001: /*
002: * Copyright 1997-2007 Sun Microsystems, Inc. All Rights Reserved.
003: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004: *
005: * This code is free software; you can redistribute it and/or modify it
006: * under the terms of the GNU General Public License version 2 only, as
007: * published by the Free Software Foundation. Sun designates this
008: * particular file as subject to the "Classpath" exception as provided
009: * by Sun in the LICENSE file that accompanied this code.
010: *
011: * This code is distributed in the hope that it will be useful, but WITHOUT
012: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014: * version 2 for more details (a copy is included in the LICENSE file that
015: * accompanied this code).
016: *
017: * You should have received a copy of the GNU General Public License version
018: * 2 along with this work; if not, write to the Free Software Foundation,
019: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020: *
021: * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022: * CA 95054 USA or visit www.sun.com if you need additional information or
023: * have any questions.
024: */
025:
026: package java.lang;
027:
028: import java.util.*;
029:
030: /**
031: * This interface imposes a total ordering on the objects of each class that
032: * implements it. This ordering is referred to as the class's <i>natural
033: * ordering</i>, and the class's <tt>compareTo</tt> method is referred to as
034: * its <i>natural comparison method</i>.<p>
035: *
036: * Lists (and arrays) of objects that implement this interface can be sorted
037: * automatically by {@link Collections#sort(List) Collections.sort} (and
038: * {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this
039: * interface can be used as keys in a {@linkplain SortedMap sorted map} or as
040: * elements in a {@linkplain SortedSet sorted set}, without the need to
041: * specify a {@linkplain Comparator comparator}.<p>
042: *
043: * The natural ordering for a class <tt>C</tt> is said to be <i>consistent
044: * with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has
045: * the same boolean value as <tt>e1.equals(e2)</tt> for every
046: * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>. Note that <tt>null</tt>
047: * is not an instance of any class, and <tt>e.compareTo(null)</tt> should
048: * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt>
049: * returns <tt>false</tt>.<p>
050: *
051: * It is strongly recommended (though not required) that natural orderings be
052: * consistent with equals. This is so because sorted sets (and sorted maps)
053: * without explicit comparators behave "strangely" when they are used with
054: * elements (or keys) whose natural ordering is inconsistent with equals. In
055: * particular, such a sorted set (or sorted map) violates the general contract
056: * for set (or map), which is defined in terms of the <tt>equals</tt>
057: * method.<p>
058: *
059: * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
060: * <tt>(!a.equals(b) && a.compareTo(b) == 0)</tt> to a sorted
061: * set that does not use an explicit comparator, the second <tt>add</tt>
062: * operation returns false (and the size of the sorted set does not increase)
063: * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
064: * perspective.<p>
065: *
066: * Virtually all Java core classes that implement <tt>Comparable</tt> have natural
067: * orderings that are consistent with equals. One exception is
068: * <tt>java.math.BigDecimal</tt>, whose natural ordering equates
069: * <tt>BigDecimal</tt> objects with equal values and different precisions
070: * (such as 4.0 and 4.00).<p>
071: *
072: * For the mathematically inclined, the <i>relation</i> that defines
073: * the natural ordering on a given class C is:<pre>
074: * {(x, y) such that x.compareTo(y) <= 0}.
075: * </pre> The <i>quotient</i> for this total order is: <pre>
076: * {(x, y) such that x.compareTo(y) == 0}.
077: * </pre>
078: *
079: * It follows immediately from the contract for <tt>compareTo</tt> that the
080: * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the
081: * natural ordering is a <i>total order</i> on <tt>C</tt>. When we say that a
082: * class's natural ordering is <i>consistent with equals</i>, we mean that the
083: * quotient for the natural ordering is the equivalence relation defined by
084: * the class's {@link Object#equals(Object) equals(Object)} method:<pre>
085: * {(x, y) such that x.equals(y)}. </pre><p>
086: *
087: * This interface is a member of the
088: * <a href="{@docRoot}/../technotes/guides/collections/index.html">
089: * Java Collections Framework</a>.
090: *
091: * @param <T> the type of objects that this object may be compared to
092: *
093: * @author Josh Bloch
094: * @version 1.33, 06/12/07
095: * @see java.util.Comparator
096: * @since 1.2
097: */
098:
099: public interface Comparable<T> {
100: /**
101: * Compares this object with the specified object for order. Returns a
102: * negative integer, zero, or a positive integer as this object is less
103: * than, equal to, or greater than the specified object.
104: *
105: * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
106: * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
107: * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
108: * <tt>y.compareTo(x)</tt> throws an exception.)
109: *
110: * <p>The implementor must also ensure that the relation is transitive:
111: * <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies
112: * <tt>x.compareTo(z)>0</tt>.
113: *
114: * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
115: * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
116: * all <tt>z</tt>.
117: *
118: * <p>It is strongly recommended, but <i>not</i> strictly required that
119: * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>. Generally speaking, any
120: * class that implements the <tt>Comparable</tt> interface and violates
121: * this condition should clearly indicate this fact. The recommended
122: * language is "Note: this class has a natural ordering that is
123: * inconsistent with equals."
124: *
125: * <p>In the foregoing description, the notation
126: * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
127: * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
128: * <tt>0</tt>, or <tt>1</tt> according to whether the value of
129: * <i>expression</i> is negative, zero or positive.
130: *
131: * @param o the object to be compared.
132: * @return a negative integer, zero, or a positive integer as this object
133: * is less than, equal to, or greater than the specified object.
134: *
135: * @throws NullPointerException if the specified object is null
136: * @throws ClassCastException if the specified object's type prevents it
137: * from being compared to this object.
138: */
139: public int compareTo(T o);
140: }
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