Java基础知识_Set集合

一、HashSet剖析

按照国际惯例，我们来看看HashSet的顶部注释

/**
 * 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
 */

从顶部注释来看我们可以归纳HashSet的要点了

　　实现Set接口

　　不保证迭代顺序

　　允许元素为null

　　底层实际上是一个HashMap实例

　　非同步

　　初始容量非常影响迭代性能

    public HashSet() {
        map = new HashMap<>();
    }

我们知道Map是一个映射，有value有key的，既然HashSet底层用的是HashMap，那么value在哪里呢？？？

 // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

value是一个Object，所有的value都是它

所以可以直接总结出：HashSet实际上就是封装了HashMap，操作HashSet元素实际上就是操作HashMap。也是面向对象的一种体现，重用性贼高。

 

二、TreeSet剖析

按照惯例，我们来看看TreeSet顶部注释

/**
 * A {@link NavigableSet} implementation based on a {@link TreeMap}.
 * The elements are ordered using their {@linkplain Comparable natural
 * ordering}, or by a {@link Comparator} provided at set creation
 * time, depending on which constructor is used.
 *
 * <p>This implementation provides guaranteed log(n) time cost for the basic
 * operations ({@code add}, {@code remove} and {@code contains}).
 *
 * <p>Note that the ordering maintained by a set (whether or not an explicit
 * comparator is provided) must be <i>consistent with equals</i> if it is to
 * correctly implement the {@code Set} interface.  (See {@code Comparable}
 * or {@code Comparator} for a precise definition of <i>consistent with
 * equals</i>.)  This is so because the {@code Set} interface is defined in
 * terms of the {@code equals} operation, but a {@code TreeSet} instance
 * performs all element comparisons using its {@code compareTo} (or
 * {@code compare}) method, so two elements that are deemed equal by this method
 * are, from the standpoint of the set, equal.  The behavior of a set
 * <i>is</i> well-defined even if its ordering is inconsistent with equals; it
 * just fails to obey the general contract of the {@code Set} interface.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a tree 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#synchronizedSortedSet Collections.synchronizedSortedSet}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the set: <pre>
 *   SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));</pre>
 *
 * <p>The iterators returned by this class's {@code iterator} 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 {@code remove}
 * 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 {@code ConcurrentModificationException} 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     Collection
 * @see     Set
 * @see     HashSet
 * @see     Comparable
 * @see     Comparator
 * @see     TreeMap
 * @since   1.2
 */

底层是Treemap,实现了NavigableSet接口，根据Comparable在实现自然排序，或者在创建对象时，使用构造方法传递Comparator对象实现排序，时间复杂度是logn，通过compareTo或者compare方法认证元素是否相等。非同步

我们来归纳一下TreeSet

　　实现NavigableSet接口

　　可以实现排序功能

　　底层实际上是一个TreeMap实例

　　非同步

private static final Object PRESENT = new Object();

    /**
     * Constructs a set backed by the specified navigable map.
     */
    TreeSet(NavigableMap<E,Object> m) {
        this.m = m;
    }

    /**
     * Constructs a new, empty tree set, sorted according to the
     * natural ordering of its elements.  All elements inserted into
     * the set must implement the {@link Comparable} interface.
     * Furthermore, all such elements must be <i>mutually
     * comparable</i>: {@code e1.compareTo(e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the set.  If the user attempts to add an element
     * to the set that violates this constraint (for example, the user
     * attempts to add a string element to a set whose elements are
     * integers), the {@code add} call will throw a
     * {@code ClassCastException}.
     */
    public TreeSet() {
        this(new TreeMap<E,Object>());
    }

Map的value是new Object();

实例是new TreeMap<E,Object>()

 

三、LinkedHashSet剖析

按照国际惯例，看看LinkedHashSet顶部注释：

/**
 * <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
 */

迭代有序，维护了一个双项链表，允许为null，性能比HashSet差一点点，因为要维护双向链表，迭代的是双向链表。非同步的。

从顶部注释来看，我们可以归纳LinkedHashSet的要点

　　迭代是有序的

　　允许为null

　　底层实际上是一个HashMap+双向链表的实例（其实就是LinkedHashMap）

　　非同步

　　性能比HashSet差一丢丢，因为要维护一个双向链表。

　　初始容量与迭代无关，LinkedHashSet迭代的是双向链表

 

四、总结

可以明显的看到，Set集合的底层就是Map，所以没有做什么分析，也没什么好分析的

下面稍微总结一下吧

HashSet 

　　无序，允许为null，底层是HashMap（散列表+红黑树），非线程同步

TreeSet

　　迭代有序，不允许为null，底层是TreeMap（红黑树），非线程同步

LinkedHashSet：

　　迭代有序，允许为null，底层是HashMap+双向链表，非线程同步