25
26 package java.util;
27
28 import java.io.Serializable;
29 import java.util.function.Function;
30 import java.util.function.ToIntFunction;
31 import java.util.function.ToLongFunction;
32 import java.util.function.ToDoubleFunction;
33 import java.util.Comparators;
34
35 /**
36 * A comparison function, which imposes a <i>total ordering</i> on some
37 * collection of objects. Comparators can be passed to a sort method (such
38 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
39 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
40 * over the sort order. Comparators can also be used to control the order of
41 * certain data structures (such as {@link SortedSet sorted sets} or {@link
42 * SortedMap sorted maps}), or to provide an ordering for collections of
43 * objects that don't have a {@link Comparable natural ordering}.<p>
44 *
45 * The ordering imposed by a comparator <tt>c</tt> on a set of elements
46 * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
47 * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
48 * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
49 * <tt>S</tt>.<p>
50 *
51 * Caution should be exercised when using a comparator capable of imposing an
52 * ordering inconsistent with equals to order a sorted set (or sorted map).
53 * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>
54 * is used with elements (or keys) drawn from a set <tt>S</tt>. If the
55 * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
56 * the sorted set (or sorted map) will behave "strangely." In particular the
57 * sorted set (or sorted map) will violate the general contract for set (or
58 * map), which is defined in terms of <tt>equals</tt>.<p>
59 *
60 * For example, suppose one adds two elements {@code a} and {@code b} such that
61 * {@code (a.equals(b) && c.compare(a, b) != 0)}
62 * to an empty {@code TreeSet} with comparator {@code c}.
63 * The second {@code add} operation will return
64 * true (and the size of the tree set will increase) because {@code a} and
65 * {@code b} are not equivalent from the tree set's perspective, even though
66 * this is contrary to the specification of the
67 * {@link Set#add Set.add} method.<p>
68 *
69 * Note: It is generally a good idea for comparators to also implement
70 * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
71 * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In
72 * order for the data structure to serialize successfully, the comparator (if
73 * provided) must implement <tt>Serializable</tt>.<p>
74 *
75 * For the mathematically inclined, the <i>relation</i> that defines the
76 * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
77 * given set of objects <tt>S</tt> is:<pre>
78 * {(x, y) such that c.compare(x, y) <= 0}.
79 * </pre> The <i>quotient</i> for this total order is:<pre>
80 * {(x, y) such that c.compare(x, y) == 0}.
81 * </pre>
82 *
83 * It follows immediately from the contract for <tt>compare</tt> that the
84 * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
85 * imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that
86 * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
87 * equals</i>, we mean that the quotient for the ordering is the equivalence
88 * relation defined by the objects' {@link Object#equals(Object)
89 * equals(Object)} method(s):<pre>
90 * {(x, y) such that x.equals(y)}. </pre>
91 *
92 * <p>Unlike {@code Comparable}, a comparator may optionally permit
93 * comparison of null arguments, while maintaining the requirements for
94 * an equivalence relation.
95 *
96 * <p>This interface is a member of the
97 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
98 * Java Collections Framework</a>.
99 *
100 * @param <T> the type of objects that may be compared by this comparator
101 *
102 * @author Josh Bloch
103 * @author Neal Gafter
104 * @see Comparable
105 * @see java.io.Serializable
106 * @since 1.2
107 */
108 @FunctionalInterface
109 public interface Comparator<T> {
110 /**
111 * Compares its two arguments for order. Returns a negative integer,
112 * zero, or a positive integer as the first argument is less than, equal
113 * to, or greater than the second.<p>
114 *
115 * In the foregoing description, the notation
116 * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
117 * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
118 * <tt>0</tt>, or <tt>1</tt> according to whether the value of
119 * <i>expression</i> is negative, zero or positive.<p>
120 *
121 * The implementor must ensure that <tt>sgn(compare(x, y)) ==
122 * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
123 * implies that <tt>compare(x, y)</tt> must throw an exception if and only
124 * if <tt>compare(y, x)</tt> throws an exception.)<p>
125 *
126 * The implementor must also ensure that the relation is transitive:
127 * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
128 * <tt>compare(x, z)>0</tt>.<p>
129 *
130 * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
131 * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
132 * <tt>z</tt>.<p>
133 *
134 * It is generally the case, but <i>not</i> strictly required that
135 * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking,
136 * any comparator that violates this condition should clearly indicate
137 * this fact. The recommended language is "Note: this comparator
138 * imposes orderings that are inconsistent with equals."
139 *
140 * @param o1 the first object to be compared.
