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Library providing immutable/persistent collection classes for Java. While collections are immutable they provide methods for adding and removing values by creating new modified copies of themselves. Each copy shares almost all of its structure with other copies to minimize memory consumption.

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//
// Burton Computer Corporation
// http://www.burton-computer.com
//
// Copyright (c) 2021, Burton Computer Corporation
// All rights reserved.
//
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//
//     Redistributions in binary form must reproduce the above copyright
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//     the documentation and/or other materials provided with the
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//     of its contributors may be used to endorse or promote products
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package org.javimmutable.collections.hash;

import org.javimmutable.collections.Holder;
import org.javimmutable.collections.Holders;
import org.javimmutable.collections.JImmutableMap;
import org.javimmutable.collections.SplitableIterator;
import org.javimmutable.collections.common.AbstractJImmutableMap;
import org.javimmutable.collections.common.StreamConstants;
import org.javimmutable.collections.iterators.EmptyIterator;
import org.javimmutable.collections.serialization.JImmutableHashMapProxy;

import javax.annotation.Nonnull;
import javax.annotation.concurrent.Immutable;
import java.io.Serializable;

/**
 * Singleton implementation of JImmutableMap that contains no elements.
 * When a value is assigned to the map a JImmutableHashMap is created that
 * manages hash collisions using a tree if key is Comparable or a list otherwise.
 */
@Immutable
public class EmptyHashMap
    extends AbstractJImmutableMap
    implements Serializable
{
    @SuppressWarnings("rawtypes")
    static final EmptyHashMap INSTANCE = new EmptyHashMap();

    private static final long serialVersionUID = -121805;

    private EmptyHashMap()
    {
    }

    @Nonnull
    @Override
    public Builder mapBuilder()
    {
        return JImmutableHashMap.builder();
    }

    @Nonnull
    @Override
    public Holder find(@Nonnull K key)
    {
        return Holders.of();
    }

    @Nonnull
    @Override
    public Holder> findEntry(@Nonnull K key)
    {
        return Holders.of();
    }

    @Nonnull
    @Override
    public JImmutableMap assign(@Nonnull K key,
                                      V value)
    {
        return JImmutableHashMap.forKey(key).assign(key, value);
    }

    @Nonnull
    @Override
    public JImmutableMap delete(@Nonnull K key)
    {
        return this;
    }

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

    @Nonnull
    @Override
    public JImmutableMap deleteAll()
    {
        return this;
    }

    @Nonnull
    @Override
    public SplitableIterator> iterator()
    {
        return EmptyIterator.of();
    }

    @Override
    public int getSpliteratorCharacteristics()
    {
        return StreamConstants.SPLITERATOR_UNORDERED;
    }

    @Override
    public V getValueOr(K key,
                        V defaultValue)
    {
        return defaultValue;
    }

    @Override
    public void checkInvariants()
    {
        //TODO: fix empty checkInvariants()
    }

    private Object writeReplace()
    {
        return new JImmutableHashMapProxy(this);
    }
}