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Commons Collections example source code file (ComparatorUtils.java)

This example Commons Collections source code file (ComparatorUtils.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

comparator, comparator, comparatorchain, comparatorchain, comparatorutils, natural_comparator, natural_comparator, nullcomparator, nullpointerexception, object, object, reversecomparator, transformer, transformingcomparator, util

The Commons Collections ComparatorUtils.java source code

/*
 *  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.collections;

import java.util.Collection;
import java.util.Comparator;

import org.apache.commons.collections.comparators.BooleanComparator;
import org.apache.commons.collections.comparators.ComparableComparator;
import org.apache.commons.collections.comparators.ComparatorChain;
import org.apache.commons.collections.comparators.NullComparator;
import org.apache.commons.collections.comparators.ReverseComparator;
import org.apache.commons.collections.comparators.TransformingComparator;

/**
 * Provides convenient static utility methods for <Code>Comparator
 * objects.
 * <p>
 * Most of the functionality in this class can also be found in the 
 * <code>comparators package. This class merely provides a 
 * convenient central place if you have use for more than one class
 * in the <code>comparators subpackage.
 *
 * @since Commons Collections 2.1
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Paul Jack
 * @author Stephen Colebourne
 */
public class ComparatorUtils {

    /**
     * ComparatorUtils should not normally be instantiated.
     */
    public ComparatorUtils() {
    }

    /**
     * Comparator for natural sort order.
     *
     * @see ComparableComparator#getInstance
     */
    public static final Comparator NATURAL_COMPARATOR = ComparableComparator.getInstance();

    /**
     * Gets a comparator that uses the natural order of the objects.
     *
     * @return  a comparator which uses natural order
     */
    public static Comparator naturalComparator() {
        return NATURAL_COMPARATOR;
    }

    /**
     * Gets a comparator that compares using two {@link Comparator}s.
     * <p>
     * The second comparator is used if the first comparator returns equal.
     *
     * @param comparator1  the first comparator to use, not null
     * @param comparator2  the first comparator to use, not null
     * @return a {@link ComparatorChain} formed from the two comparators
     * @throws NullPointerException if either comparator is null
     * @see ComparatorChain
     */
    public static Comparator chainedComparator(Comparator comparator1, Comparator comparator2) {
        return chainedComparator(new Comparator[] {comparator1, comparator2});
    }

    /**
     * Gets a comparator that compares using an array of {@link Comparator}s, applied
     * in sequence until one returns not equal or the array is exhausted.
     *
     * @param comparators  the comparators to use, not null or empty or containing nulls
     * @return a {@link ComparatorChain} formed from the input comparators
     * @throws NullPointerException if comparators array is null or contains a null
     * @see ComparatorChain
     */
    public static Comparator chainedComparator(Comparator[] comparators) {
        ComparatorChain chain = new ComparatorChain();
        for (int i = 0; i < comparators.length; i++) {
            if (comparators[i] == null) {
                throw new NullPointerException("Comparator cannot be null");
            }
            chain.addComparator(comparators[i]);
        }
        return chain;
    }

    /**
     * Gets a comparator that compares using a collection of {@link Comparator}s,
     * applied in (default iterator) sequence until one returns not equal or the 
     * collection is exhausted.
     *
     * @param comparators  the comparators to use, not null or empty or containing nulls
     * @return a {@link ComparatorChain} formed from the input comparators
     * @throws NullPointerException if comparators collection is null or contains a null
     * @throws ClassCastException if the comparators collection contains the wrong object type
     * @see ComparatorChain
     */
    public static Comparator chainedComparator(Collection comparators) {
        return chainedComparator(
            (Comparator[]) comparators.toArray(new Comparator[comparators.size()])
        );
    }

    /**
     * Gets a comparator that reverses the order of the given comparator.
     *
     * @param comparator  the comparator to reverse
     * @return  a comparator that reverses the order of the input comparator
     * @see ReverseComparator
     */
    public static Comparator reversedComparator(Comparator comparator) {
        if (comparator == null) {
            comparator = NATURAL_COMPARATOR;
        }
        return new ReverseComparator(comparator);
    }

    /**
     * Gets a Comparator that can sort Boolean objects.
     * <p>
     * The parameter specifies whether true or false is sorted first.
     * <p>
     * The comparator throws NullPointerException if a null value is compared.
     * 
     * @param trueFirst  when <code>true, sort 
     *        <code>true {@link Boolean}s before
     *        <code>false {@link Boolean}s.
     * @return  a comparator that sorts booleans
     */
    public static Comparator booleanComparator(boolean trueFirst) {
        return BooleanComparator.getBooleanComparator(trueFirst);
    }
    
    /**
     * Gets a Comparator that controls the comparison of <code>null values.
     * <p>
     * The returned comparator will consider a null value to be less than
     * any nonnull value, and equal to any other null value.  Two nonnull
     * values will be evaluated with the given comparator.
     *
     * @param comparator the comparator that wants to allow nulls
     * @return  a version of that comparator that allows nulls
     * @see NullComparator
     */
    public static Comparator nullLowComparator(Comparator comparator) {
        if (comparator == null) {
            comparator = NATURAL_COMPARATOR;
        }
        return new NullComparator(comparator, false);
    }

    /**
     * Gets a Comparator that controls the comparison of <code>null values.
     * <p>
     * The returned comparator will consider a null value to be greater than
     * any nonnull value, and equal to any other null value.  Two nonnull
     * values will be evaluated with the given comparator.
     *
     * @param comparator the comparator that wants to allow nulls
     * @return  a version of that comparator that allows nulls
     * @see NullComparator
     */
    public static Comparator nullHighComparator(Comparator comparator) {
        if (comparator == null) {
            comparator = NATURAL_COMPARATOR;
        }
        return new NullComparator(comparator, true);
    }

    /**
     * Gets a Comparator that passes transformed objects to the given comparator.
     * <p>
     * Objects passed to the returned comparator will first be transformed
     * by the given transformer before they are compared by the given
     * comparator.
     *
     * @param comparator  the sort order to use
     * @param transformer  the transformer to use
     * @return  a comparator that transforms its input objects before comparing them
     * @see  TransformingComparator
     */
    public static Comparator transformedComparator(Comparator comparator, Transformer transformer) {
        if (comparator == null) {
            comparator = NATURAL_COMPARATOR;
        }
        return new TransformingComparator(transformer, comparator);
    }

    /**
     *  Returns the smaller of the given objects according to the given 
     *  comparator, returning the second object if the comparator
     *  returns equal.
     * 
     *  @param o1  the first object to compare
     *  @param o2  the second object to compare
     *  @param comparator  the sort order to use
     *  @return  the smaller of the two objects
     */
    public static Object min(Object o1, Object o2, Comparator comparator) {
        if (comparator == null) {
            comparator = NATURAL_COMPARATOR;
        }
        int c = comparator.compare(o1, o2);
        return (c < 0) ? o1 : o2;
    }

    /**
     *  Returns the larger of the given objects according to the given 
     *  comparator, returning the second object if the comparator 
     *  returns equal.
     * 
     *  @param o1  the first object to compare
     *  @param o2  the second object to compare
     *  @param comparator  the sort order to use
     *  @return  the larger of the two objects
     */
    public static Object max(Object o1, Object o2, Comparator comparator) {
        if (comparator == null) {
            comparator = NATURAL_COMPARATOR;
        }
        int c = comparator.compare(o1, o2);
        return (c > 0) ? o1 : o2;
    }
    
}

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