org.springframework.util

## Class CollectionUtils

- java.lang.Object
- - org.springframework.util.CollectionUtils

-     public abstract class CollectionUtils
      extends Object

  Miscellaneous collection utility methods. Mainly for internal use within the framework.
  Since:
    1.1.3
  Author:
    Juergen Hoeller, Rob Harrop, Arjen Poutsma

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `CollectionUtils()`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `static List` | `arrayToList(Object source)` Convert the supplied array into a List.
    `static boolean` | `contains(Enumeration enumeration, Object element)` Check whether the given Enumeration contains the given element.
    `static boolean` | `contains(Iterator iterator, Object element)` Check whether the given Iterator contains the given element.
    `static boolean` | `containsAny(Collection source, Collection candidates)` Return `true` if any element in '`candidates`' is contained in '`source`'; otherwise returns `false`.
    `static boolean` | `containsInstance(Collection collection, Object element)` Check whether the given Collection contains the given element instance.
    `static Class<?>` | `findCommonElementType(Collection collection)` Find the common element type of the given Collection, if any.
    `static Object` | `findFirstMatch(Collection source, Collection candidates)` Return the first element in '`candidates`' that is contained in '`source`'.
    `static Object` | `findValueOfType(Collection<?> collection, Class<?>[] types)` Find a single value of one of the given types in the given Collection: searching the Collection for a value of the first type, then searching for a value of the second type, etc.
    `static <T> T` | `findValueOfType(Collection<?> collection, Class<T> type)` Find a single value of the given type in the given Collection.
    `static boolean` | `hasUniqueObject(Collection collection)` Determine whether the given Collection only contains a single unique object.
    `static boolean` | `isEmpty(Collection collection)` Return `true` if the supplied Collection is `null` or empty.
    `static boolean` | `isEmpty(Map map)` Return `true` if the supplied Map is `null` or empty.
    `static void` | `mergeArrayIntoCollection(Object array, Collection collection)` Merge the given array into the given Collection.
    `static void` | `mergePropertiesIntoMap(Properties props, Map map)` Merge the given Properties instance into the given Map, copying all properties (key-value pairs) over.
    `static <A,E extends A> A[]` | `toArray(Enumeration<E> enumeration, A[] array)` Marshal the elements from the given enumeration into an array of the given type.
    `static <E> Iterator<E>` | `toIterator(Enumeration<E> enumeration)` Adapt an enumeration to an iterator.
    `static <K,V> MultiValueMap<K,V>` | `toMultiValueMap(Map<K,List<V>> map)` Adapts a `Map<K, List<V>>` to an `MultiValueMap<K,V>`.
    `static <K,V> MultiValueMap<K,V>` | `unmodifiableMultiValueMap(MultiValueMap<? extends K,? extends V> map)` Returns an unmodifiable view of the specified multi-value map.

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

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

- - ### Constructor Detail

    - #### CollectionUtils

          public CollectionUtils()

  - ### Method Detail

    - #### isEmpty

          public static boolean isEmpty(Collection collection)

      Return `true` if the supplied Collection is `null` or empty. Otherwise, return `false`.
      Parameters:
        `collection` - the Collection to check
      Returns:
        whether the given Collection is empty

    - #### isEmpty

          public static boolean isEmpty(Map map)

      Return `true` if the supplied Map is `null` or empty. Otherwise, return `false`.
      Parameters:
        `map` - the Map to check
      Returns:
        whether the given Map is empty

    - #### arrayToList

          public static List arrayToList(Object source)

      Convert the supplied array into a List. A primitive array gets converted into a List of the appropriate wrapper type.

      A `null` source value will be converted to an empty List.

      Parameters:
        `source` - the (potentially primitive) array
      Returns:
        the converted List result
      See Also:
        `ObjectUtils.toObjectArray(Object)`

    - #### mergeArrayIntoCollection

          public static void mergeArrayIntoCollection(Object array,
                                      Collection collection)

      Merge the given array into the given Collection.
      Parameters:
        `array` - the array to merge (may be `null`)
        `collection` - the target Collection to merge the array into

    - #### mergePropertiesIntoMap

          public static void mergePropertiesIntoMap(Properties props,
                                    Map map)

      Merge the given Properties instance into the given Map, copying all properties (key-value pairs) over.

