Source Code Cross Referenced for ComparatorUtils.java in  » Library » Apache-common-Collections » org » apache » commons » collections » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » Library » Apache common Collections » org.apache.commons.collections 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:         *  Copyright 2002-2004 The Apache Software Foundation
003:         *
004:         *  Licensed under the Apache License, Version 2.0 (the "License");
005:         *  you may not use this file except in compliance with the License.
006:         *  You may obtain a copy of the License at
007:         *
008:         *      http://www.apache.org/licenses/LICENSE-2.0
009:         *
010:         *  Unless required by applicable law or agreed to in writing, software
011:         *  distributed under the License is distributed on an "AS IS" BASIS,
012:         *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013:         *  See the License for the specific language governing permissions and
014:         *  limitations under the License.
015:         */
016:        package org.apache.commons.collections;
017:
018:        import java.util.Collection;
019:        import java.util.Comparator;
020:
021:        import org.apache.commons.collections.comparators.BooleanComparator;
022:        import org.apache.commons.collections.comparators.ComparableComparator;
023:        import org.apache.commons.collections.comparators.ComparatorChain;
024:        import org.apache.commons.collections.comparators.NullComparator;
025:        import org.apache.commons.collections.comparators.ReverseComparator;
026:        import org.apache.commons.collections.comparators.TransformingComparator;
027:
028:        /**
029:         * Provides convenient static utility methods for <Code>Comparator</Code>
030:         * objects.
031:         * <p>
032:         * Most of the functionality in this class can also be found in the 
033:         * <code>comparators</code> package. This class merely provides a 
034:         * convenient central place if you have use for more than one class
035:         * in the <code>comparators</code> subpackage.
036:         *
037:         * @since Commons Collections 2.1
038:         * @version $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $
039:         * 
040:         * @author Paul Jack
041:         * @author Stephen Colebourne
042:         */
043:        public class ComparatorUtils {
044:
045:            /**
046:             * ComparatorUtils should not normally be instantiated.
047:             */
048:            public ComparatorUtils() {
049:            }
050:
051:            /**
052:             * Comparator for natural sort order.
053:             *
054:             * @see ComparableComparator#getInstance
055:             */
056:            public static final Comparator NATURAL_COMPARATOR = ComparableComparator
057:                    .getInstance();
058:
059:            /**
060:             * Gets a comparator that uses the natural order of the objects.
061:             *
062:             * @return  a comparator which uses natural order
063:             */
064:            public static Comparator naturalComparator() {
065:                return NATURAL_COMPARATOR;
066:            }
067:
068:            /**
069:             * Gets a comparator that compares using two {@link Comparator}s.
070:             * <p>
071:             * The second comparator is used if the first comparator returns equal.
072:             *
073:             * @param comparator1  the first comparator to use, not null
074:             * @param comparator2  the first comparator to use, not null
075:             * @return a {@link ComparatorChain} formed from the two comparators
076:             * @throws NullPointerException if either comparator is null
077:             * @see ComparatorChain
078:             */
079:            public static Comparator chainedComparator(Comparator comparator1,
080:                    Comparator comparator2) {
081:                return chainedComparator(new Comparator[] { comparator1,
082:                        comparator2 });
083:            }
084:
085:            /**
086:             * Gets a comparator that compares using an array of {@link Comparator}s, applied
087:             * in sequence until one returns not equal or the array is exhausted.
088:             *
089:             * @param comparators  the comparators to use, not null or empty or containing nulls
090:             * @return a {@link ComparatorChain} formed from the input comparators
091:             * @throws NullPointerException if comparators array is null or contains a null
092:             * @see ComparatorChain
093:             */
094:            public static Comparator chainedComparator(Comparator[] comparators) {
095:                ComparatorChain chain = new ComparatorChain();
096:                for (int i = 0; i < comparators.length; i++) {
097:                    if (comparators[i] == null) {
098:                        throw new NullPointerException(
099:                                "Comparator cannot be null");
100:                    }
101:                    chain.addComparator(comparators[i]);
102:                }
103:                return chain;
104:            }
105:
106:            /**
107:             * Gets a comparator that compares using a collection of {@link Comparator}s,
108:             * applied in (default iterator) sequence until one returns not equal or the 
109:             * collection is exhausted.
110:             *
111:             * @param comparators  the comparators to use, not null or empty or containing nulls
112:             * @return a {@link ComparatorChain} formed from the input comparators
113:             * @throws NullPointerException if comparators collection is null or contains a null
114:             * @throws ClassCastException if the comparators collection contains the wrong object type
115:             * @see ComparatorChain
116:             */
117:            public static Comparator chainedComparator(Collection comparators) {
118:                return chainedComparator((Comparator[]) comparators
119:                        .toArray(new Comparator[comparators.size()]));
120:            }
121:
122:            /**
123:             * Gets a comparator that reverses the order of the given comparator.
