集合之TreeSet（含JDK1.8源碼分析）

1、前言

　　前面分析了Set接口下的hashSet和linkedHashSet，下面接着來看treeSet，treeSet的底層實現是基於treeMap的。

　　四個關注點在treeSet上的答案

2、treeSet的數據結構

　　由於treeSet的底層是基於treeMap的，因此treeSet的數據結構就是treeMap的數據結構：紅黑樹，由於前面已經分析過了treeMap的數據結構，這裏再也不贅述。集合之TreeMap（含JDK1.8源碼分析）。

3、treeSet源碼分析-屬性及構造函數

　　3.1 類的繼承關係

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

　　說明：實現了NavigableSet接口，定義了一些共有的操做。

　　3.2 類的屬性

/** * 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(); //版本號     private static final long serialVersionUID = -2479143000061671589L;

　　說明：屬性m爲NavigableMap接口，treeSet的一些操做都是基於此map的，而前面分析treeMap的時候，發現treeMap實現了NavigableMap接口，因此hashSet中基於NavigableMap接口的操做實際上都是基於其實現類treeMap的操做，此處也是多態的概念。

　　treeMap的繼承實現關係：

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

　　而對於Object類型的屬性PRESENT，在分析hashSet的時候已經作了說明，由於map（treeSet中是NavigableMap）是存儲key-value鍵值對的，因此PRESENT只是配一下key-value中value的位置，起個佔位的做用，沒有什麼實際的意義，全部經過treeSet添加進來的key都對應同一個value值，PRESENT。

　　3.3 類的構造函數

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

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

　　說明：構建一個treeSet，基於navigable map（其實現類treeMap）實現的。

　　二、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>()); }

　　說明：構建一個treeSet，排序是基於插入元素的天然順序。

　　三、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)); }

　　說明：構建一個treeSet，排序是基於自定義的比較器的排序規則。

　　四、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); }

　　說明：構建一個treeSet，包含參數c中的元素，排序是基於元素的天然順序。

　　五、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); }

　　說明：構建一個treeSet，排序是基於SortedSet指定的排序規則。

4、treeSet源碼分析-核心函數

　　4.1 增：add函數----存儲元素

/** * 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; }

　　說明：添加一個原先set中未存在的元素，返回true，如果該元素已經存在，set不作改變，返回false。

　　能夠看到其方法內部調用navigableMap的put方法，由於treeMap是其實現類，因此實際執行的時候，調用的是treeMap的put方法。可參見：集合之TreeMap（含JDK1.8源碼分析）。

　　4.2 增：remove函數----刪除元素

/** * 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; }

　　說明：若set中存在要刪除的元素，刪除，返回true，不存在，返回false。

　　能夠看到其方法內部調用navigableMap的remove方法，由於treeMap是其實現類，因此實際執行的時候，調用的是treeMap的remove方法。可參見：集合之TreeMap（含JDK1.8源碼分析）。

　　4.3 contains函數----是否存在該元素

/** * 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); }

　　說明：若set中存在該元素，返回true，不存在，返回false。

　　能夠看到其方法內部調用navigableMap的containsKey方法，由於treeMap是其實現類，因此實際執行的時候，調用的是treeMap的containsKey方法。

 5、總結

　　總之，treeSet底層是基於treeMap實現的，能夠自定義比較器對元素進行排序，或是使用元素的天然順序。