CollectionUtils (Spring Framework 5.3.0 API)

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()

Constructors
Constructor and Description
`CollectionUtils()`

Method Summary

All Methods Static Methods Concrete 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 <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 <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 <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 Detail
  - CollectionUtils
```
public CollectionUtils()
```
- Method Detail
  - isEmpty
```
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
```
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.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(int)
  - 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:
    
    newHashMap(int)
  - 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:
    
    ObjectUtils.toObjectArray(Object), Arrays.asList(Object[])
  - 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:
    
    SortedSet, LinkedHashMap.keySet(), LinkedHashSet
  - 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:
    
    SortedSet, LinkedHashMap.keySet(), LinkedHashSet
  - 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

Class CollectionUtils

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Detail

CollectionUtils

Method Detail

isEmpty

isEmpty

newHashMap

newLinkedHashMap

arrayToList

mergeArrayIntoCollection

mergePropertiesIntoMap

contains

contains

containsInstance

containsAny

findFirstMatch

findValueOfType

findValueOfType

hasUniqueObject

findCommonElementType

firstElement

firstElement

lastElement

lastElement

toArray

toIterator

toMultiValueMap

unmodifiableMultiValueMap