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HashSet详解

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

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

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

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// 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

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// 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

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/**
* 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();
}