TreeSet是一个有序的集合，它的作用是提供有序的Set集合。它继承了AbstractSet抽象类，实现了NavigableSet<E>，Cloneable，Serializable接口。TreeSet是基于TreeMap实现的，TreeSet的元素支持2种排序方式：自然排序或者根据提供的Comparator进行排序。
TreeSet的接口依赖图：
从图中可以看出：
（1）TreeSet继承于AbstractSet，并且实现了NavigableSet接口。
（2）TreeSet是一个包含有序的且没有重复元素的集合，通过TreeMap实现。
TreeSet的主要函数：
boolean add(E object)
boolean addAll(Collection<? extends E> collection)
void clear()
Object clone()
boolean contains(Object object)
E first()
boolean isEmpty()
E last()
E pollFirst()
E pollLast()
E lower(E e)
E floor(E e)
E ceiling(E e)
E higher(E e)
boolean remove(Object object)
int size()
Comparator<? super E> comparator()
Iterator<E> iterator()
Iterator<E> descendingIterator()
SortedSet<E> headSet(E end)
NavigableSet<E> descendingSet()
NavigableSet<E> headSet(E end, boolean endInclusive)
SortedSet<E> subSet(E start, E end)
NavigableSet<E> subSet(E start, boolean startInclusive, E end, boolean endInclusive)
NavigableSet<E> tailSet(E start, boolean startInclusive)
SortedSet<E> tailSet(E start)
TreeSet遍历方式：
（1）迭代器顺序遍历方式：
for(Iterator iter = set.iterator(); iter.hasNext(); )
{
iter.next();
}
（2）迭代器倒序遍历方式：
for(Iterator iter = set.descendingIterator(); iter.hasNext(); )
{
iter.next();
}
（3）foreach遍历HashSet
String[] arr = (String[])set.toArray(new String[0]);
for (String str:arr)
{
System.out.printf("for each : %s\n", str);
}
TreeSet示例代码：
public class Hello {
public static void main(String[] args) {
testTreeSetAPIs();
}
// 测试TreeSet的api
public static void testTreeSetAPIs() {
String val;
// 新建TreeSet
TreeSet tSet = new TreeSet();
// 将元素添加到TreeSet中
tSet.add("aaa");
// Set中不允许重复元素，所以只会保存一个“aaa”
tSet.add("aaa");
tSet.add("bbb");
tSet.add("eee");
tSet.add("ddd");
tSet.add("ccc");
System.out.println("TreeSet:"+tSet);
// 打印TreeSet的实际大小
System.out.printf("size : %d\n", tSet.size());
// 导航方法
// floor(小于、等于)
System.out.printf("floor bbb: %s\n", tSet.floor("bbb"));
// lower(小于)
System.out.printf("lower bbb: %s\n", tSet.lower("bbb"));
// ceiling(大于、等于)
System.out.printf("ceiling bbb: %s\n", tSet.ceiling("bbb"));
System.out.printf("ceiling eee: %s\n", tSet.ceiling("eee"));
// ceiling(大于)
System.out.printf("higher bbb: %s\n", tSet.higher("bbb"));
// subSet()
System.out.printf("subSet(aaa, true, ccc, true): %s\n", tSet.subSet("aaa", true, "ccc", true));
System.out.printf("subSet(aaa, true, ccc, false): %s\n", tSet.subSet("aaa", true, "ccc", false));
System.out.printf("subSet(aaa, false, ccc, true): %s\n", tSet.subSet("aaa", false, "ccc", true));
System.out.printf("subSet(aaa, false, ccc, false): %s\n", tSet.subSet("aaa", false, "ccc", false));
// headSet()
System.out.printf("headSet(ccc, true): %s\n", tSet.headSet("ccc", true));
System.out.printf("headSet(ccc, false): %s\n", tSet.headSet("ccc", false));
// tailSet()
System.out.printf("tailSet(ccc, true): %s\n", tSet.tailSet("ccc", true));
System.out.printf("tailSet(ccc, false): %s\n", tSet.tailSet("ccc", false));
// 删除“ccc”
tSet.remove("ccc");
// 将Set转换为数组
String[] arr = (String[])tSet.toArray(new String[0]);
for (String str:arr)
System.out.printf("for each : %s\n", str);
// 打印TreeSet
System.out.printf("TreeSet:%s\n", tSet);
// 遍历TreeSet
for(Iterator iter = tSet.iterator(); iter.hasNext(); ) {
System.out.printf("iter : %s\n", iter.next());
}
// 删除并返回第一个元素
