2362N/A * Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved. 0N/A * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 0N/A * This code is free software; you can redistribute it and/or modify it 0N/A * under the terms of the GNU General Public License version 2 only, as 2362N/A * published by the Free Software Foundation. Oracle designates this 0N/A * particular file as subject to the "Classpath" exception as provided 2362N/A * by Oracle in the LICENSE file that accompanied this code. 0N/A * This code is distributed in the hope that it will be useful, but WITHOUT 0N/A * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 0N/A * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 0N/A * version 2 for more details (a copy is included in the LICENSE file that 0N/A * accompanied this code). 0N/A * You should have received a copy of the GNU General Public License version 0N/A * 2 along with this work; if not, write to the Free Software Foundation, 0N/A * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 2362N/A * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 2362N/A * or visit www.oracle.com if you need additional information or have any 0N/A * A comparison function, which imposes a <i>total ordering</i> on some 0N/A * collection of objects. Comparators can be passed to a sort method (such 0N/A * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link 0N/A * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control 0N/A * over the sort order. Comparators can also be used to control the order of 0N/A * certain data structures (such as {@link SortedSet sorted sets} or {@link 0N/A * SortedMap sorted maps}), or to provide an ordering for collections of 0N/A * objects that don't have a {@link Comparable natural ordering}.<p> 0N/A * The ordering imposed by a comparator <tt>c</tt> on a set of elements 0N/A * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if 0N/A * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as 0N/A * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in 0N/A * Caution should be exercised when using a comparator capable of imposing an 0N/A * ordering inconsistent with equals to order a sorted set (or sorted map). 0N/A * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt> 0N/A * is used with elements (or keys) drawn from a set <tt>S</tt>. If the 0N/A * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals, 0N/A * the sorted set (or sorted map) will behave "strangely." In particular the 0N/A * sorted set (or sorted map) will violate the general contract for set (or 0N/A * map), which is defined in terms of <tt>equals</tt>.<p> 0N/A * For example, suppose one adds two elements {@code a} and {@code b} such that 0N/A * {@code (a.equals(b) && c.compare(a, b) != 0)} 0N/A * to an empty {@code TreeSet} with comparator {@code c}. 0N/A * The second {@code add} operation will return 0N/A * true (and the size of the tree set will increase) because {@code a} and 0N/A * {@code b} are not equivalent from the tree set's perspective, even though 0N/A * this is contrary to the specification of the 0N/A * {@link Set#add Set.add} method.<p> 0N/A * Note: It is generally a good idea for comparators to also implement 0N/A * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in 0N/A * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In 0N/A * order for the data structure to serialize successfully, the comparator (if 0N/A * provided) must implement <tt>Serializable</tt>.<p> 0N/A * For the mathematically inclined, the <i>relation</i> that defines the 0N/A * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a 0N/A * given set of objects <tt>S</tt> is:<pre> 0N/A * {(x, y) such that c.compare(x, y) <= 0}. 0N/A * </pre> The <i>quotient</i> for this total order is:<pre> 0N/A * {(x, y) such that c.compare(x, y) == 0}. 0N/A * It follows immediately from the contract for <tt>compare</tt> that the 0N/A * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the 0N/A * imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that 0N/A * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with 0N/A * equals</i>, we mean that the quotient for the ordering is the equivalence 0N/A * relation defined by the objects' {@link Object#equals(Object) 0N/A * equals(Object)} method(s):<pre> 0N/A * {(x, y) such that x.equals(y)}. </pre> 0N/A * <p>Unlike {@code Comparable}, a comparator may optionally permit 0N/A * comparison of null arguments, while maintaining the requirements for 0N/A * an equivalence relation. 0N/A * <p>This interface is a member of the 0N/A * Java Collections Framework</a>. 0N/A * @param <T> the type of objects that may be compared by this comparator 0N/A * @author Josh Bloch 0N/A * @author Neal Gafter 0N/A * @see java.io.Serializable 0N/A * Compares its two arguments for order. Returns a negative integer, 0N/A * zero, or a positive integer as the first argument is less than, equal 0N/A * to, or greater than the second.<p> 0N/A * In the foregoing description, the notation 0N/A * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical 0N/A * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, 0N/A * <tt>0</tt>, or <tt>1</tt> according to whether the value of 0N/A * <i>expression</i> is negative, zero or positive.<p> 0N/A * The implementor must ensure that <tt>sgn(compare(x, y)) == 0N/A * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This 0N/A * implies that <tt>compare(x, y)</tt> must throw an exception if and only 0N/A * if <tt>compare(y, x)</tt> throws an exception.)<p> 0N/A * The implementor must also ensure that the relation is transitive: 0N/A * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies 0N/A * <tt>compare(x, z)>0</tt>.<p> 0N/A * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt> 0N/A * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all 0N/A * It is generally the case, but <i>not</i> strictly required that 0N/A * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking, 0N/A * any comparator that violates this condition should clearly indicate 0N/A * this fact. The recommended language is "Note: this comparator 0N/A * imposes orderings that are inconsistent with equals." 0N/A * @param o1 the first object to be compared. 0N/A * @param o2 the second object to be compared. 0N/A * @return a negative integer, zero, or a positive integer as the 0N/A * first argument is less than, equal to, or greater than the 0N/A * @throws NullPointerException if an argument is null and this 0N/A * comparator does not permit null arguments 0N/A * @throws ClassCastException if the arguments' types prevent them from 0N/A * being compared by this comparator. 0N/A * Indicates whether some other object is "equal to" this 0N/A * comparator. This method must obey the general contract of 0N/A * {@link Object#equals(Object)}. Additionally, this method can return 0N/A * <tt>true</tt> <i>only</i> if the specified object is also a comparator 0N/A * and it imposes the same ordering as this comparator. Thus, 0N/A * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1, 0N/A * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference 0N/A * <tt>o1</tt> and <tt>o2</tt>.<p> 0N/A * Note that it is <i>always</i> safe <i>not</i> to override 0N/A * <tt>Object.equals(Object)</tt>. However, overriding this method may, 0N/A * in some cases, improve performance by allowing programs to determine 0N/A * that two distinct comparators impose the same order. 0N/A * @param obj the reference object with which to compare. 0N/A * @return <code>true</code> only if the specified object is also 0N/A * a comparator and it imposes the same ordering as this 0N/A * @see Object#equals(Object) 0N/A * @see Object#hashCode()