java keysetview,Set——你真的了解吗？
JAVA 基礎(chǔ) ：Set——你真的了解嗎？
簡(jiǎn)述
Set 繼承于 Collection ，是一種集合。有元素?zé)o序、值不重復(fù)、不允許空值得特性。主要有HashSet、TreeSet兩種實(shí)現(xiàn)方式。由于Set主要基于Map實(shí)現(xiàn)，所以特點(diǎn)也由Map決定。
Set 結(jié)構(gòu)圖
例如 HashSet ,調(diào)用 HashSet 的無(wú)參構(gòu)造函數(shù)，HashSet 會(huì)使用默認(rèn)的 HashMap ，初始化 Size 為16，擴(kuò)張系數(shù)為0.75
HashSet
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HashSet 結(jié)構(gòu)圖
查看 HashSet 源碼會(huì)發(fā)現(xiàn)主要數(shù)據(jù)操作都間接調(diào)用 HashMap 的數(shù)據(jù)操作，從 add() 方法可以看出 HashSet 的值其實(shí)為 HashMap 的 Key，而 Value 是一個(gè)關(guān)鍵字為 final 類型為 Object 的 PRESENT ，遍歷的 HashSet 的值其實(shí)是遍歷 HashMap 的 KeyEntry .
HashSet 源碼
public class HashSet
extends AbstractSet
implements Set, Cloneable, java.io.Serializable
{
static final long serialVersionUID = -5024744406713321676L;
private transient HashMap 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 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 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 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 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 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 true if this set contains no elements.
*
* @return true if this set contains no elements
*/
public boolean isEmpty() {
return map.isEmpty();
}
/**
* Returns true if this set contains the specified element.
* More formally, returns true if and only if this set
* contains an element e such that
* (o==null???e==null?:?o.equals(e)).
*
* @param o element whose presence in this set is to be tested
* @return 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 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.
*
* @param e element to be added to this set
* @return 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 e such that
* (o==null???e==null?:?o.equals(e)),
* if this set contains such an element. Returns 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 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 HashSet instance: the elements
* themselves are not cloned.
*
* @return a shallow copy of this set
*/
@SuppressWarnings("unchecked")
public Object clone() {
try {
HashSet newSet = (HashSet) super.clone();
newSet.map = (HashMap) map.clone();
return newSet;
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
/**
* Save the state of this HashSet instance to a stream (that is,
* serialize it).
*
* @serialData The capacity of the backing 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 HashSet 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 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);
}
// 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);
// 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
@SuppressWarnings("unchecked")
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
/**
* Creates a late-binding
* and fail-fast {@link Spliterator} over the elements in this
* set.
*
*
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 spliterator() {
return new HashMap.KeySpliterator(map, 0, -1, 0, 0);
}
}
TreeSet
TreeSet 和 HashSet 實(shí)現(xiàn)類似，間接調(diào)用內(nèi)部的 TreeMap ，都是利用紅黑樹算法實(shí)現(xiàn)；TreeSet 會(huì)根據(jù)其元素的自然順序?qū)υ剡M(jìn)行排序,元素依然是唯一的不可重復(fù)，元素不可為 null .
TreeSet 結(jié)構(gòu)圖
LinkedHashSet
介于 HashSet 與 TreeSet 之間，在 HashSet 的基礎(chǔ)上增加了一個(gè)記錄插入順序的雙鏈表。線程不安全有序不重復(fù)集合，基于 LinkedHashMap 實(shí)現(xiàn)，是 HashMap 與雙向鏈表結(jié)合實(shí)現(xiàn)的，利用雙向鏈表記錄插入順序，以保證迭代輸出的有序性。
LinkedHashSet 結(jié)構(gòu)圖
ConcurrentSkipListSet
線程安全的有序不重復(fù)集合，適用于高并發(fā)場(chǎng)景；與 TreeSet 對(duì)比，相同點(diǎn)是都是有序集合，不同點(diǎn)有兩方面，第一 TreeSet 是非線程安全的，第二 ConcurrentSkipListSet 是基于 ConcurrentSkipListMap 通過(guò)跳表數(shù)據(jù)結(jié)構(gòu)實(shí)現(xiàn)而 TreeSet 是基于 TreeMap 通過(guò)紅黑樹算法實(shí)現(xiàn)。
ConcurrentSkipListSet 結(jié)構(gòu)圖
CopyOnWriteArraySet
線程安全的無(wú)序不重復(fù)集合，適用于高并發(fā)場(chǎng)景；與 HashSet 對(duì)比，相同點(diǎn)是都是無(wú)序集合，不同點(diǎn)有有兩個(gè)，第一 HashSet 是非線程安全的，第二 CopyOnWriteArraySet 是基于 CopyOnWriteArrayList 通過(guò)動(dòng)態(tài)數(shù)組數(shù)據(jù)結(jié)構(gòu)實(shí)現(xiàn)而 HashSet 是基于 HashMap 通過(guò)散列表數(shù)據(jù)結(jié)構(gòu)實(shí)現(xiàn)。
CopyOnWriteArraySet 結(jié)構(gòu)圖
EnumSet
Set針對(duì)枚舉類型的接口實(shí)現(xiàn)類；通過(guò)位向量實(shí)現(xiàn)；EnumSet 中所有元素都必須是指定的枚舉類型或枚舉值，由 EnumSet 創(chuàng)建時(shí)指定，集合元素為有序、不重復(fù)、非 null ，元素的順序與枚舉類元素順序相同；
EnumSet 結(jié)構(gòu)圖
JobStateReasons
JobStateReasons 結(jié)構(gòu)圖
ConcurrentHashMap.KeySetView
KeySetView 結(jié)構(gòu)圖
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