ComparatorUtil.java

ComparatorUtil.java

package com.pallasathenagroup.util;

import java.util.Comparator;
import java.util.Optional;

/**
 * Comparator utility methods for handling optional comparable.
 */
public class ComparatorUtil {

    /**
     * A comparator that sorts present {@code Optional} values first.
     *
     * @param  <T> the type of the elements to be compared
     * @param  comparator a {@code Comparator} for comparing present values
     * @return a comparator that considers {@code isPresent} to be lower than
     *         non-present, and compares present objects with the supplied
     *         {@code Comparator}.
     * @see Comparator#nullsFirst(Comparator)
     */
    public static <T> Comparator<Optional<T>> presentFirst(Comparator<? super T> comparator) {
        return (a,b) -> a.map(t1 -> b.map(t -> comparator.compare(t1, t))
            .orElse(-1)).orElseGet(() -> b.isPresent() ?
            1 : 0);
    }

    /**
     * A comparator that sorts present {@code Optional} values last.
     *
     * @param  <T> the type of the elements to be compared
     * @param  comparator a {@code Comparator} for comparing present values
     * @return a comparator that considers {@code isPresent} to be higher than
     *         non-present, and compares present objects with the supplied
     *         {@code Comparator}.
     * @see Comparator#nullsFirst(Comparator)
     */
    public static <T> Comparator<Optional<T>> presentLast(Comparator<? super T> comparator) {
        return (a,b) -> a.map(t1 -> b.map(t -> comparator.compare(t1, t))
            .orElse(1)).orElseGet(() -> b.isPresent() ?
            -1 : 0);
    }

}