# Class FixedOrderComparator<T>

java.lang.Object
org.apache.commons.collections4.comparators.FixedOrderComparator<T>

Type Parameters:
  `T` - the type of objects compared by this comparator

All Implemented Interfaces:
  `Serializable`, `Comparator<T>`

public class FixedOrderComparator<T> extends Object implements Comparator<T>, Serializable
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.

This class is Serializable from Commons Collections 4.0.

Since:
  3.0
See Also:
  - Serialized Form

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static enum`
  `FixedOrderComparator.UnknownObjectBehavior`
  Enumerates the unknown object behaviors.

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `FixedOrderComparator()`
  Constructs an empty FixedOrderComparator.
  `FixedOrderComparator(List<T> items)`
  Constructs a FixedOrderComparator which uses the order of the given list to compare the objects.
  `FixedOrderComparator(T... items)`
  Constructs a FixedOrderComparator which uses the order of the given array to compare the objects.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `add(T obj)`
  Adds an item, which compares as after all items known to the Comparator.
  `boolean`
  `addAsEqual(T existingObj, T 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(T obj1, T obj2)`
  Compares two objects according to the order of this Comparator.
  `boolean`
  `equals(Object obj)`
  `FixedOrderComparator.UnknownObjectBehavior`
  `getUnknownObjectBehavior()`
  Gets the behavior for comparing unknown objects.
  `int`
  `hashCode()`
  `boolean`
  `isLocked()`
  Returns true if modifications cannot be made to the FixedOrderComparator.
  `void`
  `setUnknownObjectBehavior(FixedOrderComparator.UnknownObjectBehavior unknownObjectBehavior)`
  Sets the behavior for comparing unknown objects.

  ### Methods inherited from class java.lang.Object

  `clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait`

  ### Methods inherited from interface java.util.Comparator

  `reversed, thenComparing, thenComparing, thenComparing, thenComparingDouble, thenComparingInt, thenComparingLong`

- ## Constructor Details

  - ### FixedOrderComparator

    public FixedOrderComparator()
    Constructs an empty FixedOrderComparator.

  - ### FixedOrderComparator

    public FixedOrderComparator(List<T> 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:
      `NullPointerException` - if the list is null

  - ### FixedOrderComparator

    public FixedOrderComparator(T... 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:
      `NullPointerException` - if the array is null

- ## Method Details

  - ### add

    public boolean add(T 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(T existingObj, T 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(T obj1, T 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 unrecognized object. This is not specified in the Comparator interface, but is the most appropriate exception.

    Specified by:
      `compare` in interface `Comparator<T>`
    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(UnknownObjectBehavior)`.

  - ### equals

    public boolean equals(Object obj)
    Specified by:
      `equals` in interface `Comparator<T>`
    Overrides:
      `equals` in class `Object`

  - ### getUnknownObjectBehavior

    public FixedOrderComparator.UnknownObjectBehavior getUnknownObjectBehavior()
    Gets the behavior for comparing unknown objects.
    Returns:
      `FixedOrderComparator.UnknownObjectBehavior`

  - ### hashCode

    public int hashCode()
    Overrides:
      `hashCode` in class `Object`

  - ### 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(FixedOrderComparator.UnknownObjectBehavior unknownObjectBehavior)
    Sets the behavior for comparing unknown objects.
    Parameters:
      `unknownObjectBehavior` - the flag for unknown behavior - UNKNOWN_AFTER, UNKNOWN_BEFORE or UNKNOWN_THROW_EXCEPTION
    Throws:
      `UnsupportedOperationException` - if a comparison has been performed
      `NullPointerException` - if unknownObjectBehavior is null