Package org.apache.commons.collections4

Class MultiSetUtils

  • public class MultiSetUtils
extends Object
    Provides utility methods and decorators for MultiSet instances.
    Since:
    4.1

    • Field Detail

      • EMPTY_MULTISET

        public static final MultiSet EMPTY_MULTISET
        An empty unmodifiable multiset.

    • Method Detail

      • synchronizedMultiSet

        public static <E> MultiSet<E> synchronizedMultiSet​(MultiSet<E> multiset)
        Returns a synchronized (thread-safe) multiset backed by the given multiset. In order to guarantee serial access, it is critical that all access to the backing multiset is accomplished through the returned multiset.

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

         MultiSet multiset = MultiSetUtils.synchronizedMultiSet(new HashMultiSet());
 ...
 synchronized(multiset) {
     Iterator i = multiset.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:
        multiset - the multiset to synchronize, must not be null
        Returns:
        a synchronized multiset backed by that multiset
        Throws:
        NullPointerException - if the MultiSet is null

      • unmodifiableMultiSet

        public static <E> MultiSet<E> unmodifiableMultiSet​(MultiSet<? extends E> multiset)
        Returns an unmodifiable view of the given multiset. Any modification attempts to the returned multiset will raise an UnsupportedOperationException.
        Type Parameters:
        E - the element type
        Parameters:
        multiset - the multiset whose unmodifiable view is to be returned, must not be null
        Returns:
        an unmodifiable view of that multiset
        Throws:
        NullPointerException - if the MultiSet is null

      • predicatedMultiSet

        public static <E> MultiSet<E> predicatedMultiSet​(MultiSet<E> multiset,
                                                 Predicate<? super E> predicate)
        Returns a predicated (validating) multiset backed by the given multiset.

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

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

      • emptyMultiSet

        public static <E> MultiSet<E> emptyMultiSet()
        Get an empty MultiSet.
        Type Parameters:
        E - the element type
        Returns:
        an empty MultiSet