author | mduigou |
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/* |
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* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. |
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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* |
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* This code is free software; you can redistribute it and/or modify it |
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* under the terms of the GNU General Public License version 2 only, as |
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* published by the Free Software Foundation. Oracle designates this |
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* particular file as subject to the "Classpath" exception as provided |
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* by Oracle in the LICENSE file that accompanied this code. |
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* |
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* This code is distributed in the hope that it will be useful, but WITHOUT |
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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* version 2 for more details (a copy is included in the LICENSE file that |
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* accompanied this code). |
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* |
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* You should have received a copy of the GNU General Public License version |
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* 2 along with this work; if not, write to the Free Software Foundation, |
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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* |
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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* or visit www.oracle.com if you need additional information or have any |
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* questions. |
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*/ |
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package java.util; |
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import java.util.function.Function; |
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import java.util.function.ToIntFunction; |
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import java.util.function.ToLongFunction; |
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import java.util.function.ToDoubleFunction; |
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/** |
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* A comparison function, which imposes a <i>total ordering</i> on some |
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* collection of objects. Comparators can be passed to a sort method (such |
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* as {@link Collections#sort(List,Comparator) Collections.sort} or {@link |
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* Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control |
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* over the sort order. Comparators can also be used to control the order of |
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* certain data structures (such as {@link SortedSet sorted sets} or {@link |
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* SortedMap sorted maps}), or to provide an ordering for collections of |
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* objects that don't have a {@link Comparable natural ordering}.<p> |
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* |
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* The ordering imposed by a comparator <tt>c</tt> on a set of elements |
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* <tt>S</tt> is said to be <i>consistent with equals</i> if and only if |
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* <tt>c.compare(e1, e2)==0</tt> has the same boolean value as |
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* <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in |
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* <tt>S</tt>.<p> |
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* |
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* Caution should be exercised when using a comparator capable of imposing an |
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* ordering inconsistent with equals to order a sorted set (or sorted map). |
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* Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt> |
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* is used with elements (or keys) drawn from a set <tt>S</tt>. If the |
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* ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals, |
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* the sorted set (or sorted map) will behave "strangely." In particular the |
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* sorted set (or sorted map) will violate the general contract for set (or |
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* map), which is defined in terms of <tt>equals</tt>.<p> |
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* |
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* For example, suppose one adds two elements {@code a} and {@code b} such that |
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* {@code (a.equals(b) && c.compare(a, b) != 0)} |
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* to an empty {@code TreeSet} with comparator {@code c}. |
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* The second {@code add} operation will return |
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* true (and the size of the tree set will increase) because {@code a} and |
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* {@code b} are not equivalent from the tree set's perspective, even though |
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* this is contrary to the specification of the |
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* {@link Set#add Set.add} method.<p> |
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* |
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* Note: It is generally a good idea for comparators to also implement |
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* <tt>java.io.Serializable</tt>, as they may be used as ordering methods in |
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* serializable data structures (like {@link TreeSet}, {@link TreeMap}). In |
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* order for the data structure to serialize successfully, the comparator (if |
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* provided) must implement <tt>Serializable</tt>.<p> |
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* |
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* For the mathematically inclined, the <i>relation</i> that defines the |
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* <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a |
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* given set of objects <tt>S</tt> is:<pre> |
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* {(x, y) such that c.compare(x, y) <= 0}. |
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* </pre> The <i>quotient</i> for this total order is:<pre> |
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* {(x, y) such that c.compare(x, y) == 0}. |
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* </pre> |
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* |
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* It follows immediately from the contract for <tt>compare</tt> that the |
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* quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the |
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* imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that |
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* the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with |
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* equals</i>, we mean that the quotient for the ordering is the equivalence |
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* relation defined by the objects' {@link Object#equals(Object) |
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* equals(Object)} method(s):<pre> |
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* {(x, y) such that x.equals(y)}. </pre> |
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* |
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* <p>Unlike {@code Comparable}, a comparator may optionally permit |
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* comparison of null arguments, while maintaining the requirements for |
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* an equivalence relation. |
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* |
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* <p>This interface is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @param <T> the type of objects that may be compared by this comparator |
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* |
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* @author Josh Bloch |
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* @author Neal Gafter |
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* @see Comparable |
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* @see java.io.Serializable |
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* @since 1.2 |
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*/ |
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@FunctionalInterface |
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public interface Comparator<T> { |
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/** |
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* Compares its two arguments for order. Returns a negative integer, |
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* zero, or a positive integer as the first argument is less than, equal |
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* to, or greater than the second.<p> |
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* |
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* In the foregoing description, the notation |
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* <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical |
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* <i>signum</i> function, which is defined to return one of <tt>-1</tt>, |
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* <tt>0</tt>, or <tt>1</tt> according to whether the value of |
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* <i>expression</i> is negative, zero or positive.<p> |
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* |
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* The implementor must ensure that <tt>sgn(compare(x, y)) == |
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* -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This |
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* implies that <tt>compare(x, y)</tt> must throw an exception if and only |
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* if <tt>compare(y, x)</tt> throws an exception.)<p> |
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* |
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* The implementor must also ensure that the relation is transitive: |
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* <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies |
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* <tt>compare(x, z)>0</tt>.<p> |
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* |
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* Finally, the implementor must ensure that <tt>compare(x, y)==0</tt> |
