org.apache.commons.collections4

## Class BagUtils

- java.lang.Object
- - org.apache.commons.collections4.BagUtils

-     public class BagUtils
      extends Object

  Provides utility methods and decorators for `Bag` and `SortedBag` instances.
  Since:
    2.1
  Version:
    $Id: BagUtils.java 1686855 2015-06-22 13:00:27Z tn $

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static Bag` | `EMPTY_BAG` An empty unmodifiable bag.
    `static Bag` | `EMPTY_SORTED_BAG` An empty unmodifiable sorted bag.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `static <E> Bag<E>` | `collectionBag(Bag<E> bag)` Returns a bag that complies to the Collection contract, backed by the given bag.
    `static <E> Bag<E>` | `emptyBag()` Get an empty `Bag`.
    `static <E> SortedBag<E>` | `emptySortedBag()` Get an empty `SortedBag`.
    `static <E> Bag<E>` | `predicatedBag(Bag<E> bag, Predicate<? super E> predicate)` Returns a predicated (validating) bag backed by the given bag.
    `static <E> SortedBag<E>` | `predicatedSortedBag(SortedBag<E> bag, Predicate<? super E> predicate)` Returns a predicated (validating) sorted bag backed by the given sorted bag.
    `static <E> Bag<E>` | `synchronizedBag(Bag<E> bag)` Returns a synchronized (thread-safe) bag backed by the given bag.
    `static <E> SortedBag<E>` | `synchronizedSortedBag(SortedBag<E> bag)` Returns a synchronized (thread-safe) sorted bag backed by the given sorted bag.
    `static <E> Bag<E>` | `transformingBag(Bag<E> bag, Transformer<? super E,? extends E> transformer)` Returns a transformed bag backed by the given bag.
    `static <E> SortedBag<E>` | `transformingSortedBag(SortedBag<E> bag, Transformer<? super E,? extends E> transformer)` Returns a transformed sorted bag backed by the given bag.
    `static <E> Bag<E>` | `unmodifiableBag(Bag<? extends E> bag)` Returns an unmodifiable view of the given bag.
    `static <E> SortedBag<E>` | `unmodifiableSortedBag(SortedBag<E> bag)` Returns an unmodifiable view of the given sorted bag.

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

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

- - ### Field Detail

    - #### EMPTY_BAG

          public static final Bag EMPTY_BAG

      An empty unmodifiable bag.

    - #### EMPTY_SORTED_BAG

          public static final Bag EMPTY_SORTED_BAG

      An empty unmodifiable sorted bag.

  - ### Method Detail

    - #### synchronizedBag

          public static <E> Bag<E> synchronizedBag(Bag<E> bag)

      Returns a synchronized (thread-safe) bag backed by the given bag. In order to guarantee serial access, it is critical that all access to the backing bag is accomplished through the returned bag.

      It is imperative that the user manually synchronize on the returned bag when iterating over it:

           Bag bag = BagUtils.synchronizedBag(new HashBag());
           ...
           synchronized(bag) {
               Iterator i = bag.iterator(); // Must be in synchronized block
               while (i.hasNext())
                   foo(i.next());
               }
           }

      Failure to follow this advice may result in non-deterministic behavior.
      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag to synchronize, must not be null
      Returns:
        a synchronized bag backed by that bag
      Throws:
        `NullPointerException` - if the Bag is null

    - #### unmodifiableBag

          public static <E> Bag<E> unmodifiableBag(Bag<? extends E> bag)

      Returns an unmodifiable view of the given bag. Any modification attempts to the returned bag will raise an `UnsupportedOperationException`.
      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag whose unmodifiable view is to be returned, must not be null
      Returns:
        an unmodifiable view of that bag
      Throws:
        `NullPointerException` - if the Bag is null

    - #### predicatedBag

          public static <E> Bag<E> predicatedBag(Bag<E> bag,
                                 Predicate<? super E> predicate)

      Returns a predicated (validating) bag backed by the given bag.

      Only objects that pass the test in the given predicate can be added to the bag. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original bag after invoking this method, as it is a backdoor for adding invalid objects.

