TreeSet 源码

概括

TreeSet 是一个有序集合，底层采用 TreeMap 实现，非线程安全。

继承结构

基本属性

View Code

/**
 * 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();

• NavigableMap

  TreeSet 底层使用NavigableMap存储元素

• PRESENT

  虚拟元素, 用来作为 value 存储在 map 中

构造方法

View Code

/**
 * 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>());
}

/**
 * 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));
}

/**
 * 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);
}

/**
 * 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);
}

• TreeSet 可指定具体的存储，只要是实现 NavigableMap ；
• TreeSet 默认使用 TreeMap 存储；

其他操作方法

View Code

/**
 * 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;
}

/**
 * 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;
}

/**
 * Removes all of the elements from this set.
 * The set will be empty after this call returns.
 */
public void clear() {
    m.clear();
}

/**
 * Adds all of the elements in the specified collection to this set.
 *
 * @param c collection containing elements to be added to this set
 * @return {@code true} if this set changed as a result of the call
 * @throws ClassCastException if the elements provided cannot be compared
 *         with the elements currently in the set
 * @throws NullPointerException if the specified collection is null or
 *         if any element is null and this set uses natural ordering, or
 *         its comparator does not permit null elements
 */
public  boolean addAll(Collection<? extends E> c) {
    // Use linear-time version if applicable
    if (m.size()==0 && c.size() > 0 &&
        c instanceof SortedSet &&
        m instanceof TreeMap) {
        SortedSet<? extends E> set = (SortedSet<? extends E>) c;
        TreeMap<E,Object> map = (TreeMap<E, Object>) m;
        Comparator<?> cc = set.comparator();
        Comparator<? super E> mc = map.comparator();
        if (cc==mc || (cc != null && cc.equals(mc))) {
            map.addAllForTreeSet(set, PRESENT);
            return true;
        }
    }
    return super.addAll(c);
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if {@code fromElement} or {@code toElement}
 *         is null and this set uses natural ordering, or its comparator
 *         does not permit null elements
 * @throws IllegalArgumentException {@inheritDoc}
 * @since 1.6
 */
public NavigableSet<E> subSet(E fromElement, boolean fromInclusive,
                              E toElement,   boolean toInclusive) {
    return new TreeSet<>(m.subMap(fromElement, fromInclusive,
                                   toElement,   toInclusive));
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if {@code toElement} is null and
 *         this set uses natural ordering, or its comparator does
 *         not permit null elements
 * @throws IllegalArgumentException {@inheritDoc}
 * @since 1.6
 */
public NavigableSet<E> headSet(E toElement, boolean inclusive) {
    return new TreeSet<>(m.headMap(toElement, inclusive));
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if {@code fromElement} is null and
 *         this set uses natural ordering, or its comparator does
 *         not permit null elements
 * @throws IllegalArgumentException {@inheritDoc}
 * @since 1.6
 */
public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
    return new TreeSet<>(m.tailMap(fromElement, inclusive));
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if {@code fromElement} or
 *         {@code toElement} is null and this set uses natural ordering,
 *         or its comparator does not permit null elements
 * @throws IllegalArgumentException {@inheritDoc}
 */
public SortedSet<E> subSet(E fromElement, E toElement) {
    return subSet(fromElement, true, toElement, false);
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if {@code toElement} is null
 *         and this set uses natural ordering, or its comparator does
 *         not permit null elements
 * @throws IllegalArgumentException {@inheritDoc}
 */
public SortedSet<E> headSet(E toElement) {
    return headSet(toElement, false);
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if {@code fromElement} is null
 *         and this set uses natural ordering, or its comparator does
 *         not permit null elements
 * @throws IllegalArgumentException {@inheritDoc}
 */
public SortedSet<E> tailSet(E fromElement) {
    return tailSet(fromElement, true);
}

public Comparator<? super E> comparator() {
    return m.comparator();
}

/**
 * @throws NoSuchElementException {@inheritDoc}
 */
public E first() {
    // 返回最小的元素
    return m.firstKey();
}

/**
 * @throws NoSuchElementException {@inheritDoc}
 */
public E last() {
    // 返回最大的元素
    return m.lastKey();
}

// NavigableSet API methods

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if the specified element is null
 *         and this set uses natural ordering, or its comparator
 *         does not permit null elements
 * @since 1.6
 */
public E lower(E e) {
    // 小于等于 e 的最大元素
    return m.lowerKey(e);
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if the specified element is null
 *         and this set uses natural ordering, or its comparator
 *         does not permit null elements
 * @since 1.6
 */
public E floor(E e) {
    // 小于等于 e 的最大元素
    return m.floorKey(e);
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if the specified element is null
 *         and this set uses natural ordering, or its comparator
 *         does not permit null elements
 * @since 1.6
 */
public E ceiling(E e) {
    // 大于等于 e 的最小元素
    return m.ceilingKey(e);
}

/**
 * @throws ClassCastException {@inheritDoc}
 * @throws NullPointerException if the specified element is null
 *         and this set uses natural ordering, or its comparator
 *         does not permit null elements
 * @since 1.6
 */
public E higher(E e) {
    // 大于等于 e 的最小元素
    return m.higherKey(e);
}

/**
 * @since 1.6
 */
public E pollFirst() {
    // 最小元素
    Map.Entry<E,?> e = m.pollFirstEntry();
    return (e == null) ? null : e.getKey();
}

/**
 * @since 1.6
 */
public E pollLast() {
    // 最大元素
    Map.Entry<E,?> e = m.pollLastEntry();
    return (e == null) ? null : e.getKey();
}

/**
 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
 * set.
 *
 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
 * {@link Spliterator#DISTINCT}, {@link Spliterator#SORTED}, and
 * {@link Spliterator#ORDERED}.  Overriding implementations should document
 * the reporting of additional characteristic values.
 *
 * <p>The spliterator's comparator (see
 * {@link java.util.Spliterator#getComparator()}) is {@code null} if
 * the tree set's comparator (see {@link #comparator()}) is {@code null}.
 * Otherwise, the spliterator's comparator is the same as or imposes the
 * same total ordering as the tree set's comparator.
 *
 * @return a {@code Spliterator} over the elements in this set
 * @since 1.8
 */
public Spliterator<E> spliterator() {
    // 分割迭代器
    return TreeMap.keySpliteratorFor(m);
}

• 源码比较简单，基本都是调用 map 相应的方法，可根据 key 取最近的元素。

总结

• TreeSet底层使用 NavigableMap 存储元素；
• TreeSet是有序的；非线程安全的；
• TreeSet实现了 NavigableSet 接口，而 NavigableSet 继承自 SortedSet 接口；

问题

TreeSet和LinkedHashSet都是有序的，那它们有何不同？

• LinkedHashSet 有序性主要依赖于 LinkedHashMap 的有序性，所以它的有序性是指按照插入顺序保证的有序性；
  而 LinkedHashMap 维护了一个双向链表，实现元素顺序的控制；
• TreeSet 实现了 SortedSet 接口，它的有序性主要依赖于 NavigableMap 的有序性，而 NavigableMap 又继承自 SortedMap；
  这个接口的有序性是指按照 key 的自然排序保证的有序性，而key的自然排序又有两种实现方式，一种是key实现Comparable接口，一种是构造方法传入Comparator比较器。

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