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 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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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;

    }

}