集合之TreeSet（含JDK1.8源码分析）

一、前言

　　前面分析了Set接口下的hashSet和linkedHashSet，下面接着来看treeSet，treeSet的底层实现是基于treeMap的。

　　四个关注点在treeSet上的答案

二、treeSet的数据结构

　　因为treeSet的底层是基于treeMap的，所以treeSet的数据结构就是treeMap的数据结构：红黑树，因为前面已经分析过了treeMap的数据结构，这里不再赘述。集合之TreeMap（含JDK1.8源码分析）。

三、treeSet源码分析-属性及构造函数

　　3.1 类的继承关系

public class TreeSet<E> extends AbstractSet<E>
    implements NavigableSet<E>, Cloneable, java.io.Serializable

　　说明：实现了NavigableSet接口，定义了一些共有的操作。

　　3.2 类的属性

/**
     * The backing map.
     */
    private transient NavigableMap<E,Object> m;

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

    //版本号
    private static final long serialVersionUID = -2479143000061671589L;

　　说明：属性m为NavigableMap接口，treeSet的一些操作都是基于此map的，而前面分析treeMap的时候，发现treeMap实现了NavigableMap接口，所以hashSet中基于NavigableMap接口的操作实际上都是基于其实现类treeMap的操作，此处也是多态的概念。

　　treeMap的继承实现关系：

public class TreeMap<K,V>
    extends AbstractMap<K,V>
    implements NavigableMap<K,V>, Cloneable, java.io.Serializable

　　而对于Object类型的属性PRESENT，在分析hashSet的时候已经做了说明，因为map（treeSet中是NavigableMap）是存储key-value键值对的，所以PRESENT只是配一下key-value中value的位置，起个占位的作用，没有什么实际的意义，所有通过treeSet添加进来的key都对应同一个value值，PRESENT。

　　3.3 类的构造函数

　　1、TreeSet(NavigableMap<E,Object> m)型

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

　　说明：构建一个treeSet，基于navigable map（其实现类treeMap）实现的。

　　2、TreeSet()型

/**
     * 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>());
    }

　　说明：构建一个treeSet，排序是基于插入元素的自然顺序。

　　3、TreeSet(Comparator<? super E> comparator)型

/**
     * Constructs a new, empty tree set, sorted according to the specified
     * comparator.  All elements inserted into the set must be <i>mutually
     * comparable</i> by the specified comparator: {@code comparator.compare(e1,
     * 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, the
     * {@code add} call will throw a {@code ClassCastException}.
     *
     * @param comparator the comparator that will be used to order this set.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the elements will be used.
     */
    public TreeSet(Comparator<? super E> comparator) {
        this(new TreeMap<>(comparator));
    }

　　说明：构建一个treeSet，排序是基于自定义的比较器的排序规则。

　　4、TreeSet(Collection<? extends E> c)型

/**
     * Constructs a new tree set containing the elements in the specified
     * collection, sorted according to the <i>natural ordering</i> 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.
     *
     * @param c collection whose elements will comprise the new set
     * @throws ClassCastException if the elements in {@code c} are
     *         not {@link Comparable}, or are not mutually comparable
     * @throws NullPointerException if the specified collection is null
     */
    public TreeSet(Collection<? extends E> c) {
        this();
        addAll(c);
    }

　　说明：构建一个treeSet，包含参数c中的元素，排序是基于元素的自然顺序。

　　5、TreeSet(SortedSet<E> s)型

/**
     * Constructs a new tree set containing the same elements and
     * using the same ordering as the specified sorted set.
     *
     * @param s sorted set whose elements will comprise the new set
     * @throws NullPointerException if the specified sorted set is null
     */
    public TreeSet(SortedSet<E> s) {
        this(s.comparator());
        addAll(s);
    }

　　说明：构建一个treeSet，排序是基于SortedSet指定的排序规则。

四、treeSet源码分析-核心函数

　　4.1 增：add函数----存储元素

/**
     * 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
     * the set contains no element {@code e2} such that
     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
     * 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
     * @throws ClassCastException if the specified object cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     *         and this set uses natural ordering, or its comparator
     *         does not permit null elements
     */
    public boolean add(E e) {
        return m.put(e, PRESENT)==null;
    }

　　说明：添加一个原先set中未存在的元素，返回true，若是该元素已经存在，set不做改变，返回false。

　　可以看到其方法内部调用navigableMap的put方法，因为treeMap是其实现类，所以实际执行的时候，调用的是treeMap的put方法。可参见：集合之TreeMap（含JDK1.8源码分析）。

　　4.2 增：remove函数----删除元素

/**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
     * 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 this set contained the specified element
     * @throws ClassCastException if the specified object cannot be compared
     *         with the elements currently in this set
     * @throws NullPointerException if the specified element is null
     *         and this set uses natural ordering, or its comparator
     *         does not permit null elements
     */
    public boolean remove(Object o) {
        return m.remove(o)==PRESENT;
    }

　　说明：若set中存在要删除的元素，删除，返回true，不存在，返回false。

　　可以看到其方法内部调用navigableMap的remove方法，因为treeMap是其实现类，所以实际执行的时候，调用的是treeMap的remove方法。可参见：集合之TreeMap（含JDK1.8源码分析）。

　　4.3 contains函数----是否存在该元素

/**
     * 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
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o object to be checked for containment in this set
     * @return {@code true} if this set contains the specified element
     * @throws ClassCastException if the specified object cannot be compared
     *         with the elements currently in the set
     * @throws NullPointerException if the specified element is null
     *         and this set uses natural ordering, or its comparator
     *         does not permit null elements
     */
    public boolean contains(Object o) {
        return m.containsKey(o);
    }

　　说明：若set中存在该元素，返回true，不存在，返回false。

　　可以看到其方法内部调用navigableMap的containsKey方法，因为treeMap是其实现类，所以实际执行的时候，调用的是treeMap的containsKey方法。

五、总结

　　总之，treeSet底层是基于treeMap实现的，可以自定义比较器对元素进行排序，或是使用元素的自然顺序。