/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.collections4.comparators;

import java.io.Serializable;
import java.util.Comparator;

import org.apache.commons.collections4.ComparatorUtils;

A Comparator that will compare nulls to be either lower or higher than
other objects.
Type parameters:
<E> – the type of objects compared by this comparator
Since:
2.0/**
 * A Comparator that will compare nulls to be either lower or higher than
 * other objects.
 *
 * @param <E> the type of objects compared by this comparator
 * @since 2.0
 */
public class NullComparator<E> implements Comparator<E>, Serializable {

    
Serialization version. /** Serialization version. */
    private static final long serialVersionUID = -5820772575483504339L;

    
 The comparator to use when comparing two non-null objects.
/**
     *  The comparator to use when comparing two non-<code>null</code> objects.
     **/
    private final Comparator<? super E> nonNullComparator;

    
 Specifies whether a null are compared as higher than
 non-null objects.
/**
     *  Specifies whether a <code>null</code> are compared as higher than
     *  non-<code>null</code> objects.
     **/
    private final boolean nullsAreHigh;

    //-----------------------------------------------------------------------
    
 Construct an instance that sorts null higher than any
 non-null object it is compared with. When comparing two
 non-null objects, the ComparableComparator is used. /**
     *  Construct an instance that sorts <code>null</code> higher than any
     *  non-<code>null</code> object it is compared with. When comparing two
     *  non-<code>null</code> objects, the {@link ComparableComparator} is
     *  used.
     **/
    public NullComparator() {
        this(ComparatorUtils.NATURAL_COMPARATOR, true);
    }

    
 Construct an instance that sorts null higher than any
 non-null object it is compared with.  When comparing two
 non-null objects, the specified Comparator is used. @param nonNullComparator the comparator to use when comparing two non-null objects.  This argument cannot be
 null
 @throws NullPointerException if nonNullComparator is
 null
/**
     *  Construct an instance that sorts <code>null</code> higher than any
     *  non-<code>null</code> object it is compared with.  When comparing two
     *  non-<code>null</code> objects, the specified {@link Comparator} is
     *  used.
     *
     *  @param nonNullComparator the comparator to use when comparing two
     *  non-<code>null</code> objects.  This argument cannot be
     *  <code>null</code>
     *
     *  @throws NullPointerException if <code>nonNullComparator</code> is
     *  <code>null</code>
     **/
    public NullComparator(final Comparator<? super E> nonNullComparator) {
        this(nonNullComparator, true);
    }

    
 Construct an instance that sorts null higher or lower than
 any non-null object it is compared with.  When comparing
 two non-null objects, the ComparableComparator is used. @param nullsAreHigh a true value indicates that
 null should be compared as higher than a
 non-null object.  A false value indicates
 that null should be compared as lower than a
 non-null object.
/**
     *  Construct an instance that sorts <code>null</code> higher or lower than
     *  any non-<code>null</code> object it is compared with.  When comparing
     *  two non-<code>null</code> objects, the {@link ComparableComparator} is
     *  used.
     *
     *  @param nullsAreHigh a <code>true</code> value indicates that
     *  <code>null</code> should be compared as higher than a
     *  non-<code>null</code> object.  A <code>false</code> value indicates
     *  that <code>null</code> should be compared as lower than a
     *  non-<code>null</code> object.
     **/
    public NullComparator(final boolean nullsAreHigh) {
        this(ComparatorUtils.NATURAL_COMPARATOR, nullsAreHigh);
    }

