TreeSet

	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.
     */
    TreeSet(NavigableMap<E,Object> m) {
        this.m = m;
    }
    
	public TreeSet() {
        this(new TreeMap<E,Object>());
    }

    public TreeSet(Comparator<? super E> comparator) {
        this(new TreeMap<>(comparator));
    }

	public TreeSet(Collection<? extends E> c) {
        this();
        addAll(c);
    }

	public TreeSet(SortedSet<E> s) {
        this(s.comparator());
        addAll(s);
    }

	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);
    }

transient：添加此关键字的属性不会被序列化；

add方法：

    public boolean add(E e) {
        return m.put(e, PRESENT)==null;
    }

本质是将数据保存着TreeMap中，key就是我们需要添加的值，value是定义的一个Object；