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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) 2014, 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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//     Neither the name of the Burton Computer Corporation nor the names
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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package org.javimmutable.collections.list;

import org.javimmutable.collections.Cursor;
import org.javimmutable.collections.JImmutableStack;
import org.javimmutable.collections.common.IteratorAdaptor;
import org.javimmutable.collections.cursors.Cursors;
import org.javimmutable.collections.cursors.SequenceCursor;
import org.javimmutable.collections.cursors.SingleValueCursor;
import org.javimmutable.collections.cursors.StandardCursor;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.Immutable;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * Singly linked list implementation of PersistentList that stores and retrieves values
 * in the reverse order of the corresponding add() method calls.  If forward or random
 * access to stored values is required use PersistentArrayList or PersistentTreeList
 * instead but this class  is significantly faster when its limitations are acceptable.
 *
 * @param 
 */
@Immutable
public abstract class JImmutableLinkedStack
        implements JImmutableStack
{
    private static final Empty EMPTY = new Empty();

    @SuppressWarnings("unchecked")
    public static  JImmutableLinkedStack of()
    {
        return (JImmutableLinkedStack)EMPTY;
    }

    public static  JImmutableLinkedStack of(T value)
    {
        return new Single(value);
    }

    public static  JImmutableLinkedStack of(List values)
    {
        JImmutableLinkedStack list = of();
        for (T value : values) {
            list = list.insert(value);
        }
        return list;
    }

    public static  JImmutableLinkedStack of(T... values)
    {
        JImmutableLinkedStack list = of();
        for (T value : values) {
            list = list.insert(value);
        }
        return list;
    }

    @Nonnull
    public abstract JImmutableLinkedStack insert(@Nullable T value);

    @Nonnull
    public abstract JImmutableLinkedStack getTail();

    public Iterator iterator()
    {
        return IteratorAdaptor.of(cursor());
    }

    public List makeList()
    {
        List answer = new ArrayList();
        if (!isEmpty()) {
            answer.add(getHead());
            for (JImmutableStack next = getTail(); !next.isEmpty(); next = next.getTail()) {
                answer.add(next.getHead());
            }
        }
        return answer;
    }

    @Override
    public boolean equals(Object o)
    {
        return (o instanceof JImmutableStack) && Cursors.areEqual(cursor(), ((JImmutableStack)o).cursor());
    }

    @Override
    public int hashCode()
    {
        return Cursors.computeHashCode(cursor());
    }

    @Override
    public String toString()
    {
        return Cursors.makeString(cursor());
    }

    @Nonnull
    @Override
    public JImmutableStack remove()
    {
        return getTail();
    }

    private static class Empty
            extends JImmutableLinkedStack
    {
        public boolean isEmpty()
        {
            return true;
        }

        public V getHead()
        {
            throw new UnsupportedOperationException();
        }

        @Nonnull
        @Override
        public JImmutableLinkedStack getTail()
        {
            return this;
        }

        @Nonnull
        @Override
        public JImmutableLinkedStack insert(@Nullable V value)
        {
            return new Single(value);
        }

        @Nonnull
        public Cursor cursor()
        {
            return StandardCursor.of();
        }
    }

    private static class Single
            extends JImmutableLinkedStack
    {
        private final V value;

        private Single(V value)
        {
            this.value = value;
        }

        public boolean isEmpty()
        {
            return false;
        }

        public V getHead()
        {
            return value;
        }

        @Nonnull
        @Override
        public JImmutableLinkedStack getTail()
        {
            return of();
        }

        @Nonnull
        @Override
        public JImmutableLinkedStack insert(@Nullable V value)
        {
            return new Chain(value, this);
        }

        @Nonnull
        public Cursor cursor()
        {
            return SingleValueCursor.of(value);
        }
    }

    private static class Chain
            extends JImmutableLinkedStack
    {
        private final V value;
        private final JImmutableLinkedStack next;

        private Chain(V value,
                      JImmutableLinkedStack next)
        {
            this.value = value;
            this.next = next;
        }

        public boolean isEmpty()
        {
            return false;
        }

        public V getHead()
        {
            return value;
        }

        @Nonnull
        @Override
        public JImmutableLinkedStack getTail()
        {
            return next;
        }

        @Override
        @Nonnull
        public JImmutableLinkedStack insert(@Nullable V value)
        {
            return new Chain(value, this);
        }

        @Override
        @Nonnull
        public Cursor cursor()
        {
            return SequenceCursor.of(this);
        }
    }
}