141 * @param o2 the second object to be compared.
142 * @return a negative integer, zero, or a positive integer as the
143 * first argument is less than, equal to, or greater than the
144 * second.
145 * @throws NullPointerException if an argument is null and this
146 * comparator does not permit null arguments
147 * @throws ClassCastException if the arguments' types prevent them from
148 * being compared by this comparator.
149 */
150 int compare(T o1, T o2);
151
152 /**
153 * Indicates whether some other object is "equal to" this
154 * comparator. This method must obey the general contract of
155 * {@link Object#equals(Object)}. Additionally, this method can return
156 * <tt>true</tt> <i>only</i> if the specified object is also a comparator
157 * and it imposes the same ordering as this comparator. Thus,
158 * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
159 * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
160 * <tt>o1</tt> and <tt>o2</tt>.<p>
161 *
162 * Note that it is <i>always</i> safe <i>not</i> to override
163 * <tt>Object.equals(Object)</tt>. However, overriding this method may,
164 * in some cases, improve performance by allowing programs to determine
165 * that two distinct comparators impose the same order.
166 *
167 * @param obj the reference object with which to compare.
168 * @return <code>true</code> only if the specified object is also
169 * a comparator and it imposes the same ordering as this
170 * comparator.
171 * @see Object#equals(Object)
172 * @see Object#hashCode()
173 */
174 boolean equals(Object obj);
175
176 /**
177 * Returns a comparator that imposes the reverse ordering of this
178 * comparator.
179 *
180 * @return a comparator that imposes the reverse ordering of this
181 * comparator.
182 * @since 1.8
183 */
184 default Comparator<T> reversed() {
185 return Collections.reverseOrder(this);
186 }
187
188 /**
|
25
26 package java.util;
27
28 import java.io.Serializable;
29 import java.util.function.Function;
30 import java.util.function.ToIntFunction;
31 import java.util.function.ToLongFunction;
32 import java.util.function.ToDoubleFunction;
33 import java.util.Comparators;
34
35 /**
36 * A comparison function, which imposes a <i>total ordering</i> on some
37 * collection of objects. Comparators can be passed to a sort method (such
38 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
39 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
40 * over the sort order. Comparators can also be used to control the order of
41 * certain data structures (such as {@link SortedSet sorted sets} or {@link
42 * SortedMap sorted maps}), or to provide an ordering for collections of
43 * objects that don't have a {@link Comparable natural ordering}.<p>
44 *
45 * The ordering imposed by a comparator {@code c} on a set of elements
46 * {@code S} is said to be <i>consistent with equals</i> if and only if
47 * {@code c.compare(e1, e2)==0} has the same boolean value as
48 * {@code e1.equals(e2)} for every {@code e1} and {@code e2} in
49 * {@code S}.<p>
50 *
51 * Caution should be exercised when using a comparator capable of imposing an
52 * ordering inconsistent with equals to order a sorted set (or sorted map).
53 * Suppose a sorted set (or sorted map) with an explicit comparator {@code c}
54 * is used with elements (or keys) drawn from a set {@code S}. If the
55 * ordering imposed by {@code c} on {@code S} is inconsistent with equals,
56 * the sorted set (or sorted map) will behave "strangely." In particular the
57 * sorted set (or sorted map) will violate the general contract for set (or
58 * map), which is defined in terms of {@code equals}.<p>
59 *
60 * For example, suppose one adds two elements {@code a} and {@code b} such that
61 * {@code (a.equals(b) && c.compare(a, b) != 0)}
62 * to an empty {@code TreeSet} with comparator {@code c}.
63 * The second {@code add} operation will return
64 * true (and the size of the tree set will increase) because {@code a} and
65 * {@code b} are not equivalent from the tree set's perspective, even though
66 * this is contrary to the specification of the
67 * {@link Set#add Set.add} method.<p>
68 *
69 * Note: It is generally a good idea for comparators to also implement
70 * {@code java.io.Serializable}, as they may be used as ordering methods in
71 * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In
72 * order for the data structure to serialize successfully, the comparator (if
73 * provided) must implement {@code Serializable}.<p>
74 *
75 * For the mathematically inclined, the <i>relation</i> that defines the
76 * <i>imposed ordering</i> that a given comparator {@code c} imposes on a
77 * given set of objects {@code S} is:<pre>
78 * {(x, y) such that c.compare(x, y) <= 0}.
79 * </pre> The <i>quotient</i> for this total order is:<pre>
80 * {(x, y) such that c.compare(x, y) == 0}.
