/*
* Copyright 2002-2005 the original author or authors.
*
* Licensed 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.springframework.util.comparator;
import java.util.Comparator;
import org.springframework.util.Assert;
/**
* A Comparator that will safely compare nulls to be lower or higher than
* other objects. Can decorate a given Comparator or work on Comparables.
*
* @author Keith Donald
* @author Juergen Hoeller
* @since 1.2.2
* @see Comparable
*/
public class NullSafeComparator<T> implements Comparator<T> {
/**
* A shared default instance of this comparator, treating nulls lower
* than non-null objects.
*/
public static final NullSafeComparator NULLS_LOW = new NullSafeComparator(true);
/**
* A shared default instance of this comparator, treating nulls higher
* than non-null objects.
*/
public static final NullSafeComparator NULLS_HIGH = new NullSafeComparator(false);
private final Comparator<T> nonNullComparator;
private final boolean nullsLow;
/**
* Create a NullSafeComparator that sorts <code>null</code> based on
* the provided flag, working on Comparables.
* <p>When comparing two non-null objects, their Comparable implementation
* will be used: this means that non-null elements (that this Comparator
* will be applied to) need to implement Comparable.
* <p>As a convenience, you can use the default shared instances:
* <code>NullSafeComparator.NULLS_LOW</code> and
* <code>NullSafeComparator.NULLS_HIGH</code>.
* @param nullsLow whether to treat nulls lower or higher than non-null objects
* @see java.lang.Comparable
* @see #NULLS_LOW
* @see #NULLS_HIGH
*/
@SuppressWarnings("unchecked")
private NullSafeComparator(boolean nullsLow) {
this.nonNullComparator = new ComparableComparator();
this.nullsLow = nullsLow;
}
/**
* Create a NullSafeComparator that sorts <code>null</code> based on the
* provided flag, decorating the given Comparator.
* <p>When comparing two non-null objects, the specified Comparator will be used.
* The given underlying Comparator must be able to handle the elements that this
* Comparator will be applied to.
* @param comparator the comparator to use when comparing two non-null objects
* @param nullsLow whether to treat nulls lower or higher than non-null objects
*/
public NullSafeComparator(Comparator<T> comparator, boolean nullsLow) {
Assert.notNull(comparator, "The non-null comparator is required");
this.nonNullComparator = comparator;
this.nullsLow = nullsLow;
}
public int compare(T o1, T o2) {
if (o1 == o2) {
return 0;
}
if (o1 == null) {
return (this.nullsLow ? -1 : 1);
}
if (o2 == null) {
return (this.nullsLow ? 1 : -1);
}
return this.nonNullComparator.compare(o1, o2);
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof NullSafeComparator)) {
return false;
}
NullSafeComparator other = (NullSafeComparator) obj;
return (this.nonNullComparator.equals(other.nonNullComparator) && this.nullsLow == other.nullsLow);
}
@Override
public int hashCode() {
return (this.nullsLow ? -1 : 1) * this.nonNullComparator.hashCode();
}
@Override
public String toString() {
return "NullSafeComparator: non-null comparator [" + this.nonNullComparator + "]; " +
(this.nullsLow ? "nulls low" : "nulls high");
}
}
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