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java.util

Interface Comparator<T>

Known Implementing Classes:
Collator, RuleBasedCollator
public interface Comparator<T>
Interface for objects that specify an ordering between objects. The ordering should be total, such that any two objects of the correct type can be compared, and the comparison is reflexive, anti-symmetric, and transitive. It is also recommended that the comparator be consistent with equals, although this is not a strict requirement. A relation is consistent with equals if these two statements always have the same results (if no exceptions occur):
compare((Object) e1, (Object) e2) == 0 and e1.equals((Object) e2)
Comparators that violate consistency with equals may cause strange behavior in sorted lists and sets. For example, a case-sensitive dictionary order comparison of Strings is consistent with equals, but if it is case-insensitive it is not, because "abc" and "ABC" compare as equal even though "abc".equals("ABC") returns false.

In general, Comparators should be Serializable, because when they are passed to Serializable data structures such as SortedMap or SortedSet, the entire data structure will only serialize correctly if the comparator is Serializable.

Since:
1.2
See Also:
Comparable, TreeMap, TreeSet, SortedMap, SortedSet, Arrays.sort(Object[], Comparator), Serializable

Method Summary

int
compare(T o1, T o2)
Return an integer that is negative, zero or positive depending on whether the first argument is less than, equal to or greater than the second according to this ordering.
boolean
equals(Object obj)
Return true if the object is equal to this object.

Method Details

compare

public int compare(T o1,
                   T o2)
Return an integer that is negative, zero or positive depending on whether the first argument is less than, equal to or greater than the second according to this ordering. This method should obey the following contract:
  • if compare(a, b) < 0 then compare(b, a) > 0
  • if compare(a, b) throws an exception, so does compare(b, a)
  • if compare(a, b) < 0 and compare(b, c) < 0 then compare(a, c) < 0
  • if compare(a, b) == 0 then compare(a, c) and compare(b, c) must have the same sign
To be consistent with equals, the following additional constraint is in place:
  • if a.equals(b) or both a and b are null, then compare(a, b) == 0.

Although it is permissible for a comparator to provide an order inconsistent with equals, that should be documented.

Parameters:
o1 - the first object
o2 - the second object
Returns:
the comparison
Throws:
ClassCastException - if the elements are not of types that can be compared by this ordering.

equals

public boolean equals(Object obj)
Return true if the object is equal to this object. To be considered equal, the argument object must satisfy the constraints of Object.equals(), be a Comparator, and impose the same ordering as this Comparator. The default implementation inherited from Object is usually adequate.
Overrides:
equals in interface Object
Parameters:
obj - The object
Returns:
true if it is a Comparator that imposes the same order
See Also:
Object.equals(Object)
Comparator.java -- Interface for objects that specify an ordering Copyright (C) 1998, 2001, 2004, 2005 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath 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 for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.

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