HashSet详解

HashSet是基于HashMap实现的一个单列存储的集合类，将所有的数据存在HashMap的key值中，而value全部使用一个Object对象存储

继承关系

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

继承了AbstractSet类，实现了Set接口、Cloneable接口和Serializable接口，所以HashSet是支持克隆和序列化的

源码分析

关键变量

// 使用HashMap存储数据  map的key为HashSet的元素值
private transient HashMap<E,Object> map;

// Dummy value to associate with an Object in the backing Map
// map中所有的值都是该Object对象
private static final Object PRESENT = new Object();

构造器

// 无参构造器，直接实例化一个HashMap
public HashSet() {
    map = new HashMap<>();
}

/**
 * 使用的是HahMap中传入初始容量的构造器
 */
public HashSet(Collection<? extends E> c) {
    map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
    addAll(c);
}

/**
 * 使用的是HahMap中传入初始容量和加载因子的构造器
 */
public HashSet(int initialCapacity, float loadFactor) {
    map = new HashMap<>(initialCapacity, loadFactor);
}

/**
 * 使用的是HahMap中传入初始容量的构造器
 */
public HashSet(int initialCapacity) {
    map = new HashMap<>(initialCapacity);
}

/**
 * 该构造器是提供给LinkedHashSet使用的，不对外暴露，实例化的是LinkedHashMap
 */
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
    map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

方法分析

HashSet的方法实现都非常简单，直接使用封装的HashMap来操作数据，真正执行的是HashMap的方法

/**
 * 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();
}

/**
 * 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();
}

/**
 * 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();
}

/**
 * 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);
}

/**
 * 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;
}

/**
 * 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;
}

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