TreeSet HashSet

TreeSet是一种排序的set，实现了SortedSet接口，它的元素要求实现Comparable接口

private transient NavigableMap<E,Object> m;

private static final Object PRESENT = new Object();

TreeSet(NavigableMap<E,Object> m) {
        this.m = m;
    }

public TreeSet() {
        this(new TreeMap<E,Object>());//默认为TreeMap
    }

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

实际上是基于TreeMap实现的，key存入元素，value存入常量Object对象

HashSet和TreeSet几乎一样，区别在于底层是HashMap实现

private transient HashMap<E,Object> map;

private static final Object PRESENT = new Object();

public HashSet() {
        map = new HashMap<>();
    }

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