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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.base;

import static org.kiwiproject.base.KiwiPreconditions.checkArgumentNotNull;

import lombok.experimental.UtilityClass;

import java.util.Optional;
import java.util.function.Consumer;
import java.util.function.Supplier;

/**
 * Utility methods for instances of {@link Optional}, and which are not already present in {@link Optional}.
 */
@SuppressWarnings("OptionalUsedAsFieldOrParameterType")
@UtilityClass
public class Optionals {

    /**
     * Takes an action if the {@code optional} is present, otherwise throws a {@link RuntimeException} that is
     * created by the given {@link Supplier}.
     * 
     * Note that only {@link RuntimeException}s can be thrown. Use
     * {@link #ifPresentOrElseThrowChecked(Optional, Consumer, Supplier)} if you need to throw a checked exception.
     *
     * @param optional          the {@link Optional} to act upon
     * @param action            action to be performed if a value is present in {@code optional}
     * @param exceptionSupplier supplier for a {@link RuntimeException} that will be thrown if {@code optional} is empty
     * @param                the type parameter
     */
    public static  void ifPresentOrElseThrow(
            Optional optional,
            Consumer action,
            Supplier exceptionSupplier) {

        checkArgumentNotNull(optional);
        checkArgumentNotNull(action);
        checkArgumentNotNull(exceptionSupplier);

        if (optional.isPresent()) {
            action.accept(optional.get());
        } else {
            throw exceptionSupplier.get();
        }
    }

    /**
     * Takes an action if the {@code optional} is present, otherwise throws a checked exception that is
     * created by the given {@link Supplier}.
     *
     * @param optional                 the {@link Optional} to act upon
     * @param action                   action to be performed if a value is present in {@code optional}
     * @param checkedExceptionSupplier supplier for the checked exception that will be thrown if {@code optional} is empty
     * @param                       the type parameter
     * @param                       the type of checked exception
     * @throws E if the {@code optional} is empty
     */
    public static  void ifPresentOrElseThrowChecked(
            Optional optional,
            Consumer action,
            Supplier checkedExceptionSupplier) throws E {

        checkArgumentNotNull(optional);
        checkArgumentNotNull(action);
        checkArgumentNotNull(checkedExceptionSupplier);

        if (optional.isPresent()) {
            action.accept(optional.get());
        } else {
            throw checkedExceptionSupplier.get();
        }
    }
}