HashSet 源码解读

Set<String> set = new HashSet<>();
set.add("aaa");

private transient HashMap<E,Object> map;
/**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

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

Set<String> set = new TreeSet<>();
        set.add("aaa");

private transient NavigableMap<E,Object> 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 Comparable interface.
 Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and 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 add call will throw a ClassCastException.
public TreeSet() {
        this(new TreeMap<E,Object>());
    }
/**
 * Constructs a set backed by the specified navigable map.
 */
TreeSet(NavigableMap<E,Object> m) {
    this.m = m;
}

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