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package org.javimmutable.collections;

import javax.annotation.concurrent.Immutable;

/**
 * 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 
 */
@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);
}