JDK7集合框架源码阅读（六） HashSet与LinkedHashSet
基于版本jdk1.7.0_80
java.util.HashSet
java.util.LinkedHashSet
代码如下
HashSet，312行
/* * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.util; /** * This class implements the <tt>Set</tt> interface, backed by a hash table * (actually a <tt>HashMap</tt> instance). It makes no guarantees as to the * iteration order of the set; in particular, it does not guarantee that the * order will remain constant over time. This class permits the <tt>null</tt> * element. * * <p>This class offers constant time performance for the basic operations * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>), * assuming the hash function disperses the elements properly among the * buckets. Iterating over this set requires time proportional to the sum of * the <tt>HashSet</tt> instance's size (the number of elements) plus the * "capacity" of the backing <tt>HashMap</tt> instance (the number of * buckets). Thus, it's very important not to set the initial capacity too * high (or the load factor too low) if iteration performance is important. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access a hash set concurrently, and at least one of * the threads modifies the set, it <i>must</i> be synchronized externally. * This is typically accomplished by synchronizing on some object that * naturally encapsulates the set. * * If no such object exists, the set should be "wrapped" using the * {@link Collections#synchronizedSet Collections.synchronizedSet} * method. This is best done at creation time, to prevent accidental * unsynchronized access to the set:<pre> * Set s = Collections.synchronizedSet(new HashSet(...));</pre> * * <p>The iterators returned by this class's <tt>iterator</tt> method are * <i>fail-fast</i>: if the set is modified at any time after the iterator is * created, in any way except through the iterator's own <tt>remove</tt> * method, the Iterator throws a {@link ConcurrentModificationException}. * Thus, in the face of concurrent modification, the iterator fails quickly * and cleanly, rather than risking arbitrary, non-deterministic behavior at * an undetermined time in the future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <E> the type of elements maintained by this set * * @author Josh Bloch * @author Neal Gafter * @see Collection * @see Set * @see TreeSet * @see HashMap * @since 1.2 */ 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 <tt>HashMap</tt> 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 <tt>HashMap</tt> 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 <tt>HashMap</tt> 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 <tt>HashMap</tt> 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 <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 ? e==null : 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 ? e2==null : 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 ? e==null : 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(); } /** * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements * themselves are not cloned. * * @return a shallow copy of this set */ 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(); } } /** * Save the state of this <tt>HashSet</tt> instance to a stream (that is, * serialize it). * * @serialData The capacity of the backing <tt>HashMap</tt> 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 <tt>HashSet</tt> 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 in HashMap capacity and load factor and create backing HashMap int capacity = s.readInt(); float loadFactor = s.readFloat(); map = (((HashSet)this) instanceof LinkedHashSet ? new LinkedHashMap<E,Object>(capacity, loadFactor) : new HashMap<E,Object>(capacity, loadFactor)); // Read in size int size = s.readInt(); // Read in all elements in the proper order. for (int i=0; i<size; i++) { E e = (E) s.readObject(); map.put(e, PRESENT); } } }
LinkedHashSet，171行
/* * Copyright (c) 2000, 2006, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.util; /** * <p>Hash table and linked list implementation of the <tt>Set</tt> interface, * with predictable iteration order. This implementation differs from * <tt>HashSet</tt> in that it maintains a doubly-linked list running through * all of its entries. This linked list defines the iteration ordering, * which is the order in which elements were inserted into the set * (<i>insertion-order</i>). Note that insertion order is <i>not</i> affected * if an element is <i>re-inserted</i> into the set. (An element <tt>e</tt> * is reinserted into a set <tt>s</tt> if <tt>s.add(e)</tt> is invoked when * <tt>s.contains(e)</tt> would return <tt>true</tt> immediately prior to * the invocation.) * * <p>This implementation spares its clients from the unspecified, generally * chaotic ordering provided by {@link HashSet}, without incurring the * increased cost associated with {@link TreeSet}. It can be used to * produce a copy of a set that has the same order as the original, regardless * of the original set's implementation: * <pre> * void foo(Set s) { * Set