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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) 2019, Burton Computer Corporation
// All rights reserved.
//
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//
//     Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//
//     Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in
//     the documentation and/or other materials provided with the
//     distribution.
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//     derived from this software without specific prior written permission.
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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package org.javimmutable.collections.tree;

import org.javimmutable.collections.Func1;
import org.javimmutable.collections.Holder;
import org.javimmutable.collections.Holders;
import org.javimmutable.collections.JImmutableMap.Entry;
import org.javimmutable.collections.Proc2;
import org.javimmutable.collections.Proc2Throws;
import org.javimmutable.collections.Sum2;
import org.javimmutable.collections.Sum2Throws;
import org.javimmutable.collections.iterators.GenericIterator;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.Immutable;
import java.util.Comparator;

/**
 * Node with no value and no children.  Used as the terminating node at the bottom
 * of every tree or the root of an empty tree.  Only one instance exists and is
 * reused everywhere.
 */
@Immutable
class FringeNode
    extends AbstractNode
{
    private static final FringeNode INSTANCE = new FringeNode();

    private FringeNode()
    {
    }

    @SuppressWarnings("unchecked")
    static  AbstractNode instance()
    {
        return INSTANCE;
    }

    @Nonnull
    @Override
    public AbstractNode assign(@Nonnull Comparator comp,
                                     @Nonnull K key,
                                     @Nullable V value)
    {
        return new ValueNode<>(key, value, this, this);
    }

    @Nonnull
    @Override
    public AbstractNode update(@Nonnull Comparator comp,
                                     @Nonnull K key,
                                     @Nonnull Func1, V> generator)
    {
        return new ValueNode<>(key, generator.apply(Holders.of()), this, this);
    }

    @Nonnull
    @Override
    public AbstractNode delete(@Nonnull Comparator comp,
                                     @Nonnull K key)
    {
        return this;
    }

    @Nonnull
    @Override
    DeleteResult deleteLeftmost()
    {
        throw new UnsupportedOperationException();
    }

    @Nonnull
    @Override
    DeleteResult deleteRightmost()
    {
        throw new UnsupportedOperationException();
    }

    @Nullable
    @Override
    public V get(@Nonnull Comparator comp,
                 @Nonnull K key,
                 V defaultValue)
    {
        return defaultValue;
    }

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

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

    @Override
    public boolean isEmpty()
    {
        return true;
    }

    @Override
    int depth()
    {
        return 0;
    }

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

    @Nonnull
    @Override
    K key()
    {
        throw new UnsupportedOperationException();
    }

    @Nullable
    @Override
    V value()
    {
        throw new UnsupportedOperationException();
    }

    @Nonnull
    @Override
    AbstractNode left()
    {
        throw new UnsupportedOperationException();
    }

    @Nonnull
    @Override
    AbstractNode right()
    {
        throw new UnsupportedOperationException();
    }

    @Nullable
    @Override
    public GenericIterator.State> iterateOverRange(@Nullable GenericIterator.State> parent,
                                                               int offset,
                                                               int limit)
    {
        assert offset == 0 && limit == 0;
        return parent;
    }

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

    @Override
    void forEach(@Nonnull Proc2 proc)
    {
    }

    @Override
     void forEachThrows(@Nonnull Proc2Throws proc)
        throws E
    {
    }

    @Override
     R reduce(R sum,
                 @Nonnull Sum2 proc)
    {
        return sum;
    }

    @Override
     R reduceThrows(R sum,
                                            @Nonnull Sum2Throws proc)
        throws E
    {
        return sum;
    }

    @Override
    public void checkInvariants(@Nonnull Comparator comp)
    {
    }
}