    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 *      http://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;
    021
    022import org.apache.commons.collections4.ComparatorUtils;
    023
    024/**
    025 * A Comparator that will compare nulls to be either lower or higher than
    026 * other objects.
    027 *
    028 * @param <E> the type of objects compared by this comparator
    029 * @since 2.0
    030 */
    031public class NullComparator<E> implements Comparator<E>, Serializable {
    032
    033    /** Serialization version. */
    034    private static final long serialVersionUID = -5820772575483504339L;
    035
    036    /**
    037     *  The comparator to use when comparing two non-<code>null</code> objects.
    038     **/
    039    private final Comparator<? super E> nonNullComparator;
    040
    041    /**
    042     *  Specifies whether a <code>null</code> are compared as higher than
    043     *  non-<code>null</code> objects.
    044     **/
    045    private final boolean nullsAreHigh;
    046
    047    //-----------------------------------------------------------------------
    048    /**
    049     *  Construct an instance that sorts <code>null</code> higher than any
    050     *  non-<code>null</code> object it is compared with. When comparing two
    051     *  non-<code>null</code> objects, the {@link ComparableComparator} is
    052     *  used.
    053     **/
    054    @SuppressWarnings("unchecked")
    055    public NullComparator() {
    056        this(ComparatorUtils.NATURAL_COMPARATOR, true);
    057    }
    058
    059    /**
    060     *  Construct an instance that sorts <code>null</code> higher than any
    061     *  non-<code>null</code> object it is compared with.  When comparing two
    062     *  non-<code>null</code> objects, the specified {@link Comparator} is
    063     *  used.
    064     *
    065     *  @param nonNullComparator the comparator to use when comparing two
    066     *  non-<code>null</code> objects.  This argument cannot be
    067     *  <code>null</code>
    068     *
    069     *  @throws NullPointerException if <code>nonNullComparator</code> is
    070     *  <code>null</code>
    071     **/
    072    public NullComparator(final Comparator<? super E> nonNullComparator) {
    073        this(nonNullComparator, true);
    074    }
    075
    076    /**
    077     *  Construct an instance that sorts <code>null</code> higher or lower than
    078     *  any non-<code>null</code> object it is compared with.  When comparing
    079     *  two non-<code>null</code> objects, the {@link ComparableComparator} is
    080     *  used.
    081     *
    082     *  @param nullsAreHigh a <code>true</code> value indicates that
    083     *  <code>null</code> should be compared as higher than a
    084     *  non-<code>null</code> object.  A <code>false</code> value indicates
    085     *  that <code>null</code> should be compared as lower than a
    086     *  non-<code>null</code> object.
    087     **/
    088    @SuppressWarnings("unchecked")
    089    public NullComparator(final boolean nullsAreHigh) {
    090        this(ComparatorUtils.NATURAL_COMPARATOR, nullsAreHigh);
    091    }
    092
    093    /**
    094     *  Construct an instance that sorts <code>null</code> higher or lower than
    095     *  any non-<code>null</code> object it is compared with.  When comparing
    096     *  two non-<code>null</code> objects, the specified {@link Comparator} is
    097     *  used.
    098     *
    099     *  @param nonNullComparator the comparator to use when comparing two
    100     *  non-<code>null</code> objects. This argument cannot be
    101     *  <code>null</code>
    102     *
    103     *  @param nullsAreHigh a <code>true</code> value indicates that
    104     *  <code>null</code> should be compared as higher than a
    105     *  non-<code>null</code> object.  A <code>false</code> value indicates
    106     *  that <code>null</code> should be compared as lower than a
    107     *  non-<code>null</code> object.
    108     *
    109     *  @throws NullPointerException if <code>nonNullComparator</code> is
    110     *  <code>null</code>
    111     **/
    112    public NullComparator(final Comparator<? super E> nonNullComparator, final boolean nullsAreHigh) {
    113        this.nonNullComparator = nonNullComparator;
    114        this.nullsAreHigh = nullsAreHigh;
    115
    116        if (nonNullComparator == null) {
    117            throw new NullPointerException("null nonNullComparator");
    118        }
    119    }
    120
    121    //-----------------------------------------------------------------------
    122    /**
    123     *  Perform a comparison between two objects.  If both objects are
    124     *  <code>null</code>, a <code>0</code> value is returned.  If one object
    125     *  is <code>null</code> and the other is not, the result is determined on
    126     *  whether the Comparator was constructed to have nulls as higher or lower
    127     *  than other objects.  If neither object is <code>null</code>, an
    128     *  underlying comparator specified in the constructor (or the default) is
    129     *  used to compare the non-<code>null</code> objects.
    130     *
    131     *  @param o1  the first object to compare
    132     *  @param o2  the object to compare it to.
    133     *  @return <code>-1</code> if <code>o1</code> is "lower" than (less than,
    134     *  before, etc.) <code>o2</code>; <code>1</code> if <code>o1</code> is
    135     *  "higher" than (greater than, after, etc.) <code>o2</code>; or
    136     *  <code>0</code> if <code>o1</code> and <code>o2</code> are equal.
    137     **/
    138    @Override
    139    public int compare(final E o1, final E o2) {
    140        if(o1 == o2) { return 0; }
    141        if(o1 == null) { return this.nullsAreHigh ? 1 : -1; }
    142        if(o2 == null) { return this.nullsAreHigh ? -1 : 1; }
    143        return this.nonNullComparator.compare(o1, o2);
    144    }
    145
    146    //-----------------------------------------------------------------------
    147    /**
    148     *  Implement a hash code for this comparator that is consistent with
    149     *  {@link #equals(Object)}.
    150     *
    151     *  @return a hash code for this comparator.
    152     **/
    153    @Override
    154    public int hashCode() {
    155        return (nullsAreHigh ? -1 : 1) * nonNullComparator.hashCode();
    156    }
    157
    158    /**
    159     *  Determines whether the specified object represents a comparator that is
    160     *  equal to this comparator.
    161     *
    162     *  @param obj  the object to compare this comparator with.
    163     *
    164     *  @return <code>true</code> if the specified object is a NullComparator
    165     *  with equivalent <code>null</code> comparison behavior
    166     *  (i.e. <code>null</code> high or low) and with equivalent underlying
    167     *  non-<code>null</code> object comparators.
    168     **/
    169    @Override
    170    public boolean equals(final Object obj) {
    171        if(obj == null) { return false; }
    172        if(obj == this) { return true; }
    173        if(!obj.getClass().equals(this.getClass())) { return false; }
    174
    175        final NullComparator<?> other = (NullComparator<?>) obj;
    176
    177        return this.nullsAreHigh == other.nullsAreHigh &&
    178                this.nonNullComparator.equals(other.nonNullComparator);
    179    }
    180
    181}