HashSet是一個沒有重複元素的集合,HashSet是由HashMap實現的,不保證元素的順序,而HashSet允許使用null元素。 HashSet不是執行緒安全的,當多個執行緒同時存取HashSet時,會出現問題,解決的方法是透過對自然封裝該Set的物件執行同步操作來完成的。也可以使用Collections.synchronizedSet方法來包裝set。
Set s=Collectins.synchronizedSet(new HashSet())。
HashSet的繼承實作圖:
#從圖中我們可以看出:
(1)HashSet繼承於AbstractSet,並實作了Set介面。
(2)HashSet本質是一個沒有重複元素的集合,透過HashMap實現的。 HashSet中有一個HashMap的map變數,HashSet的運算函數,其實都是透過map實現的。 HashSet是將值儲存為HashMap的key。
HashSet主要的函數:
add(E object) clear() Object clone() contains(Object object) isEmpty() Iteratora8093152e673feb7aba1828c43532094 iterator() remove(Object object) size()
HashSet的遍歷方式:
#
(Iterator iterator = set.iterator()iterator.hasNext()) { iterator.next()}
(1)透過Iterator迭代方式遍歷HashSet:先根據iterator()取得HashSet的迭代器,迭代取得各個元素。
String[] arr = (String[])set.toArray(String[])(String str:arr) { System.out.printf(str) }
(2)透過for-each遍歷HashSet:先透過toArray()取得HashSet的元素集合對應的數組,之後遍歷數組得到元素。
Hello { (String[] args) Exception { () } () { HashSet set = HashSet()set.add()set.add()set.add()set.add()set.add()System..printf(set.size())System..printf(set.contains())System..printf(set.contains())set.remove()String[] arr = (String[])set.toArray(String[])(String str:arr) System..printf(str)HashSet otherset = HashSet()otherset.add()otherset.add()otherset.add()HashSet removeset = (HashSet)set.clone()removeset.removeAll(otherset)System..printf(removeset)HashSet retainset = (HashSet)set.clone()retainset.retainAll(otherset)System..printf(retainset)(Iterator iterator = set.iterator()iterator.hasNext()) System..printf(iterator.next())set.clear()System..printf(set.isEmpty()?:)} }
HashSet範例程式碼:
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size : 5 HashSet contains a :true HashSet contains g :false for each : a for each : b for each : c for each : d removeset : [a, d] retainset : [b, c] iterator : a iterator : b iterator : c iterator : d set is empty
public class HashSet1a4db2c2c2313771e5742b6debf617a1 extends AbstractSet1a4db2c2c2313771e5742b6debf617a1 implements Set1a4db2c2c2313771e5742b6debf617a1, Cloneable, java.io.Serializable { static final long serialVersionUID = -5024744406713321676L; private transient HashMapc81ac021d9fe376b0afca3578d66e836 map;//HashSet是利用map来存储数据 private static final Object PRESENT = new Object(); /** * Constructs a new, empty set; the backing 78f983dbc27872ba42409adefe5049d9HashMapd98ca7951c814b9263d12f482df06c69 instance has * default initial capacity (16) and load factor (0.75). */ //构造一个空Set public HashSet() { map = new HashMapa8093152e673feb7aba1828c43532094(); } /** * Constructs a new set containing the elements in the specified * collection. The 78f983dbc27872ba42409adefe5049d9HashMapd98ca7951c814b9263d12f482df06c69 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 */ //构造一个有初始参数c的构造函数 public HashSet(Collection216930c0ab3d232ff5eb2d5ba8cef959 c) { map = new HashMapa8093152e673feb7aba1828c43532094(Math.max((int) (c.size()/.75f) + 1, 16)); addAll(c); } /** * Constructs a new, empty set; the backing 78f983dbc27872ba42409adefe5049d9HashMapd98ca7951c814b9263d12f482df06c69 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 */ //初始容量和加载因子的构造函数,其实是HashMap的构造函数 public HashSet(int initialCapacity, float loadFactor) { map = new HashMapa8093152e673feb7aba1828c43532094(initialCapacity, loadFactor); } /** * Constructs a new, empty set; the backing 78f983dbc27872ba42409adefe5049d9HashMapd98ca7951c814b9263d12f482df06c69 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 */ //构造初始容量的构造函数 public HashSet(int initialCapacity) { map = new HashMapa8093152e673feb7aba1828c43532094(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 */ //构造一个空的LinkedHashMap HashSet(int initialCapacity, float loadFactor, boolean dummy) { map = new LinkedHashMapa8093152e673feb7aba1828c43532094(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 */ //返回key的迭代 public Iterator1a4db2c2c2313771e5742b6debf617a1 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) */ //大小 public int size() { return map.size(); } /** * Returns 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if this set contains no