目录
- 一. 🦁 HashSet介绍
- 1.1 特点
- 1.2 底层实现
- 二. 🦁 结构以及对应方法分析
- 2.1 结构组成
- 2.1.1 源码实现
- 2.1.2 成员变量及构造方法
- 2.2 常用的方法
- 2.2.1 添加add(E e)方法
- 2.2.2 删除remove(Object o)方法
- 三. 最后想说
一. 🦁 HashSet介绍
1.1 特点
HashSet 是一个不保证元素的顺序且没有重复元素的集合，是线程不安全的。HashSet
允许有 null 元素。
1.2 底层实现
- HashSet底层使用 HashMap 存储元素的。
- HashMap 底层使用的是数组与链表实现元素的存储。
- 元素在数组中存放时，并不是有序存放的也不是随机存放的，而是
对元素的哈希值进行运算决定元素在数组中的位置。- 当两个元素的哈希值进行运算后得到相同的在数组中的位置时，会调用元素的 equals()方法判断两个元素是否相同。如果元素相同则不会添加该元素，如果不相同则会使用单向链表保存该元素。
二. 🦁 结构以及对应方法分析
2.1 结构组成
2.1.1 源码实现
HashSet的源码并没有很多，因为其基本是调用HashMap的方法来实现的。
package java.util;
import java.io.InvalidObjectException;
import jdk.internal.access.SharedSecrets;
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();
/**
* Constructs a new, empty set; the backing {@code HashMap} instance has
* default initial capacity (16) and load factor (0.75).
*/
public HashSet() {
map = new HashMap<>();
}
/**
* Constructs a new set containing the elements in the specified
* collection. The {@code HashMap} 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 {@code HashMap} 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);
}
/**
* Constructs a new, empty set; the backing {@code HashMap} instance has
* the specified initial capacity and default load factor (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.
*
* @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);
}
/**
* 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 {@code true} if this set contains no elements.
*
* @return {@code true} if this set contains no elements
*/
public boolean isEmpty() {
return map.isEmpty();
}
/**
* Returns {@code true} if this set contains the specified element.
* More formally, returns {@code true} if and only if this set
* contains an element {@code e} such that
* {@code Objects.equals(o, e)}.
*
* @param o element whose presence in this set is to be tested
* @return {@code true} 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 {@code e} to this set if
* this set contains no element {@code e2} such that
* {@code Objects.equals(e, e2)}.
* If this set already contains the element, the call leaves the set
* unchanged and returns {@code false}.
*
* @param e element to be added to this set
* @return {@code true} 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 {@code e} such that
* {@code Objects.equals(o, e)},
* if this set contains such an element. Returns {@code true} 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 {@code true} 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();
}
/**
* Returns a shallow copy of this {@code HashSet} instance: the elements
* themselves are not cloned.
*
* @return a shallow copy of this set
*/
@SuppressWarnings("unchecked")
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(e);
}
}
/**
* Save the state of this {@code HashSet} instance to a stream (that is,
* serialize it).
*
* @serialData The capacity of the backing {@code HashMap} 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 {@code HashSet} instance from a stream (that is,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Consume and ignore stream fields (currently zero).
s.readFields();
// Read capacity and verify non-negative.
int capacity = s.readInt();
if (capacity < 0) {
throw new InvalidObjectException("Illegal capacity: " +
capacity);
}
// Read load factor and verify positive and non NaN.
float loadFactor = s.readFloat();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
}
// Clamp load factor to range of 0.25...4.0.
loadFactor = Math.min(Math.max(0.25f, loadFactor), 4.0f);
// Read size and verify non-negative.
int size = s.readInt();
if (size < 0) {
throw new InvalidObjectException("Illegal size: " + size);
}
// Set the capacity according to the size and load factor ensuring that
// the HashMap is at least 25% full but clamping to maximum capacity.
capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
HashMap.MAXIMUM_CAPACITY);
// Constructing the backing map will lazily create an array when the first element is
// added, so check it before construction. Call HashMap.tableSizeFor to compute the
// actual allocation size. Check Map.Entry[].class since it's the nearest public type to
// what is actually created.
SharedSecrets.getJavaObjectInputStreamAccess()
.checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
// Create backing HashMap
map = (((HashSet<?>)this) instanceof LinkedHashSet ?
new LinkedHashMap<>(capacity, loadFactor) :
new HashMap<>(capacity, loadFactor));
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
@SuppressWarnings("unchecked")
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
/**
* Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
* and <em>fail-fast</em> {@link Spliterator} over the elements in this
* set.
*
* <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
* {@link Spliterator#DISTINCT}. Overriding implementations should document
* the reporting of additional characteristic values.
*
* @return a {@code Spliterator} over the elements in this set
* @since 1.8
*/
public Spliterator<E> spliterator() {
return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);
}
}
2.1.2 成员变量及构造方法
- HashMap<E,Object> map：存储元素的容器
- Object PRESENT = new Object()：充当map中的value
- 构造方法：这个无参构造方法里new了一个HashMap容器。说明了HashSet的底层是由HashMap实现的。
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();
/**
* Constructs a new, empty set; the backing {@code HashMap} instance has
* default initial capacity (16) and load factor (0.75).
*/
public HashSet() {
map = new HashMap<>();
}
2.2 常用的方法
2.2.1 添加add(E e)方法
/**
* Adds the specified element to this set if it is not already present.
* More formally, adds the specified element {@code e} to this set if
* this set contains no element {@code e2} such that
* {@code Objects.equals(e, e2)}.
* If this set already contains the element, the call leaves the set
* unchanged and returns {@code false}.
*
* @param e element to be added to this set
* @return {@code true} if this set did not already contain the specified
* element
*/
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
由源码可以看到，添加方法是通过返回一个布尔类型，而有趣的是它的实现方法居然是直接调用map的put()方法，而参数e则是放在了key的位置，value位置则是前面看到的成员变量。这里很好的说明了HashSet的值为什么不能重复，而且是无序的，因为HashMap的key就是无序的，而且不能重复。
2.2.2 删除remove(Object o)方法
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}这里的删除方法是调用了HashMap的remove(),这个方法在HashMap中是直接根据主键删除对应的元素，非常直观。
三. 最后想说
学习源码知识，有助于帮助我们扎实内功,提升程序员的涵养，如果您不想直接在idea查看源码，也想了解他，可以关注博主，都给您整理好啦，好了，文章到这里就结束啦，咱们下期见，喜欢可以一键三连哦😄
