Java tutorial
/* * Copyright 2001-2004 The Apache Software Foundation * * 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.apache.commons.collections.comparators; import java.io.Serializable; import java.util.Comparator; /** * A Comparator that will compare nulls to be either lower or higher than * other objects. * * @since Commons Collections 2.0 * @version $Revision: 1.13 $ $Date: 2004/05/15 13:24:11 $ * * @author Michael A. Smith */ public class NullComparator implements Comparator, Serializable { /** Serialization version. */ private static final long serialVersionUID = -5820772575483504339L; /** * The comparator to use when comparing two non-<code>null</code> objects. **/ private Comparator nonNullComparator; /** * Specifies whether a <code>null</code> are compared as higher than * non-<code>null</code> objects. **/ private boolean nullsAreHigh; //----------------------------------------------------------------------- /** * 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(ComparableComparator.getInstance(), true); } /** * 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> * * @exception NullPointerException if <code>nonNullComparator</code> is * <code>null</code> **/ public NullComparator(Comparator nonNullComparator) { this(nonNullComparator, true); } /** * 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(boolean nullsAreHigh) { this(ComparableComparator.getInstance(), nullsAreHigh); } /** * 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. * * @exception NullPointerException if <code>nonNullComparator</code> is * <code>null</code> **/ public NullComparator(Comparator nonNullComparator, 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 * <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. **/ public int compare(Object o1, Object 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. **/ 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 <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. **/ public boolean equals(Object obj) { if (obj == null) { return false; } if (obj == this) { return true; } if (!obj.getClass().equals(this.getClass())) { return false; } NullComparator other = (NullComparator) obj; return ((this.nullsAreHigh == other.nullsAreHigh) && (this.nonNullComparator.equals(other.nonNullComparator))); } }