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GS Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework.

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/*
 * Copyright 2015 Goldman Sachs.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.gs.collections.impl.set.fixed;

import java.io.Serializable;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;

import com.gs.collections.api.block.function.Function;
import com.gs.collections.api.block.procedure.Procedure;
import com.gs.collections.api.block.procedure.Procedure2;
import com.gs.collections.api.block.procedure.primitive.ObjectIntProcedure;
import com.gs.collections.api.ordered.OrderedIterable;
import com.gs.collections.api.set.MutableSet;
import com.gs.collections.api.tuple.Pair;
import com.gs.collections.impl.EmptyIterator;
import com.gs.collections.impl.factory.Sets;
import net.jcip.annotations.Immutable;

/**
 * This class is a memory efficient list with no elements.  It is created by calling Lists.fixedSize.of() which
 * actually returns a singleton instance.
 */
@Immutable
final class EmptySet
        extends AbstractMemoryEfficientMutableSet
        implements Serializable
{
    private static final long serialVersionUID = 1L;

    private Object readResolve()
    {
        return Sets.fixedSize.of();
    }

    // Weird implementation of clone() is ok on final classes

    @Override
    public EmptySet clone()
    {
        return this;
    }

    public int size()
    {
        return 0;
    }

    @Override
    public boolean contains(Object obj)
    {
        return false;
    }

    public T getFirst()
    {
        return null;
    }

    public T getLast()
    {
        return null;
    }

    public void each(Procedure procedure)
    {
    }

    @Override
    public void forEachWithIndex(ObjectIntProcedure objectIntProcedure)
    {
    }

    @Override
    public  void forEachWith(Procedure2 procedure, P parameter)
    {
    }

    public Iterator iterator()
    {
        return EmptyIterator.getInstance();
    }

    @Override
    public boolean equals(Object other)
    {
        if (other == this)
        {
            return true;
        }

        if (!(other instanceof Set))
        {
            return false;
        }

        Set set = (Set) other;
        return set.isEmpty();
    }

    @Override
    public int hashCode()
    {
        return 0;
    }

    @Override
    public T min(Comparator comparator)
    {
        throw new NoSuchElementException();
    }

    @Override
    public T max(Comparator comparator)
    {
        throw new NoSuchElementException();
    }

    @Override
    public T min()
    {
        throw new NoSuchElementException();
    }

    @Override
    public T max()
    {
        throw new NoSuchElementException();
    }

    @Override
    public > T minBy(Function function)
    {
        throw new NoSuchElementException();
    }

    @Override
    public > T maxBy(Function function)
    {
        throw new NoSuchElementException();
    }

    /**
     * @deprecated in 6.0. Use {@link OrderedIterable#zip(Iterable)} instead.
     */
    @Deprecated
    @Override
    public  MutableSet> zip(Iterable that)
    {
        return Sets.fixedSize.of();
    }

    @Override
    public >> R zip(Iterable that, R target)
    {
        return target;
    }

    /**
     * @deprecated in 6.0. Use {@link OrderedIterable#zipWithIndex()} instead.
     */
    @Deprecated
    @Override
    public MutableSet> zipWithIndex()
    {
        return Sets.fixedSize.of();
    }

    @Override
    public >> R zipWithIndex(R target)
    {
        return target;
    }

    public MutableSet with(T element)
    {
        return new SingletonSet(element);
    }

    public MutableSet without(T element)
    {
        return this;
    }
}