1 /*
2 * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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10 *
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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).
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24 */
25
26 package java.util;
27
28 import java.util.function.Function;
29 import java.util.function.IntFunction;
30 import java.util.function.LongFunction;
31 import java.util.function.DoubleFunction;
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
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 * Construct a lexicographic order comparator with another comparator. For
188 * example, a {@code Comparator<Person> byLastName} can be composed with
189 * another {@code Comparator<Person> byFirstName}, then {@code
190 * byLastName.compose(byFirstName)} creates a {@code Comparator<Person>}
191 * which sorts by last name, and for equal last names sorts by first name.
192 *
193 * @param other the other comparator used when equals on this.
194 * @throws NullPointerException if the argument is null.
195 * @since 1.8
196 */
197 default Comparator<T> thenComparing(Comparator<? super T> other) {
198 return Comparators.compose(this, other);
199 }
200
201 /**
202 * Construct a lexicographic order comparator with a function that extracts
203 * a {@code Comparable} key. This is essentially calling {@code
204 * thenComparing(this, Comparators.comparing(keyExtractor))}.
205 *
206 * @param <U> the {@link Comparable} type for comparison
207 * @param keyExtractor the function used to extract the {@link Comparable} sort key
208 * @throws NullPointerException if the argument is null.
209 * @see Comparators#comparing(Function)
210 * @see #thenComparing(Comparator)
211 * @since 1.8
212 */
213 default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor) {
214 return Comparators.compose(this, Comparators.comparing(keyExtractor));
215 }
216
217 /**
218 * Construct a lexicographic order comparator with a function that extracts
219 * a {@code int} value. This is essentially calling {@code
220 * thenComparing(this, Comparators.comparing(keyExtractor))}.
221 *
222 * @param keyExtractor the function used to extract the integer value
223 * @throws NullPointerException if the argument is null.
224 * @see Comparators#comparing(IntFunction)
225 * @see #thenComparing(Comparator)
226 * @since 1.8
227 */
228 default Comparator<T> thenComparing(IntFunction<? super T> keyExtractor) {
229 return Comparators.compose(this, Comparators.comparing(keyExtractor));
230 }
231
232 /**
233 * Construct a lexicographic order comparator with a function that extracts
234 * a {@code long} value. This is essentially calling {@code
235 * thenComparing(this, Comparators.comparing(keyExtractor))}.
236 *
237 * @param keyExtractor the function used to extract the long value
238 * @throws NullPointerException if the argument is null.
239 * @see Comparators#comparing(LongFunction)
240 * @see #thenComparing(Comparator)
241 * @since 1.8
242 */
243 default Comparator<T> thenComparing(LongFunction<? super T> keyExtractor) {
244 return Comparators.compose(this, Comparators.comparing(keyExtractor));
245 }
246
247 /**
248 * Construct a lexicographic order comparator with a function that extracts
249 * a {@code double} value. This is essentially calling {@code
250 * thenComparing(this, Comparators.comparing(keyExtractor))}.
251 *
252 * @param keyExtractor the function used to extract the double value
253 * @throws NullPointerException if the argument is null.
254 * @see Comparators#comparing(DoubleFunction)
255 * @see #thenComparing(Comparator)
256 * @since 1.8
257 */
258 default Comparator<T> thenComparing(DoubleFunction<? super T> keyExtractor) {
259 return Comparators.compose(this, Comparators.comparing(keyExtractor));
260 }
261 }