81 * </pre>
82 *
83 * It follows immediately from the contract for {@code compare} that the
84 * quotient is an <i>equivalence relation</i> on {@code S}, and that the
85 * imposed ordering is a <i>total order</i> on {@code S}. When we say that
86 * the ordering imposed by {@code c} on {@code S} is <i>consistent with
87 * equals</i>, we mean that the quotient for the ordering is the equivalence
88 * relation defined by the objects' {@link Object#equals(Object)
89 * equals(Object)} method(s):<pre>
90 * {(x, y) such that x.equals(y)}. </pre>
91 *
92 * <p>Unlike {@code Comparable}, a comparator may optionally permit
93 * comparison of null arguments, while maintaining the requirements for
94 * an equivalence relation.
95 *
96 * <p>This interface is a member of the
97 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
98 * Java Collections Framework</a>.
99 *
100 * @param <T> the type of objects that may be compared by this comparator
101 *
102 * @author Josh Bloch
103 * @author Neal Gafter
104 * @see Comparable
105 * @see java.io.Serializable
106 * @since 1.2
107 */
108 @FunctionalInterface
109 public interface Comparator<T> {
110 /**
111 * Compares its two arguments for order. Returns a negative integer,
112 * zero, or a positive integer as the first argument is less than, equal
113 * to, or greater than the second.<p>
114 *
115 * In the foregoing description, the notation
116 * {@code sgn(}<i>expression</i>{@code )} designates the mathematical
117 * <i>signum</i> function, which is defined to return one of {@code -1},
118 * {@code 0}, or {@code 1} according to whether the value of
119 * <i>expression</i> is negative, zero or positive.<p>
120 *
121 * The implementor must ensure that {@code sgn(compare(x, y)) ==
122 * -sgn(compare(y, x))} for all {@code x} and {@code y}. (This
123 * implies that {@code compare(x, y)} must throw an exception if and only
124 * if {@code compare(y, x)} throws an exception.)<p>
125 *
126 * The implementor must also ensure that the relation is transitive:
127 * {@code ((compare(x, y)>0) && (compare(y, z)>0))} implies
128 * {@code compare(x, z)>0}.<p>
129 *
130 * Finally, the implementor must ensure that {@code compare(x, y)==0}
131 * implies that {@code sgn(compare(x, z))==sgn(compare(y, z))} for all
132 * {@code z}.<p>
133 *
134 * It is generally the case, but <i>not</i> strictly required that
135 * {@code (compare(x, y)==0) == (x.equals(y))}. Generally speaking,
136 * any comparator that violates this condition should clearly indicate
137 * this fact. The recommended language is "Note: this comparator
138 * imposes orderings that are inconsistent with equals."
139 *
140 * @param o1 the first object to be compared.
141 * @param o2 the second object to be compared.
142 * @return a negative integer, zero, or a positive integer as the
143 * first argument is less than, equal to, or greater than the
144 * second.
145 * @throws NullPointerException if an argument is null and this
146 * comparator does not permit null arguments
147 * @throws ClassCastException if the arguments' types prevent them from
148 * being compared by this comparator.
149 */
150 int compare(T o1, T o2);
151
152 /**
153 * Indicates whether some other object is "equal to" this
154 * comparator. This method must obey the general contract of
155 * {@link Object#equals(Object)}. Additionally, this method can return
156 * {@code true} <i>only</i> if the specified object is also a comparator
157 * and it imposes the same ordering as this comparator. Thus,
158 * {@code comp1.equals(comp2)} implies that {@code sgn(comp1.compare(o1,
159 * o2))==sgn(comp2.compare(o1, o2))} for every object reference
160 * {@code o1} and {@code o2}.<p>
161 *
162 * Note that it is <i>always</i> safe <i>not</i> to override
163 * {@code Object.equals(Object)}. However, overriding this method may,
164 * in some cases, improve performance by allowing programs to determine
165 * that two distinct comparators impose the same order.
166 *
167 * @param obj the reference object with which to compare.
168 * @return {@code true} only if the specified object is also
169 * a comparator and it imposes the same ordering as this
170 * comparator.
171 * @see Object#equals(Object)
172 * @see Object#hashCode()
173 */
174 boolean equals(Object obj);
175
176 /**
177 * Returns a comparator that imposes the reverse ordering of this
178 * comparator.
179 *
180 * @return a comparator that imposes the reverse ordering of this
181 * comparator.
182 * @since 1.8
183 */
184 default Comparator<T> reversed() {
185 return Collections.reverseOrder(this);
186 }
187
188 /**
|