Package 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
    Description
    CollectionUtils()
     

  • Method Summary

    Modifier and Type
    Method
    Description
    static List<?>
    arrayToList(Object source)
    Convert the supplied array into a List.
    static <K, V> Map<K,V>
    compositeMap(Map<K,V> first, Map<K,V> second)
    Return a (partially unmodifiable) map that combines the provided two maps.
    static <K, V> Map<K,V>
    compositeMap(Map<K,V> first, Map<K,V> second, BiFunction<K,V,V> putFunction, Consumer<Map<K,V>> putAllFunction)
    Return a map that combines the provided maps.
    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 <E> E
    findFirstMatch(Collection<?> source, Collection<E> 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 <T> T
    firstElement(List<T> list)
    Retrieve the first element of the given List, accessing the zero index.
    static <T> T
    firstElement(Set<T> set)
    Retrieve the first element of the given Set, using SortedSet.first() or otherwise using the iterator.
    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 <T> T
    lastElement(List<T> list)
    Retrieve the last element of the given List, accessing the highest index.
    static <T> T
    lastElement(Set<T> set)
    Retrieve the last element of the given Set, using SortedSet.last() or otherwise iterating over all elements (assuming a linked set).
    static <E> void
    mergeArrayIntoCollection(Object array, Collection<E> collection)
    Merge the given array into the given Collection.
    static <K, V> void
    mergePropertiesIntoMap(Properties props, Map<K,V> map)
    Merge the given Properties instance into the given Map, copying all properties (key-value pairs) over.
    static <K, V> HashMap<K,V>
    newHashMap(int expectedSize)
    Instantiate a new HashMap with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.
    static <E> HashSet<E>
    newHashSet(int expectedSize)
    Instantiate a new HashSet with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.
    static <K, V> LinkedHashMap<K,V>
    newLinkedHashMap(int expectedSize)
    Instantiate a new LinkedHashMap with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.
    static <E> LinkedHashSet<E>
    newLinkedHashSet(int expectedSize)
    Instantiate a new LinkedHashSet with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.
    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>> targetMap)
    Adapt a Map<K, List<V>> to an MultiValueMap<K, V>.
    static <K, V> MultiValueMap<K,V>
    unmodifiableMultiValueMap(MultiValueMap<? extends K,? extends V> targetMap)
    Return 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 Details

    • CollectionUtils

      public CollectionUtils()

  • Method Details

    • isEmpty

      @Contract("null -> true") public static boolean isEmpty(@Nullable 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

      @Contract("null -> true") public static boolean isEmpty(@Nullable 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

    • newHashMap

      public static <K, V> HashMap<K,V> newHashMap(int expectedSize)
      Instantiate a new HashMap with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.

      This differs from the regular HashMap constructor which takes an initial capacity relative to a load factor but is effectively aligned with the JDK's ConcurrentHashMap(int).

      Parameters:
      expectedSize - the expected number of elements (with a corresponding capacity to be derived so that no resize/rehash operations are needed)
      Since:
      5.3
      See Also:

    • newLinkedHashMap

      public static <K, V> LinkedHashMap<K,V> newLinkedHashMap(int expectedSize)
      Instantiate a new LinkedHashMap with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.

      This differs from the regular LinkedHashMap constructor which takes an initial capacity relative to a load factor but is aligned with Spring's own LinkedCaseInsensitiveMap and LinkedMultiValueMap constructor semantics as of 5.3.

      Parameters:
      expectedSize - the expected number of elements (with a corresponding capacity to be derived so that no resize/rehash operations are needed)
      Since:
      5.3
      See Also:

    • newHashSet

      public static <E> HashSet<E> newHashSet(int expectedSize)
      Instantiate a new HashSet with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.
      Parameters:
      expectedSize - the expected number of elements (with a corresponding capacity to be derived so that no resize/rehash operations are needed)
      Since:
      6.2
      See Also:

    • newLinkedHashSet

      public static <E> LinkedHashSet<E> newLinkedHashSet(int expectedSize)
      Instantiate a new LinkedHashSet with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected.
      Parameters:
      expectedSize - the expected number of elements (with a corresponding capacity to be derived so that no resize/rehash operations are needed)
      Since:
      6.2
      See Also:

    • arrayToList

      public static List<?> arrayToList(@Nullable Object source)
      Convert the supplied array into a List. A primitive array gets converted into a List of the appropriate wrapper type.

      NOTE: Generally prefer the standard Arrays.asList(T...) method. This arrayToList method is just meant to deal with an incoming Object value that might be an Object[] or a primitive array at runtime.

