HashSet实现基于HashMap

HashSet的部分源码：

static final long serialVersionUID = -5024744406713321676L;

private transient HashMap<E,Object> map;

private static final Object PRESENT = new Object();

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

public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}

public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}

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

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

public Iterator<E> iterator() {
return map.keySet().iterator();
}

public int size() {
return map.size();
}

public boolean isEmpty() {
return map.isEmpty();
}

public boolean contains(Object o) {
return map.containsKey(o);
}

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

public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}

public void clear() {
map.clear();
}

这是里面其中的一部分代码（去掉了注释）.

可以看出HashSet是通过储存map的key来并且value是

private static final Object PRESENT = new Object();

达到键值对的。

每次的存储，都是通过

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

如果该set中不存在该key的元素则返回 true否者返回false

其他的的contains 、size 、remove等都是通过使用map对应的方法实现。