      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag to predicate, must not be null
        `predicate` - the predicate for the bag, must not be null
      Returns:
        a predicated bag backed by the given bag
      Throws:
        `NullPointerException` - if the Bag or Predicate is null

    - #### transformingBag

          public static <E> Bag<E> transformingBag(Bag<E> bag,
                                   Transformer<? super E,? extends E> transformer)

      Returns a transformed bag backed by the given bag.

      Each object is passed through the transformer as it is added to the Bag. It is important not to use the original bag after invoking this method, as it is a backdoor for adding untransformed objects.

      Existing entries in the specified bag will not be transformed. If you want that behaviour, see `TransformedBag.transformedBag(Bag, Transformer)`.

      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag to predicate, must not be null
        `transformer` - the transformer for the bag, must not be null
      Returns:
        a transformed bag backed by the given bag
      Throws:
        `NullPointerException` - if the Bag or Transformer is null

    - #### collectionBag

          public static <E> Bag<E> collectionBag(Bag<E> bag)

      Returns a bag that complies to the Collection contract, backed by the given bag.
      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag to decorate, must not be null
      Returns:
        a Bag that complies to the Collection contract
      Throws:
        `NullPointerException` - if bag is null
      Since:
        4.0

    - #### synchronizedSortedBag

          public static <E> SortedBag<E> synchronizedSortedBag(SortedBag<E> bag)

      Returns a synchronized (thread-safe) sorted bag backed by the given sorted bag. In order to guarantee serial access, it is critical that all access to the backing bag is accomplished through the returned bag.

      It is imperative that the user manually synchronize on the returned bag when iterating over it:

           SortedBag bag = BagUtils.synchronizedSortedBag(new TreeBag());
           ...
           synchronized(bag) {
               Iterator i = bag.iterator(); // Must be in synchronized block
               while (i.hasNext())
                   foo(i.next());
               }
           }

      Failure to follow this advice may result in non-deterministic behavior.
      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag to synchronize, must not be null
      Returns:
        a synchronized bag backed by that bag
      Throws:
        `NullPointerException` - if the SortedBag is null

    - #### unmodifiableSortedBag

          public static <E> SortedBag<E> unmodifiableSortedBag(SortedBag<E> bag)

      Returns an unmodifiable view of the given sorted bag. Any modification attempts to the returned bag will raise an `UnsupportedOperationException`.
      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag whose unmodifiable view is to be returned, must not be null
      Returns:
        an unmodifiable view of that bag
      Throws:
        `NullPointerException` - if the SortedBag is null

    - #### predicatedSortedBag

          public static <E> SortedBag<E> predicatedSortedBag(SortedBag<E> bag,
                                             Predicate<? super E> predicate)

      Returns a predicated (validating) sorted bag backed by the given sorted bag.

      Only objects that pass the test in the given predicate can be added to the bag. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original bag after invoking this method, as it is a backdoor for adding invalid objects.

      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the sorted bag to predicate, must not be null
        `predicate` - the predicate for the bag, must not be null
      Returns:
        a predicated bag backed by the given bag
      Throws:
        `NullPointerException` - if the SortedBag or Predicate is null

    - #### transformingSortedBag

          public static <E> SortedBag<E> transformingSortedBag(SortedBag<E> bag,
                                               Transformer<? super E,? extends E> transformer)

      Returns a transformed sorted bag backed by the given bag.

      Each object is passed through the transformer as it is added to the Bag. It is important not to use the original bag after invoking this method, as it is a backdoor for adding untransformed objects.

      Existing entries in the specified bag will not be transformed. If you want that behaviour, see `TransformedSortedBag.transformedSortedBag(SortedBag, Transformer)`.

      Type Parameters:
        `E` - the element type
      Parameters:
        `bag` - the bag to predicate, must not be null
        `transformer` - the transformer for the bag, must not be null
      Returns:
        a transformed bag backed by the given bag
      Throws:
        `NullPointerException` - if the Bag or Transformer is null

    - #### emptyBag

          public static <E> Bag<E> emptyBag()

      Get an empty `Bag`.
      Type Parameters:
        `E` - the element type
      Returns:
        an empty Bag

    - #### emptySortedBag

          public static <E> SortedBag<E> emptySortedBag()

      Get an empty `SortedBag`.
      Type Parameters:
        `E` - the element type
      Returns:
        an empty sorted Bag

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