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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) 2017, Burton Computer Corporation
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
//     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
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

package org.javimmutable.collections;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

/**
 * Provides static utility methods for constructing Holder instances.
 *
 * @param 
 */
public abstract class Holders
        implements Holder
{
    private static final Empty EMPTY = new Empty();

    /**
     * Creates an empty Holder for the specified type.
     * Shares a single instance for all empty Holders to save memory.
     *
     * @param 
     * @return the Holder
     */
    @SuppressWarnings("unchecked")
    @Nonnull
    public static  Holders of()
    {
        return (Holders)EMPTY;
    }

    /**
     * Creates a filled Holder for the specified type and (possibly null) value.
     *
     * @param value
     * @param 
     * @return the Holder
     */
    @Nonnull
    public static  Holders of(@Nullable V value)
    {
        return new Filled(value);
    }

    /**
     * Creates an empty Holder if value is null or a filled Holder if value is non-null.
     *
     * @param value
     * @param 
     * @return the Holder
     */
    @Nonnull
    public static  Holders fromNullable(@Nullable V value)
    {
        if (value == null) {
            return of();
        } else {
            return of(value);
        }
    }

    private Holders()
    {
        // prevent unrelated derived classes
    }

    @Override
    public int hashCode()
    {
        return Holders.hashCode(this);
    }

    @Override
    @SuppressWarnings("unchecked")
    public boolean equals(Object o)
    {
        return (o instanceof Holder) && Holders.areEqual(this, (Holder)o);
    }

    private static class Empty
            extends Holders
    {

        public boolean isEmpty()
        {
            return true;
        }

        public boolean isFilled()
        {
            return false;
        }

        public V getValue()
        {
            throw new UnsupportedOperationException("cannot get empty value");
        }

        @Override
        public V getValueOrNull()
        {
            return null;
        }

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

        @Override
        public String toString()
        {
            return "[]";
        }
    }

    private static class Filled
            extends Holders
    {
        private final V value;

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

        public boolean isEmpty()
        {
            return false;
        }

        public boolean isFilled()
        {
            return true;
        }

        public V getValue()
        {
            return value;
        }

        @Override
        public V getValueOrNull()
        {
            return value;
        }

        @Override
        public V getValueOr(V defaultValue)
        {
            return value;
        }

        @Override
        public String toString()
        {
            return String.format("[%s]", value);
        }
    }

    public static  boolean areEqual(Holder a,
                                       Holder b)
    {
        if ((a == null) || (b == null)) {
            return (a == null) && (b == null);
        } else if (a.isEmpty()) {
            return b.isEmpty();
        } else if (b.isEmpty()) {
            return false;
        } else {
            T v1 = a.getValue();
            T v2 = b.getValue();
            if ((v1 == null) || (v2 == null)) {
                return (v1 == null) && (v2 == null);
            } else {
                return v1.equals(v2);
            }
        }
    }

    public static  int hashCode(Holder a)
    {
        if (a.isEmpty()) {
            return -1;
        }
        if (a.getValue() == null) {
            return 1;
        }
        return a.getValue().hashCode();
    }
}