Package org.apache.commons.collections4

Class BagUtils

  • public class BagUtils
extends java.lang.Object
    Provides utility methods and decorators for Bag and SortedBag instances.
    Since:
    2.1

    • 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:
        java.lang.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:
        java.lang.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:
        java.lang.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:
        java.lang.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:
        java.lang.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:
        java.lang.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:
        java.lang.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:
        java.lang.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:
        java.lang.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