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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.
//
// 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.
//
//     Neither the name of the Burton Computer Corporation nor the names
//     of its contributors may be used to endorse or promote products
//     derived from this software without specific prior written permission.
//
// 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
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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package org.javimmutable.collections;

import javax.annotation.Nonnull;
import javax.annotation.concurrent.Immutable;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

/**
 * Instances are immutable containers for at most a single object.  A Holder is either empty or filled
 * and always remain in the same state once created, i.e. value returned by isEmpty() and isFilled()
 * and getValue() must not change over time for a single instance.  null is a legitimate value for a
 * Holder and a filled Holder could return null from getValue().
 */
@Immutable
public interface Holder
{
    /**
     * @return true iff this Holder has no value to return
     */
    boolean isEmpty();

    /**
     * @return true iff this Holder has a value to return
     */
    boolean isFilled();

    /**
     * Retrieve the value of a filled Holder.  Must throw if Holder is empty.
     *
     * @return the (possibly null) value
     * @throws UnsupportedOperationException if Holder is empty
     */
    T getValue();

    /**
     * Retrieve the value of a filled Holder or null if Holder is empty.
     *
     * @return null (empty) or value (filled)
     */
    T getValueOrNull();

    /**
     * Retrieve the value of a filled Holder or the defaultValue if Holder is empty
     *
     * @param defaultValue value to return if Holder is empty
     * @return value or defaultValue
     */
    T getValueOr(T defaultValue);

    /**
     * Call consumer with my value if I am filled.  Otherwise do nothing.
     */
    default void ifPresent(@Nonnull Consumer consumer)
    {
        if (isFilled()) {
            consumer.accept(getValue());
        }
    }

    /**
     * Apply the transform function to my value (if I am filled) and return a new Holder containing the result.
     * If I am empty return an empty Holder.
     */
    default  Holder map(@Nonnull Function transforminator)
    {
        return isFilled() ? Holders.of(transforminator.apply(getValue())) : Holders.of();
    }

    /**
     * Return my value if I am filled.  Otherwise return defaultValue.
     */
    default T orElse(T defaultValue)
    {
        return getValueOr(defaultValue);
    }

    /**
     * Return my value if I am filled.  Otherwise call supplier and return its result.
     */
    default T orElseGet(@Nonnull Supplier supplier)
    {
        return isFilled() ? getValue() : supplier.get();
    }

    /**
     * Return my value if I am filled.  Otherwise call supplier and throw its result.
     */
    default  T orElseThrow(@Nonnull Supplier supplier)
        throws X
    {
        if (isFilled()) {
            return getValue();
        } else {
            throw supplier.get();
        }
    }
}