/*

* Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved.

* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*/

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) &lt;= 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)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies

* <tt>compare(x, z)&gt;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 &quot;equal to&quot; 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);

}