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package java.lang;

import java.util.*;

/**

* This interface imposes a total ordering on the objects of each class that

* implements it.

This ordering is referred to as the class's <i>natural

* ordering</i>, and the class's <tt>compareTo</tt> method is referred to as

* its <i>natural comparison method</i>.<p>

*

* Lists (and arrays) of objects that implement this interface can be sorted

* automatically by {@link Collections#sort(List) Collections.sort} (and

* {@link Arrays#sort(Object[]) Arrays.sort}).

Objects that implement this

* interface can be used as keys in a {@linkplain SortedMap sorted map} or as

* elements in a {@linkplain SortedSet sorted set}, without the need to

* specify a {@linkplain Comparator comparator}.<p>

*

* The natural ordering for a class <tt>C</tt> is said to be <i>consistent

* with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has

* the same boolean value as <tt>e1.equals(e2)</tt> for every

* <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>.

Note that <tt>null</tt>

* is not an instance of any class, and <tt>e.compareTo(null)</tt> should

* throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt>

* returns <tt>false</tt>.<p>

*

* It is strongly recommended (though not required) that natural orderings be

* consistent with equals.

This is so because sorted sets (and sorted maps)

* without explicit comparators behave "strangely" when they are used with

* elements (or keys) whose natural ordering is inconsistent with equals.

In

* particular, such a sorted set (or sorted map) violates the general contract

* for set (or map), which is defined in terms of the <tt>equals</tt>

* method.<p>

*

* For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that

* {@code (!a.equals(b) && a.compareTo(b) == 0)} to a sorted

* set that does not use an explicit comparator, the second <tt>add</tt>

* operation returns false (and the size of the sorted set does not increase)

* because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's

* perspective.<p>

*

* Virtually all Java core classes that implement <tt>Comparable</tt> have natural

* orderings that are consistent with equals.

One exception is

* <tt>java.math.BigDecimal</tt>, whose natural ordering equates

* <tt>BigDecimal</tt> objects with equal values and different precisions

* (such as 4.0 and 4.00).<p>

*

* For the mathematically inclined, the <i>relation</i> that defines

* the natural ordering on a given class C is:<pre>

*

{(x, y) such that x.compareTo(y) <= 0}.

* </pre> The <i>quotient</i> for this total order is: <pre>

*

{(x, y) such that x.compareTo(y) == 0}.

* </pre>

*

* It follows immediately from the contract for <tt>compareTo</tt> that the

* quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the

* natural ordering is a <i>total order</i> on <tt>C</tt>.

When we say that a

* class's natural ordering is <i>consistent with equals</i>, we mean that the

* quotient for the natural ordering is the equivalence relation defined by

* the class's {@link Object#equals(Object) equals(Object)} method:<pre>

*

{(x, y) such that x.equals(y)}. </pre><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 this object may be compared to

*

* @author

Josh Bloch

*

* @since 1.2

*/

public interface Comparable<T> {

/**

* Compares this object with the specified object for order.

Returns a

* negative integer, zero, or a positive integer as this object is less

* than, equal to, or greater than the specified object.

*

* <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==

* -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.

(This

* implies that <tt>x.compareTo(y)</tt> must throw an exception iff

* <tt>y.compareTo(x)</tt> throws an exception.)

*

* <p>The implementor must also ensure that the relation is transitive:

* <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies

* <tt>x.compareTo(z)>0</tt>.

*

* <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>

* implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for

* all <tt>z</tt>.

*

* <p>It is strongly recommended, but <i>not</i> strictly required that

* <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.

Generally speaking, any

* class that implements the <tt>Comparable</tt> interface and violates

* this condition should clearly indicate this fact.

The recommended

* language is "Note: this class has a natural ordering that is

* inconsistent with equals."

*

* <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.

*

* @param

o the object to be compared.

* @return

a negative integer, zero, or a positive integer as this object

*

is less than, equal to, or greater than the specified object.

*

* @throws NullPointerException if the specified object is null

* @throws ClassCastException if the specified object's type prevents it

*

from being compared to this object.

*/

public int compareTo(T o);

}