--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/share/classes/java/util/Comparator.java Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,168 @@
+/*
+ * Copyright 1997-2007 Sun Microsystems, Inc. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation. Sun designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Sun in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+package java.util;
+
+/**
+ * A comparison function, which imposes a <i>total ordering</i> on some
+ * collection of objects. Comparators can be passed to a sort method (such
+ * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
+ * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
+ * over the sort order. Comparators can also be used to control the order of
+ * certain data structures (such as {@link SortedSet sorted sets} or {@link
+ * SortedMap sorted maps}), or to provide an ordering for collections of
+ * objects that don't have a {@link Comparable natural ordering}.<p>
+ *
+ * The ordering imposed by a comparator <tt>c</tt> on a set of elements
+ * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
+ * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
+ * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
+ * <tt>S</tt>.<p>
+ *
+ * Caution should be exercised when using a comparator capable of imposing an
+ * ordering inconsistent with equals to order a sorted set (or sorted map).
+ * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>
+ * is used with elements (or keys) drawn from a set <tt>S</tt>. If the
+ * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
+ * the sorted set (or sorted map) will behave "strangely." In particular the
+ * sorted set (or sorted map) will violate the general contract for set (or
+ * map), which is defined in terms of <tt>equals</tt>.<p>
+ *
+ * For example, suppose one adds two elements {@code a} and {@code b} such that
+ * {@code (a.equals(b) && c.compare(a, b) != 0)}
+ * to an empty {@code TreeSet} with comparator {@code c}.
+ * The second {@code add} operation will return
+ * true (and the size of the tree set will increase) because {@code a} and
+ * {@code b} are not equivalent from the tree set's perspective, even though
+ * this is contrary to the specification of the
+ * {@link Set#add Set.add} method.<p>
+ *
+ * Note: It is generally a good idea for comparators to also implement
+ * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
+ * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In
+ * order for the data structure to serialize successfully, the comparator (if
+ * provided) must implement <tt>Serializable</tt>.<p>
+ *
+ * For the mathematically inclined, the <i>relation</i> that defines the
+ * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
+ * given set of objects <tt>S</tt> is:<pre>
+ * {(x, y) such that c.compare(x, y) <= 0}.
+ * </pre> The <i>quotient</i> for this total order is:<pre>
+ * {(x, y) such that c.compare(x, y) == 0}.
+ * </pre>
+ *
+ * It follows immediately from the contract for <tt>compare</tt> that the
+ * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
+ * imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that
+ * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
+ * equals</i>, we mean that the quotient for the ordering is the equivalence
+ * relation defined by the objects' {@link Object#equals(Object)
+ * equals(Object)} method(s):<pre>
+ * {(x, y) such that x.equals(y)}. </pre>
+ *
+ * <p>Unlike {@code Comparable}, a comparator may optionally permit
+ * comparison of null arguments, while maintaining the requirements for
+ * an equivalence relation.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @param <T> the type of objects that may be compared by this comparator
+ *
+ * @author Josh Bloch
+ * @author Neal Gafter
+ * @see Comparable
+ * @see java.io.Serializable
+ * @since 1.2
+ */
+
+public interface Comparator<T> {
+ /**
+ * Compares its two arguments for order. Returns a negative integer,
+ * zero, or a positive integer as the first argument is less than, equal
+ * to, or greater than the second.<p>
+ *
+ * In the foregoing description, the notation
+ * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
+ * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
+ * <tt>0</tt>, or <tt>1</tt> according to whether the value of
+ * <i>expression</i> is negative, zero or positive.<p>
+ *
+ * The implementor must ensure that <tt>sgn(compare(x, y)) ==
+ * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
+ * implies that <tt>compare(x, y)</tt> must throw an exception if and only
+ * if <tt>compare(y, x)</tt> throws an exception.)<p>
+ *
+ * The implementor must also ensure that the relation is transitive:
+ * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
+ * <tt>compare(x, z)>0</tt>.<p>
+ *
+ * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
+ * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
+ * <tt>z</tt>.<p>
+ *
+ * It is generally the case, but <i>not</i> strictly required that
+ * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking,
+ * any comparator that violates this condition should clearly indicate
+ * this fact. The recommended language is "Note: this comparator
+ * imposes orderings that are inconsistent with equals."
+ *
+ * @param o1 the first object to be compared.
+ * @param o2 the second object to be compared.
+ * @return a negative integer, zero, or a positive integer as the
+ * first argument is less than, equal to, or greater than the
+ * second.
+ * @throws NullPointerException if an argument is null and this
+ * comparator does not permit null arguments
+ * @throws ClassCastException if the arguments' types prevent them from
+ * being compared by this comparator.
+ */
+ int compare(T o1, T o2);
+
+ /**
+ * Indicates whether some other object is "equal to" this
+ * comparator. This method must obey the general contract of
+ * {@link Object#equals(Object)}. Additionally, this method can return
+ * <tt>true</tt> <i>only</i> if the specified object is also a comparator
+ * and it imposes the same ordering as this comparator. Thus,
+ * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
+ * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
+ * <tt>o1</tt> and <tt>o2</tt>.<p>
+ *
+ * Note that it is <i>always</i> safe <i>not</i> to override
+ * <tt>Object.equals(Object)</tt>. However, overriding this method may,
+ * in some cases, improve performance by allowing programs to determine
+ * that two distinct comparators impose the same order.
+ *
+ * @param obj the reference object with which to compare.
+ * @return <code>true</code> only if the specified object is also
+ * a comparator and it imposes the same ordering as this
+ * comparator.
+ * @see Object#equals(Object)
+ * @see Object#hashCode()
+ */
+ boolean equals(Object obj);
+}