|Collection
||List
||Set
||Queue
我们来看看Set的JDK，因为它继承Collection，我们主要关心它和Collection的区别(相同的部分就略过不写注释，可参照Collection)：
//We can know that Set is interface and extends Collection
public interface Set<E> extends Collection<E> {
///// Query Operations
int size();
boolean isEmpty();
boolean contains(Object o);
Iterator<E> iterator();
Object[] toArray();
<T> T[] toArray(T[] a);
///// Modification Operations
//Return true if added, the same element can't be added to a Set
//Also you can add a null element but only once
boolean add(E e);
boolean remove(Object o);
///// Bulk Operations
boolean containsAll(Collection<?> c);
boolean retainAll(Collection<?> c);
boolean removeAll(Collection<?> c);
void clear();
//Add all the elements of c which are not contained in this Set
//Return true if added
boolean addAll(Collection<? extends E> c);
///// Comparison and hashing
boolean equals(Object o);
int hashCode();
default Spliterator<E> spliterator() {
}
Set在Collection的基础上加了一点定义。Set，从数学上讲也是不允许重复元素的，而且可以包含null，即空元素。所以对于Set, add操作是不能添加已有的元素的。。