val = (String)tSet.pollFirst();
System.out.printf("pollFirst=%s, set=%s\n", val, tSet);
// 删除并返回最后一个元素
val = (String)tSet.pollLast();
System.out.printf("pollLast=%s, set=%s\n", val, tSet);
// 清空HashSet
tSet.clear();
// 输出HashSet是否为空
System.out.printf("%s\n", tSet.isEmpty()?"set is empty":"set is not empty");
}
}
基于Java8的TreeSet源码分析：
public class Hello {
public static void main(String[] args) {
testTreeSetAPIs();
}
// 测试TreeSet的api
public static void testTreeSetAPIs() {
String val;
// 新建TreeSet
TreeSet tSet = new TreeSet();
// 将元素添加到TreeSet中
tSet.add("aaa");
// Set中不允许重复元素，所以只会保存一个“aaa”
tSet.add("aaa");
tSet.add("bbb");
tSet.add("eee");
tSet.add("ddd");
tSet.add("ccc");
System.out.println("TreeSet:"+tSet);
// 打印TreeSet的实际大小
System.out.printf("size : %d\n", tSet.size());
// 导航方法
// floor(小于、等于)
System.out.printf("floor bbb: %s\n", tSet.floor("bbb"));
// lower(小于)
System.out.printf("lower bbb: %s\n", tSet.lower("bbb"));
// ceiling(大于、等于)
System.out.printf("ceiling bbb: %s\n", tSet.ceiling("bbb"));
System.out.printf("ceiling eee: %s\n", tSet.ceiling("eee"));
// ceiling(大于)
System.out.printf("higher bbb: %s\n", tSet.higher("bbb"));
// subSet()
System.out.printf("subSet(aaa, true, ccc, true): %s\n", tSet.subSet("aaa", true, "ccc", true));
System.out.printf("subSet(aaa, true, ccc, false): %s\n", tSet.subSet("aaa", true, "ccc", false));
System.out.printf("subSet(aaa, false, ccc, true): %s\n", tSet.subSet("aaa", false, "ccc", true));
System.out.printf("subSet(aaa, false, ccc, false): %s\n", tSet.subSet("aaa", false, "ccc", false));
// headSet()
System.out.printf("headSet(ccc, true): %s\n", tSet.headSet("ccc", true));
System.out.printf("headSet(ccc, false): %s\n", tSet.headSet("ccc", false));
// tailSet()
System.out.printf("tailSet(ccc, true): %s\n", tSet.tailSet("ccc", true));
System.out.printf("tailSet(ccc, false): %s\n", tSet.tailSet("ccc", false));
// 删除“ccc”
tSet.remove("ccc");
// 将Set转换为数组
]);
for (String str:arr)
System.out.printf("for each : %s\n", str);
// 打印TreeSet
System.out.printf("TreeSet:%s\n", tSet);
// 遍历TreeSet
for(Iterator iter = tSet.iterator(); iter.hasNext(); ) {
System.out.printf("iter : %s\n", iter.next());
}
// 删除并返回第一个元素
val = (String)tSet.pollFirst();
System.out.printf("pollFirst=%s, set=%s\n", val, tSet);
// 删除并返回最后一个元素
val = (String)tSet.pollLast();
System.out.printf("pollLast=%s, set=%s\n", val, tSet);
// 清空HashSet
tSet.clear();
// 输出HashSet是否为空
System.out.printf("%s\n", tSet.isEmpty()?"set is empty":"set is not empty");
}
}
基于Java8的TreeSet源码分析：
public class TreeSet<E> extends AbstractSet<E>
implements NavigableSet<E>, Cloneable, java.io.Serializable
{
/**
* 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();
/**
* Constructs a set backed by the specified navigable map.
*/
//构造函数
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}.
*/
//构造空的tree set 根据自然顺序对元素进行排序
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.
*/
//构造函数，根据特定的比较器对元素进行排序
public TreeSet(Comparator<? super E> comparator) {
this(new TreeMap<>(comparator));
}
//构造函数，参数为已有的集合
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
*/
//构造函数，参数为已经排序的集合
public TreeSet(SortedSet<E> s) {
this(s.comparator());
addAll(s);
}
/**
* Returns an iterator over the elements in this set in ascending order.
