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package java.util;

/**
 * This class provides a skeletal implementation of the {@code Set}
 * interface to minimize the effort required to implement this
 * interface. <p>
 *
 * The process of implementing a set by extending this class is identical
 * to that of implementing a Collection by extending AbstractCollection,
 * except that all of the methods and constructors in subclasses of this
 * class must obey the additional constraints imposed by the {@code Set}
 * interface (for instance, the add method must not permit addition of
 * multiple instances of an object to a set).<p>
 *
 * Note that this class does not override any of the implementations from
 * the {@code AbstractCollection} class. It merely adds implementations
 * for {@code equals} and {@code hashCode}.<p>
 *
 * This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements maintained by this set
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Collection
 * @see AbstractCollection
 * @see Set
 * @since 1.2
 */

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
 * {@code true} 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 {@code equals} method works
 * properly across different implementations of the {@code Set}
 * interface.<p>
 *
 * This implementation first checks if the specified object is this
 * set; if so it returns {@code true}. 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
 * {@code containsAll((Collection) o)}.
 *
 * @param o object to be compared for equality with this set
 * @return {@code true} 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 | 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 {@code null} element is defined to be zero.
 * This ensures that {@code s1.equals(s2)} implies that
 * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
 * and {@code s2}, as required by the general contract of
 * {@link Object#hashCode}.
 *
 * <p>This implementation iterates over the set, calling the
 * {@code hashCode} 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 {@code size}
 * 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 {@code remove} 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 {@code remove} method.
 *
 * <p>Note that this implementation will throw an
 * {@code UnsupportedOperationException} if the iterator returned by the
 * {@code iterator} method does not implement the {@code remove} method.
 *
 * @param c collection containing elements to be removed from this set
 * @return {@code true} if this set changed as a result of the call
 * @throws UnsupportedOperationException if the {@code removeAll} 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 (Object e : c)
 modified |= remove(e);
 } else {
 for (Iterator<?> i = iterator(); i.hasNext(); ) {
 if (c.contains(i.next())) {
 i.remove();
 modified = true;
 }
 }
 }
 return modified;
 }

}