public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable

public interface Set<E> extends Collection<E> {// Query Operationsint size();boolean isEmpty();boolean contains(Object o);Iterator<E> iterator();Object[] toArray();<T> T[] toArray(T[] a);// Modification Operationsboolean add(E e);boolean remove(Object o);// Bulk Operationsboolean containsAll(Collection<?> c);boolean addAll(Collection<? extends E> c);boolean retainAll(Collection<?> c);boolean removeAll(Collection<?> c);void clear();// Comparison and hashingboolean equals(Object o);int hashCode();
}

// 底层使用HashMap来保存HashSet的元素private transient HashMap<E,Object> map;// Dummy value to associate with an Object in the backing Map// 由于Set只使用到了HashMap的key，所以此处定义一个静态的常量Object类，来充当HashMap的value//用于避免java.lang.NullPointerException异常private static final Object PRESENT = new Object();

/*** 使用HashMap的默认容量大小16和默认加载因子0.75初始化map，构造一个HashSet*/public HashSet() {map = new HashMap<E,Object>();}/*** 构造一个指定Collection参数的HashSet，这里不仅仅是Set，只要实现Collection接口的容器都可以*/public HashSet(Collection<? extends E> c) {map = new HashMap<E,Object>(Math. max((int) (c.size()/.75f) + 1, 16));// 使用Collection实现的Iterator迭代器，将集合c的元素一个个加入HashSet中
       addAll(c);}/*** 使用指定的初始容量大小和加载因子初始化map，构造一个HashSet*/public HashSet( int initialCapacity, float loadFactor) {map = new HashMap<E,Object>(initialCapacity, loadFactor);}/*** 使用指定的初始容量大小和默认的加载因子0.75初始化map，构造一个HashSet*/public HashSet( int initialCapacity) {map = new HashMap<E,Object>(initialCapacity);}/*** 不对外公开的一个构造方法（默认default修饰），底层构造的是LinkedHashMap，dummy只是一个标示参数，无具体意义*/HashSet( int initialCapacity, float loadFactor, boolean dummy) {map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor);
}

/*** 利用HashMap的put方法实现add方法*/public boolean add(E e) {return map .put(e, PRESENT)== null;}/*** 利用HashMap的remove方法实现remove方法*/public boolean remove(Object o) {return map .remove(o)==PRESENT;}/*** 添加一个集合到HashSet中，该方法在AbstractCollection中*/public boolean addAll(Collection<? extends E> c) {boolean modified = false;// 取得集合c迭代器IteratorIterator<? extends E> e = c.iterator();// 遍历迭代器while (e.hasNext()) {// 将集合c的每个元素加入到HashSet中if (add(e.next()))modified = true;}return modified;}/*** 删除指定集合c中的所有元素，该方法在AbstractSet中*/public boolean removeAll(Collection<?> c) {boolean modified = false;// 判断当前HashSet元素个数和指定集合c的元素个数，目的是减少遍历次数if (size() > c.size()) {// 如果当前HashSet元素多，则遍历集合c，将集合c中的元素一个个删除for (Iterator<?> i = c.iterator(); i.hasNext(); )modified |= remove(i.next());} else {// 如果集合c元素多，则遍历当前HashSet，将集合c中包含的元素一个个删除for (Iterator<?> i = iterator(); i.hasNext(); ) {if (c.contains(i.next())) {i.remove();modified = true;}}}return modified;
}