copy = new LinkedHashSet(s); * ... * } * </pre> * This technique is particularly useful if a module takes a set on input, * copies it, and later returns results whose order is determined by that of * the copy. (Clients generally appreciate having things returned in the same * order they were presented.) * * <p>This class provides all of the optional <tt>Set</tt> operations, and * permits null elements. Like <tt>HashSet</tt>, it provides constant-time * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and * <tt>remove</tt>), assuming the hash function disperses elements * properly among the buckets. Performance is likely to be just slightly * below that of <tt>HashSet</tt>, due to the added expense of maintaining the * linked list, with one exception: Iteration over a <tt>LinkedHashSet</tt> * requires time proportional to the <i>size</i> of the set, regardless of * its capacity. Iteration over a <tt>HashSet</tt> is likely to be more * expensive, requiring time proportional to its <i>capacity</i>. * * <p>A linked hash set has two parameters that affect its performance: * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely * as for <tt>HashSet</tt>. Note, however, that the penalty for choosing an * excessively high value for initial capacity is less severe for this class * than for <tt>HashSet</tt>, as iteration times for this class are unaffected * by capacity. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access a linked hash set concurrently, and at least * one of the threads modifies the set, it <em>must</em> be synchronized * externally. This is typically accomplished by synchronizing on some * object that naturally encapsulates the set. * * If no such object exists, the set should be "wrapped" using the * {@link Collections#synchronizedSet Collections.synchronizedSet} * method. This is best done at creation time, to prevent accidental * unsynchronized access to the set: <pre> * Set s = Collections.synchronizedSet(new LinkedHashSet(...));</pre> * * <p>The iterators returned by this class's <tt>iterator</tt> method are * <em>fail-fast</em>: if the set is modified at any time after the iterator * is created, in any way except through the iterator's own <tt>remove</tt> * method, the iterator will throw a {@link ConcurrentModificationException}. * Thus, in the face of concurrent modification, the iterator fails quickly * and cleanly, rather than risking arbitrary, non-deterministic behavior at * an undetermined time in the future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <E> the type of elements maintained by this set * * @author Josh Bloch * @see Object#hashCode() * @see Collection * @see Set * @see HashSet * @see TreeSet * @see Hashtable * @since 1.4 */ public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, java.io.Serializable { private static final long serialVersionUID = -2851667679971038690L; /** * Constructs a new, empty linked hash set with the specified initial * capacity and load factor. * * @param initialCapacity the initial capacity of the linked hash set * @param loadFactor the load factor of the linked hash set * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */ public LinkedHashSet(int initialCapacity, float loadFactor) { super(initialCapacity, loadFactor, true); } /** * Constructs a new, empty linked hash set with the specified initial * capacity and the default load factor (0.75). * * @param initialCapacity the initial capacity of the LinkedHashSet * @throws IllegalArgumentException if the initial capacity is less * than zero */ public LinkedHashSet(int initialCapacity) { super(initialCapacity, .75f, true); } /** * Constructs a new, empty linked hash set with the default initial * capacity (16) and load factor (0.75). */ public LinkedHashSet() { super(16, .75f, true); } /** * Constructs a new linked hash set with the same elements as the * specified collection. The linked hash set is created with an initial * capacity sufficient to hold the elements in the specified collection * and the default load factor (0.75). * * @param c the collection whose elements are to be placed into * this set * @throws NullPointerException if the specified collection is null */ public LinkedHashSet(Collection<? extends E> c) { super(Math.max(2*c.size(), 11), .75f, true); addAll(c); } }
这两个类都很简单，于是合在一起讲了
HashSet
1. 接口分析
继承于AbstractSet抽象类
Set，Cloneable，java.io.Serializable接口
2. 实现原理
内置了一个HashMap，任何操作都是将传入的key与一个dummy对象组成一对，然后对这个内置的HashMap操作
随意列举几个关键方法如下
public int size() { return map.size(); } public boolean contains(Object o) { return map.containsKey(o); } public boolean add(E e) { return map.put(e, PRESENT)==null; } public boolean remove(Object o) { return map.remove(o)==PRESENT; }
3. 对LinkedHashSet的支持
HashSet有一个friendly访问权限的构造方法，其中创建了LinkedHashMap用来存储对象
这个构造方法只有LinkedHashSet会调用
4. 迭代器
返回的迭代器是内置map的keyset的迭代器
public Iterator<E> iterator() { return map.keySet().iterator(); }
LinkedHashSet
1. 接口分析
继承于HashSet类
Set，Cloneable，java.io.Serializable接口
2. 实现原理
全部调用父类HashSet的方法
父类HashSet再利用内部集成的LinkedHashMap来实现按元素的最后访问次序来迭代遍历