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* implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all |
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* <tt>z</tt>.<p> |
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* |
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* It is generally the case, but <i>not</i> strictly required that |
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* <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking, |
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* any comparator that violates this condition should clearly indicate |
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* this fact. The recommended language is "Note: this comparator |
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* imposes orderings that are inconsistent with equals." |
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* |
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* @param o1 the first object to be compared. |
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* @param o2 the second object to be compared. |
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* @return a negative integer, zero, or a positive integer as the |
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* first argument is less than, equal to, or greater than the |
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* second. |
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* @throws NullPointerException if an argument is null and this |
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* comparator does not permit null arguments |
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* @throws ClassCastException if the arguments' types prevent them from |
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* being compared by this comparator. |
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*/ |
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int compare(T o1, T o2); |
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/** |
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* Indicates whether some other object is "equal to" this |
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* comparator. This method must obey the general contract of |
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* {@link Object#equals(Object)}. Additionally, this method can return |
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* <tt>true</tt> <i>only</i> if the specified object is also a comparator |
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* and it imposes the same ordering as this comparator. Thus, |
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* <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1, |
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* o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference |
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* <tt>o1</tt> and <tt>o2</tt>.<p> |
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* |
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* Note that it is <i>always</i> safe <i>not</i> to override |
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* <tt>Object.equals(Object)</tt>. However, overriding this method may, |
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* in some cases, improve performance by allowing programs to determine |
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* that two distinct comparators impose the same order. |
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* |
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* @param obj the reference object with which to compare. |
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* @return <code>true</code> only if the specified object is also |
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* a comparator and it imposes the same ordering as this |
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* comparator. |
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* @see Object#equals(Object) |
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* @see Object#hashCode() |
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*/ |
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boolean equals(Object obj); |
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/** |
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* Returns a comparator that imposes the reverse ordering of this |
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* comparator. |
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* |
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* @return A comparator that imposes the reverse ordering of this |
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* comparator. |
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* @since 1.8 |
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*/ |
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default Comparator<T> reverseOrder() { |
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return Collections.reverseOrder(this); |
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} |
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/** |
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* Constructs a lexicographic order comparator with another comparator. |
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* For example, a {@code Comparator<Person> byLastName} can be composed |
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* with another {@code Comparator<Person> byFirstName}, then {@code |
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* byLastName.thenComparing(byFirstName)} creates a {@code |
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* Comparator<Person>} which sorts by last name, and for equal last names |
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* sorts by first name. |
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* |
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* @param other the other comparator used when equals on this. |
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* @throws NullPointerException if the argument is null. |
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* @since 1.8 |
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*/ |
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default Comparator<T> thenComparing(Comparator<? super T> other) { |
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return Comparators.compose(this, other); |
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} |
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/** |
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* Constructs a lexicographic order comparator with a function that |
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* extracts a {@code Comparable} key. This default implementation calls |
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* {@code thenComparing(this, Comparators.comparing(keyExtractor))}. |
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* |
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* @param <U> the {@link Comparable} type for comparison |
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* @param keyExtractor the function used to extract the {@link Comparable} sort key |
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* @throws NullPointerException if the argument is null. |
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* @see Comparators#comparing(Function) |
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* @see #thenComparing(Comparator) |
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* @since 1.8 |
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*/ |
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default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor) { |
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return thenComparing(Comparators.comparing(keyExtractor)); |
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} |
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|
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/** |
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* Constructs a lexicographic order comparator with a function that |
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* extracts a {@code int} value. This default implementation calls {@code |
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* thenComparing(this, Comparators.comparing(keyExtractor))}. |
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* |
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* @param keyExtractor the function used to extract the integer value |
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* @throws NullPointerException if the argument is null. |
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* @see Comparators#comparing(ToIntFunction) |
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* @see #thenComparing(Comparator) |
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* @since 1.8 |
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*/ |
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default Comparator<T> thenComparing(ToIntFunction<? super T> keyExtractor) { |
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return thenComparing(Comparators.comparing(keyExtractor)); |
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} |
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/** |
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* Constructs a lexicographic order comparator with a function that |
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* extracts a {@code long} value. This default implementation calls |
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* {@code thenComparing(this, Comparators.comparing(keyExtractor))}. |
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* |
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* @param keyExtractor the function used to extract the long value |
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* @throws NullPointerException if the argument is null. |
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* @see Comparators#comparing(ToLongFunction) |
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* @see #thenComparing(Comparator) |
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* @since 1.8 |
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*/ |
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default Comparator<T> thenComparing(ToLongFunction<? super T> keyExtractor) { |
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return thenComparing(Comparators.comparing(keyExtractor)); |
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} |
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|
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/** |
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* Constructs a lexicographic order comparator with a function that |
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* extracts a {@code double} value. This default implementation calls |
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* {@code thenComparing(this, Comparators.comparing(keyExtractor))}. |
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* |
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* @param keyExtractor the function used to extract the double value |
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* @throws NullPointerException if the argument is null. |
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* @see Comparators#comparing(ToDoubleFunction) |
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* @see #thenComparing(Comparator) |
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* @since 1.8 |
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*/ |
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default Comparator<T> thenComparing(ToDoubleFunction<? super T> keyExtractor) { |
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return thenComparing(Comparators.comparing(keyExtractor)); |
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} |
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} |