1 /* 2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package java.util; 27 28 import java.util.function.Function; 29 import java.util.function.ToIntFunction; 30 import java.util.function.ToLongFunction; 31 import java.util.function.ToDoubleFunction; 32 33 /** 34 * A comparison function, which imposes a <i>total ordering</i> on some 35 * collection of objects. Comparators can be passed to a sort method (such 36 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link 37 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control 38 * over the sort order. Comparators can also be used to control the order of 39 * certain data structures (such as {@link SortedSet sorted sets} or {@link 40 * SortedMap sorted maps}), or to provide an ordering for collections of 41 * objects that don't have a {@link Comparable natural ordering}.<p> 42 * 43 * The ordering imposed by a comparator <tt>c</tt> on a set of elements 44 * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if 45 * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as 46 * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in 47 * <tt>S</tt>.<p> 48 * 49 * Caution should be exercised when using a comparator capable of imposing an 50 * ordering inconsistent with equals to order a sorted set (or sorted map). 51 * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt> 52 * is used with elements (or keys) drawn from a set <tt>S</tt>. If the 53 * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals, 54 * the sorted set (or sorted map) will behave "strangely." In particular the 55 * sorted set (or sorted map) will violate the general contract for set (or 56 * map), which is defined in terms of <tt>equals</tt>.<p> 57 * 58 * For example, suppose one adds two elements {@code a} and {@code b} such that 59 * {@code (a.equals(b) && c.compare(a, b) != 0)} 60 * to an empty {@code TreeSet} with comparator {@code c}. 61 * The second {@code add} operation will return 62 * true (and the size of the tree set will increase) because {@code a} and 63 * {@code b} are not equivalent from the tree set's perspective, even though 64 * this is contrary to the specification of the 65 * {@link Set#add Set.add} method.<p> 66 * 67 * Note: It is generally a good idea for comparators to also implement 68 * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in 69 * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In 70 * order for the data structure to serialize successfully, the comparator (if 71 * provided) must implement <tt>Serializable</tt>.<p> 72 * 73 * For the mathematically inclined, the <i>relation</i> that defines the 74 * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a 75 * given set of objects <tt>S</tt> is:<pre> 76 * {(x, y) such that c.compare(x, y) <= 0}. 77 * </pre> The <i>quotient</i> for this total order is:<pre> 78 * {(x, y) such that c.compare(x, y) == 0}. 79 * </pre> 80 * 81 * It follows immediately from the contract for <tt>compare</tt> that the 82 * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the 83 * imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that 84 * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with 85 * equals</i>, we mean that the quotient for the ordering is the equivalence 86 * relation defined by the objects' {@link Object#equals(Object) 87 * equals(Object)} method(s):<pre> 88 * {(x, y) such that x.equals(y)}. </pre> 89 * 90 * <p>Unlike {@code Comparable}, a comparator may optionally permit 91 * comparison of null arguments, while maintaining the requirements for 92 * an equivalence relation. 93 * 94 * <p>This interface is a member of the 95 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 96 * Java Collections Framework</a>. 97 * 98 * @param <T> the type of objects that may be compared by this comparator 99 * 100 * @author Josh Bloch 101 * @author Neal Gafter 102 * @see Comparable 103 * @see java.io.Serializable 104 * @since 1.2 105 */ 106 @FunctionalInterface 107 public interface Comparator<T> { 108 /** 109 * Compares its two arguments for order. Returns a negative integer, 110 * zero, or a positive integer as the first argument is less than, equal 111 * to, or greater than the second.<p> 112 * 113 * In the foregoing description, the notation 114 * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical 115 * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, 116 * <tt>0</tt>, or <tt>1</tt> according to whether the value of 117 * <i>expression</i> is negative, zero or positive.<p> 118 * 119 * The implementor must ensure that <tt>sgn(compare(x, y)) == 120 * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This 121 * implies that <tt>compare(x, y)</tt> must throw an exception if and only 122 * if <tt>compare(y, x)</tt> throws an exception.)<p> 123 * 124 * The implementor must also ensure that the relation is transitive: 125 * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies 126 * <tt>compare(x, z)>0</tt>.<p> 127 * 128 * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt> 129 * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all 130 * <tt>z</tt>.<p> 131 * 132 * It is generally the case, but <i>not</i> strictly required that 133 * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking, 134 * any comparator that violates this condition should clearly indicate 135 * this fact. The recommended language is "Note: this comparator 136 * imposes orderings that are inconsistent with equals." 