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

      Parameters:
      source - the (potentially primitive) array
      Returns:
      the converted List result
      See Also:

    • mergeArrayIntoCollection

      public static <E> void mergeArrayIntoCollection(@Nullable Object array, Collection<E> 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 <K, V> void mergePropertiesIntoMap(@Nullable Properties props, Map<K,V> 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(@Nullable 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 otherwise

    • contains

      public static boolean contains(@Nullable 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 otherwise

    • containsInstance

      public static boolean containsInstance(@Nullable 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 otherwise

    • 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

      @Nullable public static <E> E findFirstMatch(Collection<?> source, Collection<E> 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

      @Nullable public static <T> T findValueOfType(Collection<?> collection, @Nullable 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

      @Nullable 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 otherwise

    • findCommonElementType

      @Nullable 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)

    • firstElement

      @Nullable public static <T> T firstElement(@Nullable Set<T> set)
      Retrieve the first element of the given Set, using SortedSet.first() or otherwise using the iterator.
      Parameters:
      set - the Set to check (may be null or empty)
      Returns:
      the first element, or null if none
      Since:
      5.2.3
      See Also:

    • firstElement

      @Nullable public static <T> T firstElement(@Nullable List<T> list)
      Retrieve the first element of the given List, accessing the zero index.
      Parameters:
      list - the List to check (may be null or empty)
      Returns:
      the first element, or null if none
      Since:
      5.2.3

    • lastElement

      @Nullable public static <T> T lastElement(@Nullable Set<T> set)
      Retrieve the last element of the given Set, using SortedSet.last() or otherwise iterating over all elements (assuming a linked set).
      Parameters:
      set - the Set to check (may be null or empty)
      Returns:
      the last element, or null if none
      Since:
      5.0.3
      See Also:

    • lastElement

      @Nullable public static <T> T lastElement(@Nullable List<T> list)
      Retrieve the last element of the given List, accessing the highest index.
      Parameters:
      list - the List to check (may be null or empty)
      Returns:
      the last element, or null if none
      Since:
      5.0.3

    • 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(@Nullable Enumeration<E> enumeration)
      Adapt an Enumeration to an Iterator.
      Parameters:
      enumeration - the original Enumeration
      Returns:
      the adapted Iterator

    • toMultiValueMap

      public static <K, V> MultiValueMap<K,V> toMultiValueMap(Map<K,List<V>> targetMap)
      Adapt a Map<K, List<V>> to an MultiValueMap<K, V>.
      Parameters:
      targetMap - the original map
      Returns:
      the adapted multi-value map (wrapping the original map)
      Since:
      3.1

    • unmodifiableMultiValueMap

      public static <K, V> MultiValueMap<K,V> unmodifiableMultiValueMap(MultiValueMap<? extends K,? extends V> targetMap)
      Return an unmodifiable view of the specified multi-value map.
      Parameters:
      targetMap - the map for which an unmodifiable view is to be returned.
      Returns:
      an unmodifiable view of the specified multi-value map
      Since:
      3.1

    • compositeMap

      public static <K, V> Map<K,V> compositeMap(Map<K,V> first, Map<K,V> second)
      Return a (partially unmodifiable) map that combines the provided two maps. Invoking Map.put(Object, Object) or Map.putAll(Map) on the returned map results in an UnsupportedOperationException.

      In the case of a key collision, first takes precedence over second. In other words, entries in second with a key that is also mapped by first are effectively ignored.

      Parameters:
      first - the first map to compose
      second - the second map to compose
      Returns:
      a new map that composes the given two maps
      Since:
      6.2

    • compositeMap

      public static <K, V> Map<K,V> compositeMap(Map<K,V> first, Map<K,V> second, @Nullable BiFunction<K,V,V> putFunction, @Nullable Consumer<Map<K,V>> putAllFunction)
      Return a map that combines the provided maps. Invoking Map.put(Object, Object) on the returned map will apply putFunction, or will throw an UnsupportedOperationException putFunction is null. The same applies to Map.putAll(Map) and putAllFunction.

      In the case of a key collision, first takes precedence over second. In other words, entries in second with a key that is also mapped by first are effectively ignored.

      Parameters:
      first - the first map to compose
      second - the second map to compose
      putFunction - applied when Map::put is invoked. If null, Map::put throws an UnsupportedOperationException.
      putAllFunction - applied when Map::putAll is invoked. If null, Map::putAll throws an UnsupportedOperationException.
      Returns:
      a new map that composes the give maps
      Since:
      6.2