Set.HashSet

HashSet

• Set實現Collections接口，內部持有HashMap實例，全部操做都藉助於Map

public HashSet() {
        map = new HashMap<>();
}
//private static final Object PRESENT = new Object();
public boolean add(E e) {
        return map.put(e, PRESENT)==null;
}
public Iterator<E> iterator() {
        return map.keySet().iterator();
}