    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      https://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4.comparators;
    018
    019import java.io.Serializable;
    020import java.util.Comparator;
    021import java.util.Objects;
    022
    023import org.apache.commons.collections4.ComparatorUtils;
    024
    025/**
    026 * A Comparator that will compare nulls to be either lower or higher than
    027 * other objects.
    028 *
    029 * @param <E> The type of objects compared by this comparator
    030 * @since 2.0
    031 */
    032public class NullComparator<E> implements Comparator<E>, Serializable {
    033
    034    /** Serialization version. */
    035    private static final long serialVersionUID = -5820772575483504339L;
    036
    037    /**
    038     * The comparator to use when comparing two non-{@code null} objects.
    039     */
    040    private final Comparator<? super E> nonNullComparator;
    041
    042    /**
    043     * Specifies whether a {@code null} are compared as higher than
    044     * non-{@code null} objects.
    045     */
    046    private final boolean nullsAreHigh;
    047
    048    /**
    049     * Construct an instance that sorts {@code null} higher than any
    050     * non-{@code null} object it is compared with. When comparing two
    051     * non-{@code null} objects, the {@link ComparableComparator} is
    052     * used.
    053     */
    054    public NullComparator() {
    055        this(ComparatorUtils.NATURAL_COMPARATOR, true);
    056    }
    057
    058    /**
    059     * Construct an instance that sorts {@code null} higher or lower than
    060     * any non-{@code null} object it is compared with.  When comparing
    061     * two non-{@code null} objects, the {@link ComparableComparator} is
    062     * used.
    063     *
    064     * @param nullsAreHigh A {@code true} value indicates that
    065     * {@code null} should be compared as higher than a
    066     * non-{@code null} object.  A {@code false} value indicates
    067     * that {@code null} should be compared as lower than a
    068     * non-{@code null} object.
    069     */
    070    public NullComparator(final boolean nullsAreHigh) {
    071        this(ComparatorUtils.NATURAL_COMPARATOR, nullsAreHigh);
    072    }
    073
    074    /**
    075     * Construct an instance that sorts {@code null} higher than any
    076     * non-{@code null} object it is compared with.  When comparing two
    077     * non-{@code null} objects, the specified {@link Comparator} is
    078     * used.
    079     *
    080     * @param nonNullComparator The comparator to use when comparing two
    081     * non-{@code null} objects.  This argument cannot be
    082     * {@code null}
    083     * @throws NullPointerException if {@code nonNullComparator} is
    084     * {@code null}
    085     */
    086    public NullComparator(final Comparator<? super E> nonNullComparator) {
    087        this(nonNullComparator, true);
    088    }
    089
    090    /**
    091     * Construct an instance that sorts {@code null} higher or lower than
    092     * any non-{@code null} object it is compared with.  When comparing
    093     * two non-{@code null} objects, the specified {@link Comparator} is
    094     * used.
    095     *
    096     * @param nonNullComparator The comparator to use when comparing two
    097     * non-{@code null} objects. This argument cannot be
    098     * {@code null}
    099     * @param nullsAreHigh A {@code true} value indicates that
    100     * {@code null} should be compared as higher than a
    101     * non-{@code null} object.  A {@code false} value indicates
    102     * that {@code null} should be compared as lower than a
    103     * non-{@code null} object.
    104     * @throws NullPointerException if {@code nonNullComparator} is
    105     * {@code null}
    106     */
    107    public NullComparator(final Comparator<? super E> nonNullComparator, final boolean nullsAreHigh) {
    108        this.nonNullComparator = Objects.requireNonNull(nonNullComparator, "nonNullComparator");
    109        this.nullsAreHigh = nullsAreHigh;
    110    }
    111
    112    /**
    113     * Perform a comparison between two objects.  If both objects are
    114     * {@code null}, a {@code 0} value is returned.  If one object
    115     * is {@code null} and the other is not, the result is determined on
    116     * whether the Comparator was constructed to have nulls as higher or lower
    117     * than other objects.  If neither object is {@code null}, an
    118     * underlying comparator specified in the constructor (or the default) is
    119     * used to compare the non-{@code null} objects.
    120     *
    121     * @param o1  The first object to compare
    122     * @param o2  The object to compare it to.
    123     * @return {@code -1} if {@code o1} is "lower" than (less than,
    124     * before, etc.) {@code o2}; {@code 1} if {@code o1} is
    125     * "higher" than (greater than, after, etc.) {@code o2}; or
    126     * {@code 0} if {@code o1} and {@code o2} are equal.
    127     */
    128    @Override
    129    public int compare(final E o1, final E o2) {
    130        if (o1 == o2) {
    131            return 0;
    132        }
    133        if (o1 == null) {
    134            return nullsAreHigh ? 1 : -1;
    135        }
    136        if (o2 == null) {
    137            return nullsAreHigh ? -1 : 1;
    138        }
    139        return nonNullComparator.compare(o1, o2);
    140    }
    141
    142    /**
    143     * Determines whether the specified object represents a comparator that is
    144     * equal to this comparator.
    145     *
    146     * @param obj  The object to compare this comparator with.
    147     * @return {@code true} if the specified object is a NullComparator
    148     * with equivalent {@code null} comparison behavior
    149     * (i.e. {@code null} high or low) and with equivalent underlying
    150     * non-{@code null} object comparators.
    151     */
    152    @Override
    153    public boolean equals(final Object obj) {
    154        if (obj == null) {
    155            return false;
    156        }
    157        if (obj == this) {
    158            return true;
    159        }
    160        if (!obj.getClass().equals(this.getClass())) {
    161            return false;
    162        }
    163
    164        final NullComparator<?> other = (NullComparator<?>) obj;
    165
    166        return nullsAreHigh == other.nullsAreHigh &&
    167                nonNullComparator.equals(other.nonNullComparator);
    168    }
    169
    170    /**
    171     * Implement a hash code for this comparator that is consistent with
    172     * {@link #equals(Object)}.
    173     *
    174     * @return A hash code for this comparator.
    175     */
    176    @Override
    177    public int hashCode() {
    178        return (nullsAreHigh ? -1 : 1) * nonNullComparator.hashCode();
    179    }
    180}