124:             *
125:             * @param comparator  the comparator to reverse
126:             * @return  a comparator that reverses the order of the input comparator
127:             * @see ReverseComparator
128:             */
129:            public static Comparator reversedComparator(Comparator comparator) {
130:                if (comparator == null) {
131:                    comparator = NATURAL_COMPARATOR;
132:                }
133:                return new ReverseComparator(comparator);
134:            }
135:
136:            /**
137:             * Gets a Comparator that can sort Boolean objects.
138:             * <p>
139:             * The parameter specifies whether true or false is sorted first.
140:             * <p>
141:             * The comparator throws NullPointerException if a null value is compared.
142:             * 
143:             * @param trueFirst  when <code>true</code>, sort 
144:             *        <code>true</code> {@link Boolean}s before
145:             *        <code>false</code> {@link Boolean}s.
146:             * @return  a comparator that sorts booleans
147:             */
148:            public static Comparator booleanComparator(boolean trueFirst) {
149:                return BooleanComparator.getBooleanComparator(trueFirst);
150:            }
151:
152:            /**
153:             * Gets a Comparator that controls the comparison of <code>null</code> values.
154:             * <p>
155:             * The returned comparator will consider a null value to be less than
156:             * any nonnull value, and equal to any other null value.  Two nonnull
157:             * values will be evaluated with the given comparator.
158:             *
159:             * @param comparator the comparator that wants to allow nulls
160:             * @return  a version of that comparator that allows nulls
161:             * @see NullComparator
162:             */
163:            public static Comparator nullLowComparator(Comparator comparator) {
164:                if (comparator == null) {
165:                    comparator = NATURAL_COMPARATOR;
166:                }
167:                return new NullComparator(comparator, false);
168:            }
169:
170:            /**
171:             * Gets a Comparator that controls the comparison of <code>null</code> values.
172:             * <p>
173:             * The returned comparator will consider a null value to be greater than
174:             * any nonnull value, and equal to any other null value.  Two nonnull
175:             * values will be evaluated with the given comparator.
176:             *
177:             * @param comparator the comparator that wants to allow nulls
178:             * @return  a version of that comparator that allows nulls
179:             * @see NullComparator
180:             */
181:            public static Comparator nullHighComparator(Comparator comparator) {
182:                if (comparator == null) {
183:                    comparator = NATURAL_COMPARATOR;
184:                }
185:                return new NullComparator(comparator, true);
186:            }
187:
188:            /**
189:             * Gets a Comparator that passes transformed objects to the given comparator.
190:             * <p>
191:             * Objects passed to the returned comparator will first be transformed
192:             * by the given transformer before they are compared by the given
193:             * comparator.
194:             *
195:             * @param comparator  the sort order to use
196:             * @param transformer  the transformer to use
197:             * @return  a comparator that transforms its input objects before comparing them
198:             * @see  TransformingComparator
199:             */
200:            public static Comparator transformedComparator(
201:                    Comparator comparator, Transformer transformer) {
202:                if (comparator == null) {
203:                    comparator = NATURAL_COMPARATOR;
204:                }
205:                return new TransformingComparator(transformer, comparator);
206:            }
207:
208:            /**
209:             *  Returns the smaller of the given objects according to the given 
210:             *  comparator, returning the second object if the comparator
211:             *  returns equal.
212:             * 
213:             *  @param o1  the first object to compare
214:             *  @param o2  the second object to compare
215:             *  @param comparator  the sort order to use
216:             *  @return  the smaller of the two objects
217:             */
218:            public static Object min(Object o1, Object o2, Comparator comparator) {
219:                if (comparator == null) {
220:                    comparator = NATURAL_COMPARATOR;
221:                }
222:                int c = comparator.compare(o1, o2);
223:                return (c < 0) ? o1 : o2;
224:            }
225:
226:            /**
227:             *  Returns the larger of the given objects according to the given 
228:             *  comparator, returning the second object if the comparator 
229:             *  returns equal.
230:             * 
231:             *  @param o1  the first object to compare
232:             *  @param o2  the second object to compare
233:             *  @param comparator  the sort order to use
234:             *  @return  the larger of the two objects
235:             */
236:            public static Object max(Object o1, Object o2, Comparator comparator) {
237:                if (comparator == null) {
238:                    comparator = NATURAL_COMPARATOR;
239:                }
240:                int c = comparator.compare(o1, o2);
241:                return (c > 0) ? o1 : o2;
242:            }
243:
244:        }
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