137 * 138 * @param o1 the first object to be compared. 139 * @param o2 the second object to be compared. 140 * @return a negative integer, zero, or a positive integer as the 141 * first argument is less than, equal to, or greater than the 142 * second. 143 * @throws NullPointerException if an argument is null and this 144 * comparator does not permit null arguments 145 * @throws ClassCastException if the arguments' types prevent them from 146 * being compared by this comparator. 147 */ 148 int compare(T o1, T o2); 149 150 /** 151 * Indicates whether some other object is "equal to" this 152 * comparator. This method must obey the general contract of 153 * {@link Object#equals(Object)}. Additionally, this method can return 154 * <tt>true</tt> <i>only</i> if the specified object is also a comparator 155 * and it imposes the same ordering as this comparator. Thus, 156 * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1, 157 * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference 158 * <tt>o1</tt> and <tt>o2</tt>.<p> 159 * 160 * Note that it is <i>always</i> safe <i>not</i> to override 161 * <tt>Object.equals(Object)</tt>. However, overriding this method may, 162 * in some cases, improve performance by allowing programs to determine 163 * that two distinct comparators impose the same order. 164 * 165 * @param obj the reference object with which to compare. 166 * @return <code>true</code> only if the specified object is also 167 * a comparator and it imposes the same ordering as this 168 * comparator. 169 * @see Object#equals(Object) 170 * @see Object#hashCode() 171 */ 172 boolean equals(Object obj); 173 174 /** 175 * Returns a comparator that imposes the reverse ordering of this 176 * comparator. 177 * 178 * @return A comparator that imposes the reverse ordering of this 179 * comparator. 180 * @since 1.8 181 */ 182 default Comparator<T> reverseOrder() { 183 return Collections.reverseOrder(this); 184 } 185 186 /** 187 * Constructs a lexicographic order comparator with another comparator. 188 * For example, a {@code Comparator<Person> byLastName} can be composed 189 * with another {@code Comparator<Person> byFirstName}, then {@code 190 * byLastName.thenComparing(byFirstName)} creates a {@code 191 * Comparator<Person>} which sorts by last name, and for equal last names 192 * sorts by first name. 193 * 194 * @param other the other comparator used when equals on this. 195 * @throws NullPointerException if the argument is null. 196 * @since 1.8 197 */ 198 default Comparator<T> thenComparing(Comparator<? super T> other) { 199 return Comparators.compose(this, other); 200 } 201 202 /** 203 * Constructs a lexicographic order comparator with a function that 204 * extracts a {@code Comparable} key. This default implementation calls 205 * {@code thenComparing(this, Comparators.comparing(keyExtractor))}. 206 * 207 * @param <U> the {@link Comparable} type for comparison 208 * @param keyExtractor the function used to extract the {@link Comparable} sort key 209 * @throws NullPointerException if the argument is null. 210 * @see Comparators#comparing(Function) 211 * @see #thenComparing(Comparator) 212 * @since 1.8 213 */ 214 default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor) { 215 return thenComparing(Comparators.comparing(keyExtractor)); 216 } 217 218 /** 219 * Constructs a lexicographic order comparator with a function that 220 * extracts a {@code int} value. This default implementation calls {@code 221 * thenComparing(this, Comparators.comparing(keyExtractor))}. 222 * 223 * @param keyExtractor the function used to extract the integer value 224 * @throws NullPointerException if the argument is null. 225 * @see Comparators#comparing(ToIntFunction) 226 * @see #thenComparing(Comparator) 227 * @since 1.8 228 */ 229 default Comparator<T> thenComparing(ToIntFunction<? super T> keyExtractor) { 230 return thenComparing(Comparators.comparing(keyExtractor)); 231 } 232 233 /** 234 * Constructs a lexicographic order comparator with a function that 235 * extracts a {@code long} value. This default implementation calls 236 * {@code thenComparing(this, Comparators.comparing(keyExtractor))}. 237 * 238 * @param keyExtractor the function used to extract the long value 239 * @throws NullPointerException if the argument is null. 240 * @see Comparators#comparing(ToLongFunction) 241 * @see #thenComparing(Comparator) 242 * @since 1.8 243 */ 244 default Comparator<T> thenComparing(ToLongFunction<? super T> keyExtractor) { 245 return thenComparing(Comparators.comparing(keyExtractor)); 246 } 247 248 /** 249 * Constructs a lexicographic order comparator with a function that 250 * extracts a {@code double} value. This default implementation calls 251 * {@code thenComparing(this, Comparators.comparing(keyExtractor))}. 252 * 253 * @param keyExtractor the function used to extract the double value 254 * @throws NullPointerException if the argument is null. 255 * @see Comparators#comparing(ToDoubleFunction) 256 * @see #thenComparing(Comparator) 257 * @since 1.8 258 */ 259 default Comparator<T> thenComparing(ToDoubleFunction<? super T> keyExtractor) { 260 return thenComparing(Comparators.comparing(keyExtractor)); 261 } 262 }