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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
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/*
* 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.
*
* @param the type of objects compared by this comparator
* @since 2.0
*/
public class NullComparator implements Comparator, Serializable {
/** Serialization version. */
private static final long serialVersionUID = -5820772575483504339L;
/**
* The comparator to use when comparing two non-null objects.
**/
private final Comparator nonNullComparator;
/**
* Specifies whether a null are compared as higher than
* non-null 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 {@link ComparableComparator} is
* used.
**/
@SuppressWarnings("unchecked")
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 {@link 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
**/
public NullComparator(final Comparator 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 {@link 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.
**/
@SuppressWarnings("unchecked")
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 {@link 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
**/
public NullComparator(final Comparator 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.
**/
@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
* {@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.
**/
@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);
}
}