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