      Uses `Properties.propertyNames()` to even catch default properties linked into the original Properties instance.

      Parameters:
        `props` - the Properties instance to merge (may be `null`)
        `map` - the target Map to merge the properties into

    - #### contains

          public static boolean contains(Iterator iterator,
                         Object element)

      Check whether the given Iterator contains the given element.
      Parameters:
        `iterator` - the Iterator to check
        `element` - the element to look for
      Returns:
        `true` if found, `false` else

    - #### contains

          public static boolean contains(Enumeration enumeration,
                         Object element)

      Check whether the given Enumeration contains the given element.
      Parameters:
        `enumeration` - the Enumeration to check
        `element` - the element to look for
      Returns:
        `true` if found, `false` else

    - #### containsInstance

          public static boolean containsInstance(Collection collection,
                                 Object element)

      Check whether the given Collection contains the given element instance.

      Enforces the given instance to be present, rather than returning `true` for an equal element as well.

      Parameters:
        `collection` - the Collection to check
        `element` - the element to look for
      Returns:
        `true` if found, `false` else

    - #### containsAny

          public static boolean containsAny(Collection source,
                            Collection candidates)

      Return `true` if any element in '`candidates`' is contained in '`source`'; otherwise returns `false`.
      Parameters:
        `source` - the source Collection
        `candidates` - the candidates to search for
      Returns:
        whether any of the candidates has been found

    - #### findFirstMatch

          public static Object findFirstMatch(Collection source,
                              Collection candidates)

      Return the first element in '`candidates`' that is contained in '`source`'. If no element in '`candidates`' is present in '`source`' returns `null`. Iteration order is `Collection` implementation specific.
      Parameters:
        `source` - the source Collection
        `candidates` - the candidates to search for
      Returns:
        the first present object, or `null` if not found

    - #### findValueOfType

          public static <T> T findValueOfType(Collection<?> collection,
                              Class<T> type)

      Find a single value of the given type in the given Collection.
      Parameters:
        `collection` - the Collection to search
        `type` - the type to look for
      Returns:
        a value of the given type found if there is a clear match, or `null` if none or more than one such value found

    - #### findValueOfType

          public static Object findValueOfType(Collection<?> collection,
                               Class<?>[] types)

      Find a single value of one of the given types in the given Collection: searching the Collection for a value of the first type, then searching for a value of the second type, etc.
      Parameters:
        `collection` - the collection to search
        `types` - the types to look for, in prioritized order
      Returns:
        a value of one of the given types found if there is a clear match, or `null` if none or more than one such value found

    - #### hasUniqueObject

          public static boolean hasUniqueObject(Collection collection)

      Determine whether the given Collection only contains a single unique object.
      Parameters:
        `collection` - the Collection to check
      Returns:
        `true` if the collection contains a single reference or multiple references to the same instance, `false` else

    - #### findCommonElementType

          public static Class<?> findCommonElementType(Collection collection)

      Find the common element type of the given Collection, if any.
      Parameters:
        `collection` - the Collection to check
      Returns:
        the common element type, or `null` if no clear common type has been found (or the collection was empty)

    - #### toArray

          public static <A,E extends A> A[] toArray(Enumeration<E> enumeration,
                                    A[] array)

      Marshal the elements from the given enumeration into an array of the given type. Enumeration elements must be assignable to the type of the given array. The array returned will be a different instance than the array given.

    - #### toIterator

          public static <E> Iterator<E> toIterator(Enumeration<E> enumeration)

      Adapt an enumeration to an iterator.
      Parameters:
        `enumeration` - the enumeration
      Returns:
        the iterator

    - #### toMultiValueMap

          public static <K,V> MultiValueMap<K,V> toMultiValueMap(Map<K,List<V>> map)

      Adapts a `Map<K, List<V>>` to an `MultiValueMap<K,V>`.
      Parameters:
        `map` - the map
      Returns:
        the multi-value map

    - #### unmodifiableMultiValueMap

          public static <K,V> MultiValueMap<K,V> unmodifiableMultiValueMap(MultiValueMap<? extends K,? extends V> map)

      Returns an unmodifiable view of the specified multi-value map.
      Parameters:
        `map` - the map for which an unmodifiable view is to be returned.
      Returns:
        an unmodifiable view of the specified multi-value map.