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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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package org.javimmutable.collections;

/**
 * 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().
 *
 * @param 
 */
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);
}