org.apache.commons.collections.comparators

## Class FixedOrderComparator

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

- All Implemented Interfaces:
    Comparator

      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: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
  Author:
    David Leppik, Stephen Colebourne, Janek Bogucki

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `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

    Constructors
    Constructor and Description
    `FixedOrderComparator()` Constructs an empty FixedOrderComparator.
    `FixedOrderComparator(List items)` Constructs a FixedOrderComparator which uses the order of the given list to compare the objects.
    `FixedOrderComparator(Object[] items)` Constructs a FixedOrderComparator which uses the order of the given array to compare the objects.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `add(Object obj)` Adds an item, which compares as after all items known to the Comparator.
    `boolean` | `addAsEqual(Object existingObj, 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(Object obj1, 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(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

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

    - #### 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

    - #### 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.

      Specified by:
        `compare` in interface `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:
        `IllegalArgumentException` - if obj1 or obj2 are not known to this Comparator and an alternative behavior has not been set via `setUnknownObjectBehavior(int)`.

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