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Some stellar general purpose utils.
There is a newer version: 0.7
/**
 * Copyright (C) Posten Norge AS
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *         http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package no.digipost;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collector;

import static java.util.Collections.unmodifiableList;
import static java.util.stream.Collectors.toCollection;

/**
 * Utilities for working with {@link Optional}s.
 */
public final class DiggOptionals {

    /**
     * Unwrap an {@link Optional} into a zero or one element {@link Collections#unmodifiableList(List) unmodifiable} list.
     *
     * @param  The type of the optional value.
     * @param optional The optional value to be wrapped in a list.
     *
     * @return list containing either exactly one or zero elements depending of the
     *         {@code Optional} being present or not.
     */
    public static  List toList(Optional optional) {
        return unmodifiableList(collect(optional, toCollection(() -> new ArrayList<>(1))));
    }

    /**
     * Unwrap an {@link Optional} into another container using a {@link Collector}.
     *
     * @param  The type of the optional value.
     * @param  The resulting container type.
     *
     * @param optional The optional value to be wrapped in another container.
     * @param collector The collector to use for wrapping into another container.
     *
     * @return The new container with either exactly one or zero elements depending of the
     *         {@code Optional} being present or not.
     */
    public static  R collect(Optional optional, Collector collector) {
        A container = collector.supplier().get();
        optional.ifPresent(o -> collector.accumulator().accept(container, o));
        return collector.finisher().apply(container);
    }


    public DiggOptionals() {}
}