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  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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  25 
  26 package java.lang;
  27 import java.util.*;
  28 
  29 /**
  30  * This interface imposes a total ordering on the objects of each class that
  31  * implements it.  This ordering is referred to as the class's <i>natural
  32  * ordering</i>, and the class's <tt>compareTo</tt> method is referred to as
  33  * its <i>natural comparison method</i>.<p>
  34  *
  35  * Lists (and arrays) of objects that implement this interface can be sorted
  36  * automatically by {@link Collections#sort(List) Collections.sort} (and
  37  * {@link Arrays#sort(Object[]) Arrays.sort}).  Objects that implement this
  38  * interface can be used as keys in a {@linkplain SortedMap sorted map} or as
  39  * elements in a {@linkplain SortedSet sorted set}, without the need to
  40  * specify a {@linkplain Comparator comparator}.<p>
  41  *
  42  * The natural ordering for a class <tt>C</tt> is said to be <i>consistent
  43  * with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has
  44  * the same boolean value as <tt>e1.equals(e2)</tt> for every
  45  * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>.  Note that <tt>null</tt>
  46  * is not an instance of any class, and <tt>e.compareTo(null)</tt> should
  47  * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt>
  48  * returns <tt>false</tt>.<p>
  49  *
  50  * It is strongly recommended (though not required) that natural orderings be
  51  * consistent with equals.  This is so because sorted sets (and sorted maps)
  52  * without explicit comparators behave "strangely" when they are used with
  53  * elements (or keys) whose natural ordering is inconsistent with equals.  In
  54  * particular, such a sorted set (or sorted map) violates the general contract
  55  * for set (or map), which is defined in terms of the <tt>equals</tt>
  56  * method.<p>
  57  *
  58  * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
  59  * <tt>(!a.equals(b) && a.compareTo(b) == 0)</tt> to a sorted
  60  * set that does not use an explicit comparator, the second <tt>add</tt>
  61  * operation returns false (and the size of the sorted set does not increase)
  62  * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
  63  * perspective.<p>
  64  *
  65  * Virtually all Java core classes that implement <tt>Comparable</tt> have natural
  66  * orderings that are consistent with equals.  One exception is
  67  * <tt>java.math.BigDecimal</tt>, whose natural ordering equates
  68  * <tt>BigDecimal</tt> objects with equal values and different precisions
  69  * (such as 4.0 and 4.00).<p>
  70  *
  71  * For the mathematically inclined, the <i>relation</i> that defines
  72  * the natural ordering on a given class C is:<pre>
  73  *       {(x, y) such that x.compareTo(y) &lt;= 0}.
  74  * </pre> The <i>quotient</i> for this total order is: <pre>
  75  *       {(x, y) such that x.compareTo(y) == 0}.
  76  * </pre>
  77  *
  78  * It follows immediately from the contract for <tt>compareTo</tt> that the
  79  * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the
  80  * natural ordering is a <i>total order</i> on <tt>C</tt>.  When we say that a
  81  * class's natural ordering is <i>consistent with equals</i>, we mean that the
  82  * quotient for the natural ordering is the equivalence relation defined by
  83  * the class's {@link Object#equals(Object) equals(Object)} method:<pre>
  84  *     {(x, y) such that x.equals(y)}. </pre><p>
  85  *
  86  * This interface is a member of the
  87  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  88  * Java Collections Framework</a>.
  89  *
  90  * @param <T> the type of objects that this object may be compared to
  91  *
  92  * @author  Josh Bloch
  93  * @see java.util.Comparator
  94  * @since 1.2
  95  */
  96 @FunctionalInterface
  97 public interface Comparable<T> {
  98     /**
  99      * Compares this object with the specified object for order.  Returns a
 100      * negative integer, zero, or a positive integer as this object is less
 101      * than, equal to, or greater than the specified object.
 102      *
 103      * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
 104      * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
 105      * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
 106      * <tt>y.compareTo(x)</tt> throws an exception.)
 107      *
 108      * <p>The implementor must also ensure that the relation is transitive:
 109      * <tt>(x.compareTo(y)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies
 110      * <tt>x.compareTo(z)&gt;0</tt>.
 111      *
 112      * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
 113      * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
 114      * all <tt>z</tt>.
 115      *
 116      * <p>It is strongly recommended, but <i>not</i> strictly required that
 117      * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any
 118      * class that implements the <tt>Comparable</tt> interface and violates
 119      * this condition should clearly indicate this fact.  The recommended
 120      * language is "Note: this class has a natural ordering that is
 121      * inconsistent with equals."
 122      *
 123      * <p>In the foregoing description, the notation
 124      * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
 125      * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
 126      * <tt>0</tt>, or <tt>1</tt> according to whether the value of
 127      * <i>expression</i> is negative, zero or positive.
 128      *
 129      * @param   o the object to be compared.
 130      * @return  a negative integer, zero, or a positive integer as this object
 131      *          is less than, equal to, or greater than the specified object.
 132      *
 133      * @throws NullPointerException if the specified object is null
 134      * @throws ClassCastException if the specified object's type prevents it
 135      *         from being compared to this object.
 136      */
 137     public int compareTo(T o);
 138 }