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 ? 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
*/
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 ? 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
*/
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 ? 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
*/
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。