org.apache.commons.collections.comparators

Class FixedOrderComparator

public class FixedOrderComparator extends Object implements 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: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $

Author: David Leppik Stephen Colebourne Janek Bogucki

Field Summary
static intUNKNOWN_AFTER
Behavior when comparing unknown Objects: unknown objects compare as after known Objects.
static intUNKNOWN_BEFORE
Behavior when comparing unknown Objects: unknown objects compare as before known Objects.
static intUNKNOWN_THROW_EXCEPTION
Behavior when comparing unknown Objects: unknown objects cause a IllegalArgumentException to be thrown.
Constructor Summary
FixedOrderComparator()
Constructs an empty FixedOrderComparator.
FixedOrderComparator(Object[] items)
Constructs a FixedOrderComparator which uses the order of the given array to compare the objects.
FixedOrderComparator(List items)
Constructs a FixedOrderComparator which uses the order of the given list to compare the objects.
Method Summary
booleanadd(Object obj)
Adds an item, which compares as after all items known to the Comparator.
booleanaddAsEqual(Object existingObj, Object newObj)
Adds a new item, which compares as equal to the given existing item.
protected voidcheckLocked()
Checks to see whether the comparator is now locked against further changes.
intcompare(Object obj1, Object obj2)
Compares two objects according to the order of this Comparator.
intgetUnknownObjectBehavior()
Gets the behavior for comparing unknown objects.
booleanisLocked()
Returns true if modifications cannot be made to the FixedOrderComparator.
voidsetUnknownObjectBehavior(int unknownObjectBehavior)
Sets the behavior for comparing unknown objects.

Field Detail

UNKNOWN_AFTER

public static final int UNKNOWN_AFTER
Behavior when comparing unknown Objects: unknown objects compare as after known Objects.

UNKNOWN_BEFORE

public static final int UNKNOWN_BEFORE
Behavior when comparing unknown Objects: unknown objects compare as before known Objects.

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.

Constructor Detail

FixedOrderComparator

public FixedOrderComparator()
Constructs an empty FixedOrderComparator.

FixedOrderComparator

public FixedOrderComparator(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: IllegalArgumentException if the array is null

FixedOrderComparator

public FixedOrderComparator(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: IllegalArgumentException if the list is null

Method Detail

add

public boolean add(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: UnsupportedOperationException if a comparison has already been made

addAsEqual

public boolean addAsEqual(Object existingObj, 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: IllegalArgumentException if existingObject is not in the Comparator's set of known objects. UnsupportedOperationException if a comparison has already been made

checkLocked

protected void checkLocked()
Checks to see whether the comparator is now locked against further changes.

Throws: UnsupportedOperationException if the comparator is locked

compare

public int compare(Object obj1, 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.

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: IllegalArgumentException if obj1 or obj2 are not known to this Comparator and an alternative behavior has not been set via FixedOrderComparator.

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

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.

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: UnsupportedOperationException if a comparison has been performed IllegalArgumentException if the unknown flag is not valid

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