet使用map的key，存的值都是PRESENT。所以，map.remove(o)==PRESENT就表示是否找到并删除元素`

方法clear的实现

源代码

    /**
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
     */
    public void clear() {
        map.clear();
    }

HashSet直接使用map.clear()来移除所有元素

方法clone的实现

源代码

    /**
     * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
     * themselves are not cloned.
     *
     * @return a shallow copy of this set
     */
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
    }

返回此HashSet实例的浅表副本：并没有复制这些元素本身。底层实际调用HashMap的clone()方法，获取HashMap的浅表副本，并设置到HashSet中。

序列化&反序列化的实现

源代码

    /**
     * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
     * serialize it).
     *
     * @serialData The capacity of the backing <tt>HashMap</tt> instance
     *             (int), and its load factor (float) are emitted, followed by
     *             the size of the set (the number of elements it contains)
     *             (int), followed by all of its elements (each an Object) in
     *             no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());

        // Write out size
        s.writeInt(map.size());

        // Write out all elements in the proper order.
        for (E e : map.keySet())
            s.writeObject(e);
    }

    /**
     * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read in HashMap capacity and load factor and create backing HashMap
        int capacity = s.readInt();
        float loadFactor = s.readFloat();
        map = (((HashSet)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));

        // Read in size
        int size = s.readInt();

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }

小结

从上述代码的观察可以看出：

HashSet底层由HashMap实现
HashSet的值存放于HashMap的key上
HashMap的value统一为PRESENT

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原始发表：2020-04-21，如有侵权请联系 cloudcommunity@tencent.com 删除

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