org.apache.commons.collections.comparators
Class FixedOrderComparator

java.lang.Object
  org.apache.commons.collections.comparators.FixedOrderComparator

All Implemented Interfaces:

java.util.Comparator

public class FixedOrderComparator
extends java.lang.Object
implements java.util.Comparator

A Comparator which imposes a specific order on a specific set of Objects. Objects are presented to the FixedOrderComparator in a specified order and subsequent calls to compare yield that order. For example:

 String[] planets = {"Mercury", "Venus", "Earth", "Mars"};
 FixedOrderComparator distanceFromSun = new FixedOrderComparator(planets);
 Arrays.sort(planets);                     // Sort to alphabetical order
 Arrays.sort(planets, distanceFromSun);    // Back to original order
 

Once compare has been called, the FixedOrderComparator is locked and attempts to modify it yield an UnsupportedOperationException.

Instances of FixedOrderComparator are not synchronized. The class is not thread-safe at construction time, but it is thread-safe to perform multiple comparisons after all the setup operations are complete.

Since:

Commons Collections 3.0

Version:

$Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $

Author:

David Leppik, Stephen Colebourne, Janek Bogucki

Field Summary

static int	UNKNOWN_AFTER Behavior when comparing unknown Objects: unknown objects compare as after known Objects.
static int	UNKNOWN_BEFORE Behavior when comparing unknown Objects: unknown objects compare as before known Objects.
static int	UNKNOWN_THROW_EXCEPTION Behavior when comparing unknown Objects: unknown objects cause a IllegalArgumentException to be thrown.

Constructor Summary

FixedOrderComparator()
Constructs an empty FixedOrderComparator.

FixedOrderComparator(java.util.List items)
Constructs a FixedOrderComparator which uses the order of the given list to compare the objects.

FixedOrderComparator(java.lang.Object[] items)
Constructs a FixedOrderComparator which uses the order of the given array to compare the objects.

Method Summary

boolean	add(java.lang.Object obj) Adds an item, which compares as after all items known to the Comparator.
boolean	addAsEqual(java.lang.Object existingObj, java.lang.Object newObj) Adds a new item, which compares as equal to the given existing item.
protected void	checkLocked() Checks to see whether the comparator is now locked against further changes.
int	compare(java.lang.Object obj1, java.lang.Object obj2) Compares two objects according to the order of this Comparator.
int	getUnknownObjectBehavior() Gets the behavior for comparing unknown objects.
boolean	isLocked() Returns true if modifications cannot be made to the FixedOrderComparator.
void	setUnknownObjectBehavior(int unknownObjectBehavior) Sets the behavior for comparing unknown objects.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.util.Comparator

equals

Field Detail

UNKNOWN_BEFORE

public static final int UNKNOWN_BEFORE

Behavior when comparing unknown Objects: unknown objects compare as before known Objects.

See Also:

Constant Field Values

UNKNOWN_AFTER

public static final int UNKNOWN_AFTER

Behavior when comparing unknown Objects: unknown objects compare as after known Objects.

See Also:

Constant Field Values

UNKNOWN_THROW_EXCEPTION

public static final int UNKNOWN_THROW_EXCEPTION

Behavior when comparing unknown Objects: unknown objects cause a IllegalArgumentException to be thrown. This is the default behavior.

See Also:

Constant Field Values

Constructor Detail

FixedOrderComparator

public FixedOrderComparator()

Constructs an empty FixedOrderComparator.

FixedOrderComparator

public FixedOrderComparator(java.lang.Object[] items)

Constructs a FixedOrderComparator which uses the order of the given array to compare the objects.

The array is copied, so later changes will not affect the comparator.

Parameters:

items - the items that the comparator can compare in order

Throws:

java.lang.IllegalArgumentException - if the array is null

FixedOrderComparator

public FixedOrderComparator(java.util.List items)

Constructs a FixedOrderComparator which uses the order of the given list to compare the objects.

The list is copied, so later changes will not affect the comparator.

Parameters:

items - the items that the comparator can compare in order

Throws:

java.lang.IllegalArgumentException - if the list is null

Method Detail

isLocked

public boolean isLocked()

Returns true if modifications cannot be made to the FixedOrderComparator. FixedOrderComparators cannot be modified once they have performed a comparison.

Returns:

true if attempts to change the FixedOrderComparator yield an UnsupportedOperationException, false if it can be changed.

checkLocked

protected void checkLocked()

Checks to see whether the comparator is now locked against further changes.

Throws:

java.lang.UnsupportedOperationException - if the comparator is locked

getUnknownObjectBehavior

public int getUnknownObjectBehavior()

Gets the behavior for comparing unknown objects.

Returns:

the flag for unknown behaviour - UNKNOWN_AFTER, UNKNOWN_BEFORE or UNKNOWN_THROW_EXCEPTION

setUnknownObjectBehavior

public void setUnknownObjectBehavior(int unknownObjectBehavior)

Sets the behavior for comparing unknown objects.

Parameters:

unknownObjectBehavior - the flag for unknown behaviour - UNKNOWN_AFTER, UNKNOWN_BEFORE or UNKNOWN_THROW_EXCEPTION

Throws:

java.lang.UnsupportedOperationException - if a comparison has been performed

java.lang.IllegalArgumentException - if the unknown flag is not valid

add

public boolean add(java.lang.Object obj)

Adds an item, which compares as after all items known to the Comparator. If the item is already known to the Comparator, its old position is replaced with the new position.

Parameters:

obj - the item to be added to the Comparator.

Returns:

true if obj has been added for the first time, false if it was already known to the Comparator.

Throws:

java.lang.UnsupportedOperationException - if a comparison has already been made

addAsEqual

public boolean addAsEqual(java.lang.Object existingObj,
                          java.lang.Object newObj)

Adds a new item, which compares as equal to the given existing item.

Parameters:

existingObj - an item already in the Comparator's set of known objects

newObj - an item to be added to the Comparator's set of known objects

Returns:

true if newObj has been added for the first time, false if it was already known to the Comparator.

Throws:

java.lang.IllegalArgumentException - if existingObject is not in the Comparator's set of known objects.

java.lang.UnsupportedOperationException - if a comparison has already been made

compare

public int compare(java.lang.Object obj1,
                   java.lang.Object obj2)

Compares two objects according to the order of this Comparator.

It is important to note that this class will throw an IllegalArgumentException in the case of an unrecognised object. This is not specified in the Comparator interface, but is the most appropriate exception.

Specified by:

compare in interface java.util.Comparator

Parameters:

obj1 - the first object to compare

obj2 - the second object to compare

Returns:

negative if obj1 is less, positive if greater, zero if equal

Throws:

java.lang.IllegalArgumentException - if obj1 or obj2 are not known to this Comparator and an alternative behavior has not been set via setUnknownObjectBehavior(int).