*
* @return an iterator over the elements in this set in ascending order
*/
//返回顺序排好的迭代器
public Iterator<E> iterator() {
return m.navigableKeySet().iterator();
}
/**
* Returns an iterator over the elements in this set in descending order.
*
* @return an iterator over the elements in this set in descending order
* @since 1.6
*/
//返回倒序排好的迭代器
public Iterator<E> descendingIterator() {
return m.descendingKeySet().iterator();
}
/**
* @since 1.6
*/
//倒序排好的集合
public NavigableSet<E> descendingSet() {
return new TreeSet<>(m.descendingMap());
}
/**
* Returns the number of elements in this set (its cardinality).
*
* @return the number of elements in this set (its cardinality)
*/
//大小
public int size() {
return m.size();
}
/**
* Returns {@code true} if this set contains no elements.
*
* @return {@code true} if this set contains no elements
*/
//判断是否为空
public boolean isEmpty() {
return m.isEmpty();
}
/**
* 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 ? e==null : 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
*/
//判断是否包含某个值
public boolean contains(Object o) {
return m.containsKey(o);
}
/**
* 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 ? e2==null : 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
*/
//添加元素
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 ? e==null : 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
*/
//删除某个对象
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.
*/
//清空对象
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
*/
//添加已经存在的元素到集合中
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
*/
//截取集合从fromElement到toElement
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
*/
//获得从集合开始到toElement的元素
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
*/
//获得从fromElement开始到结尾的元素
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}
*/
//截取元素，从fromElement到toElement
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}
*/
//从头开始到toElement，不包含toElement
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}
*/
//从fromElement开始(包含)到结尾
public SortedSet<E> tailSet(E fromElement) {
return tailSet(fromElement, true);
}
//比较器
public Comparator<? super E> comparator() {
return m.comparator();
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
//得到第一个元素
public E first() {
return m.firstKey();
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
//获得最后一个元素
public E last() {
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
*/
//比e小的一个元素
public E lower(E e) {
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
*/
//比e小于等于
public E floor(E e) {
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
*/
//比e大于等于
public E ceiling(E e) {
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
*/
//比e大的
public E higher(E e) {
return m.higherKey(e);
}
/**
* @since 1.6
*/
//获得第一个元素删除并返回
public E pollFirst() {
Map.Entry<E,?> e = m.pollFirstEntry();
return (e == null) ? null : e.getKey();
}
/**
* @since 1.6
*/
//获得最后一个元素删除并返回
public E pollLast() {
Map.Entry<E,?> e = m.pollLastEntry();
return (e == null) ? null : e.getKey();
}
/**
* Returns a shallow copy of this {@code TreeSet} instance. (The elements
* themselves are not cloned.)
*
* @return a shallow copy of this set
*/
//浅拷贝
@SuppressWarnings("unchecked")
public Object clone() {
TreeSet<E> clone;
try {
clone = (TreeSet<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
clone.m = new TreeMap<>(m);
return clone;
}
/**
* Save the state of the {@code TreeSet} instance to a stream (that is,
* serialize it).
*
* @serialData Emits the comparator used to order this set, or
* {@code null} if it obeys its elements' natural ordering
* (Object), followed by the size of the set (the number of
* elements it contains) (int), followed by all of its
* elements (each an Object) in order (as determined by the
* set's Comparator, or by the elements' natural ordering if
* the set has no Comparator).
*/
//序列化写对象
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden stuff
s.defaultWriteObject();
// Write out Comparator
s.writeObject(m.comparator());
// Write out size
s.writeInt(m.size());
// Write out all elements in the proper order.
for (E e : m.keySet())
s.writeObject(e);
}
/**
* Reconstitute the {@code TreeSet} instance from a stream (that is,
* deserialize it).
*/
//序列化读对象
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in any hidden stuff
s.defaultReadObject();
// Read in Comparator
@SuppressWarnings("unchecked")
Comparator<? super E> c = (Comparator<? super E>) s.readObject();
// Create backing TreeMap
TreeMap<E,Object> tm = new TreeMap<>(c);
m = tm;
// Read in size
int size = s.readInt();
tm.readTreeSet(size, s, PRESENT);
}
/**
* 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() {
return TreeMap.keySpliteratorFor(m);
}
private static final long serialVersionUID = -2479143000061671589L;
}