elements. * * @return 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if this set contains no elements */ //判断是否为空 public boolean isEmpty() { return map.isEmpty(); } /** * Returns 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if this set contains the specified element. * More formally, returns 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if and only if this set * contains an element 78f983dbc27872ba42409adefe5049d9ed98ca7951c814b9263d12f482df06c69 such that * 78f983dbc27872ba42409adefe5049d9(o==null ? e==null : o.equals(e))d98ca7951c814b9263d12f482df06c69. * * @param o element whose presence in this set is to be tested * @return 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if this set contains the specified element */ //判断是否包含某个对象 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 78f983dbc27872ba42409adefe5049d9ed98ca7951c814b9263d12f482df06c69 to this set if * this set contains no element 78f983dbc27872ba42409adefe5049d9e2d98ca7951c814b9263d12f482df06c69 such that * 78f983dbc27872ba42409adefe5049d9(e==null ? e2==null : e.equals(e2))d98ca7951c814b9263d12f482df06c69. * If this set already contains the element, the call leaves the set * unchanged and returns 78f983dbc27872ba42409adefe5049d9falsed98ca7951c814b9263d12f482df06c69. * * @param e element to be added to this set * @return 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if this set did not already contain the specified * element */ //增加对象 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 78f983dbc27872ba42409adefe5049d9ed98ca7951c814b9263d12f482df06c69 such that * 78f983dbc27872ba42409adefe5049d9(o==null ? e==null : o.equals(e))d98ca7951c814b9263d12f482df06c69, * if this set contains such an element. Returns 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 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 78f983dbc27872ba42409adefe5049d9trued98ca7951c814b9263d12f482df06c69 if the set contained the specified element */ //删除对象 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. */ //清空set public void clear() { map.clear(); } /** * Returns a shallow copy of this 78f983dbc27872ba42409adefe5049d9HashSetd98ca7951c814b9263d12f482df06c69 instance: the elements * themselves are not cloned. * * @return a shallow copy of this set */ //深拷贝 @SuppressWarnings("unchecked") public Object clone() { try { HashSet1a4db2c2c2313771e5742b6debf617a1 newSet = (HashSet1a4db2c2c2313771e5742b6debf617a1) super.clone(); newSet.map = (HashMap8e751a6654859f4ad3ab548dad16afed) map.clone(); return newSet; } catch (CloneNotSupportedException e) { throw new InternalError(e); } } /** * Save the state of this 78f983dbc27872ba42409adefe5049d9HashSetd98ca7951c814b9263d12f482df06c69 instance to a stream (that is, * serialize it). * * @serialData The capacity of the backing 78f983dbc27872ba42409adefe5049d9HashMapd98ca7951c814b9263d12f482df06c69 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. */ //序列化写 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 78f983dbc27872ba42409adefe5049d9HashSetd98ca7951c814b9263d12f482df06c69 instance from a stream (that is, * deserialize it). */ //序列化读 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 d4fe061c272c30507d0f695857f4f355)this) instanceof LinkedHashSet ? new LinkedHashMapc81ac021d9fe376b0afca3578d66e836(capacity, loadFactor) : new HashMapc81ac021d9fe376b0afca3578d66e836(capacity, loadFactor)); // Read in all elements in the proper order. for (int i=0; ia999b12ce7dfa09cdfcdb48cd9e1e64e47bdf838fe1f241804499a8943350a02late-binding5db79b134e9f6b82c0b36e0489ee08edd1c6776b927dc33c5d9114750b586338 * and 907fae80ddef53131f3292ee4f81644bfail-fastd1c6776b927dc33c5d9114750b586338 {@link Spliterator} over the elements in this * set. * * e388a4556c0f65e1904146cc1a846beeThe {@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 Spliterator1a4db2c2c2313771e5742b6debf617a1 spliterator() { return new HashMap.KeySpliteratorc81ac021d9fe376b0afca3578d66e836(map, 0, -1, 0, 0); } }###基於Java8的HashSet原始碼:############rrreee
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