本文基于Java1.8
set 和list 一样都是Collection 的子类
set 存放的数据不可以重复
三个实现类
hashSet linkedHashSet treeSet AbstractSet
hashSet 特点：元素不允许重复，可以为null，无序的，线程不安全
（1）hashSet 是基于hashMap实现的，默认的构建函数是初始化一个hashMap,如下：
其实就是相当于把 存入HashSet的值，当成了HashMap的key值来存储，value 存的是present对象。
（2）HashSet实现set接口，由哈希表（实际上是一个HashMap实例）支持。它不保证set 的迭代顺序；特别是它不保证该顺序恒久不变。此类允许使用null元素。
LinkedHashSet 继承HashSet 同时有基于linkedHashMap的实现原理
特点：保留插入顺序
底层使用LinkedHashMap 来保存元素，加了链表的数据结构功能。
所以特点是在hashSet 的基础上，无序变成了有序的，
代码就几个构造方法，但是是直接向上，调用父类的构造方法，父类hashSet 专门给他的构造方法如下，新建了一个LinkedHashMap,初始大小是16，负载因子0.75；
treeSet 特点：排序
treeSet 的实现原理是利用treeMap
支持自然排序和定制排序 默认自然排序
treeSet会调用集合元素的compareTo(Object obj)方法来比较元素之间大小关系，然后将集合元素按升序排列，这种方式就是自然排序。（比较的前提：两个对象的类型相同）
TreeSet的自然排序是根据集合元素的大小，TreeSet将他们以升序排列。如果需要实现定制排序，例如降序，则可以使用Comparator接口。该接口里包含一个int compare（T o1， T o2）方法，该方法用于比较o1和o2的大小。 如果需要实现定制排序，则需要在创建TreeSet集合对象时，并提供一个Comparator对象与该TreeSet集合关联，由该Comparator对象负责集合元素的排序逻辑。
AbstractSet 是以上子类的父类，重写了equals和hashCode方法以及removeAll方法。源码如下
public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractSet() {
}
// Comparison and hashing
/**
* Compares the specified object with this set for equality. Returns
* <tt>true</tt> if the given object is also a set, the two sets have
* the same size, and every member of the given set is contained in
* this set. This ensures that the <tt>equals</tt> method works
* properly across different implementations of the <tt>Set</tt>
* interface.<p>
*
* This implementation first checks if the specified object is this
* set; if so it returns <tt>true</tt>. Then, it checks if the
* specified object is a set whose size is identical to the size of
* this set; if not, it returns false. If so, it returns
* <tt>containsAll((Collection) o)</tt>.
*
* @param o object to be compared for equality with this set
* @return <tt>true</tt> if the specified object is equal to this set
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection<?> c = (Collection<?>) o;
if (c.size() != size())
return false;
try {
return containsAll(c);
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
}
/**
* Returns the hash code value for this set. The hash code of a set is
* defined to be the sum of the hash codes of the elements in the set,
* where the hash code of a <tt>null</tt> element is defined to be zero.
* This ensures that <tt>s1.equals(s2)</tt> implies that
* <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
* and <tt>s2</tt>, as required by the general contract of
* {@link Object#hashCode}.
*
* <p>This implementation iterates over the set, calling the
* <tt>hashCode</tt> method on each element in the set, and adding up
* the results.
*
* @return the hash code value for this set
* @see Object#equals(Object)
* @see Set#equals(Object)
*/
public int hashCode() {
int h = 0;
Iterator<E> i = iterator();
while (i.hasNext()) {
E obj = i.next();
if (obj != null)
h += obj.hashCode();
}
return h;
}
/**
* Removes from this set all of its elements that are contained in the
* specified collection (optional operation). If the specified
* collection is also a set, this operation effectively modifies this
* set so that its value is the <i>asymmetric set difference</i> of
* the two sets.
*
* <p>This implementation determines which is the smaller of this set
* and the specified collection, by invoking the <tt>size</tt>
* method on each. If this set has fewer elements, then the
* implementation iterates over this set, checking each element
* returned by the iterator in turn to see if it is contained in
* the specified collection. If it is so contained, it is removed
* from this set with the iterator's <tt>remove</tt> method. If
* the specified collection has fewer elements, then the
* implementation iterates over the specified collection, removing
* from this set each element returned by the iterator, using this
* set's <tt>remove</tt> method.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the iterator returned by the
* <tt>iterator</tt> method does not implement the <tt>remove</tt> method.
*
* @param c collection containing elements to be removed from this set
* @return <tt>true</tt> if this set changed as a result of the call
* @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
* is not supported by this set
* @throws ClassCastException if the class of an element of this set
* is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this set contains a null element and the
* specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
boolean modified = false;
if (size() > c.size()) {
for (Iterator<?> i = c.iterator(); i.hasNext(); )
modified |= remove(i.next());
} else {
for (Iterator<?> i = iterator(); i.hasNext(); ) {
if (c.contains(i.next())) {
i.remove();
modified = true;
}
}
}
return modified;
}
}