    
 Construct an instance that sorts null higher or lower than
 any non-null object it is compared with.  When comparing
 two non-null objects, the specified Comparator is used. @param nonNullComparator the comparator to use when comparing two non-null objects. This argument cannot be
 null
 @param nullsAreHigh a true value indicates that
 null should be compared as higher than a
 non-null object.  A false value indicates
 that null should be compared as lower than a
 non-null object.
 @throws NullPointerException if nonNullComparator is
 null
/**
     *  Construct an instance that sorts <code>null</code> higher or lower than
     *  any non-<code>null</code> object it is compared with.  When comparing
     *  two non-<code>null</code> objects, the specified {@link Comparator} is
     *  used.
     *
     *  @param nonNullComparator the comparator to use when comparing two
     *  non-<code>null</code> objects. This argument cannot be
     *  <code>null</code>
     *
     *  @param nullsAreHigh a <code>true</code> value indicates that
     *  <code>null</code> should be compared as higher than a
     *  non-<code>null</code> object.  A <code>false</code> value indicates
     *  that <code>null</code> should be compared as lower than a
     *  non-<code>null</code> object.
     *
     *  @throws NullPointerException if <code>nonNullComparator</code> is
     *  <code>null</code>
     **/
    public NullComparator(final Comparator<? super E> nonNullComparator, final boolean nullsAreHigh) {
        this.nonNullComparator = nonNullComparator;
        this.nullsAreHigh = nullsAreHigh;

        if (nonNullComparator == null) {
            throw new NullPointerException("null nonNullComparator");
        }
    }

    //-----------------------------------------------------------------------
    
 Perform a comparison between two objects.  If both objects are
 null, a 0 value is returned.  If one object
 is null and the other is not, the result is determined on
 whether the Comparator was constructed to have nulls as higher or lower
 than other objects.  If neither object is null, an
 underlying comparator specified in the constructor (or the default) is
 used to compare the non-null objects.
 @param o1  the first object to compare
 @param o2  the object to compare it to.
 @return -1 if o1 is "lower" than (less than,
 before, etc.) o2; 1 if o1 is
 "higher" than (greater than, after, etc.) o2; or
 0 if o1 and o2 are equal.
/**
     *  Perform a comparison between two objects.  If both objects are
     *  <code>null</code>, a <code>0</code> value is returned.  If one object
     *  is <code>null</code> and the other is not, the result is determined on
     *  whether the Comparator was constructed to have nulls as higher or lower
     *  than other objects.  If neither object is <code>null</code>, an
     *  underlying comparator specified in the constructor (or the default) is
     *  used to compare the non-<code>null</code> objects.
     *
     *  @param o1  the first object to compare
     *  @param o2  the object to compare it to.
     *  @return <code>-1</code> if <code>o1</code> is "lower" than (less than,
     *  before, etc.) <code>o2</code>; <code>1</code> if <code>o1</code> is
     *  "higher" than (greater than, after, etc.) <code>o2</code>; or
     *  <code>0</code> if <code>o1</code> and <code>o2</code> are equal.
     **/
    @Override
    public int compare(final E o1, final E o2) {
        if(o1 == o2) { return 0; }
        if(o1 == null) { return this.nullsAreHigh ? 1 : -1; }
        if(o2 == null) { return this.nullsAreHigh ? -1 : 1; }
        return this.nonNullComparator.compare(o1, o2);
    }

    //-----------------------------------------------------------------------
    
 Implement a hash code for this comparator that is consistent with equals(Object). @return a hash code for this comparator. /**
     *  Implement a hash code for this comparator that is consistent with
     *  {@link #equals(Object)}.
     *
     *  @return a hash code for this comparator.
     **/
    @Override
    public int hashCode() {
        return (nullsAreHigh ? -1 : 1) * nonNullComparator.hashCode();
    }

    
 Determines whether the specified object represents a comparator that is
 equal to this comparator.
 @param obj  the object to compare this comparator with.
 @return true if the specified object is a NullComparator
 with equivalent null comparison behavior
 (i.e. null high or low) and with equivalent underlying
 non-null object comparators.
/**
     *  Determines whether the specified object represents a comparator that is
     *  equal to this comparator.
     *
     *  @param obj  the object to compare this comparator with.
     *
     *  @return <code>true</code> if the specified object is a NullComparator
     *  with equivalent <code>null</code> comparison behavior
     *  (i.e. <code>null</code> high or low) and with equivalent underlying
     *  non-<code>null</code> object comparators.
     **/
    @Override
    public boolean equals(final Object obj) {
        if(obj == null) { return false; }
        if(obj == this) { return true; }
        if(!obj.getClass().equals(this.getClass())) { return false; }

        final NullComparator<?> other = (NullComparator<?>) obj;

        return this.nullsAreHigh == other.nullsAreHigh &&
                this.nonNullComparator.equals(other.nonNullComparator);
    }

}