Package util

Class CollectionUtils

java.lang.Object
util.CollectionUtils
public class CollectionUtils extends Object
A collection of utility methods that prevent you from having to do unsafe casts of Collection classes due to runtime type erasure.

Be sure to check Apache collection utils before using this class, as it's a standard utility and often more efficient.

Some examples:

  1. CollectionUtils
  2. IterableUtils
  3. IteratorUtils
  4. StringUtils.join(Iterable, char) - for pretty printing collections with newlines
  5. Apache CollectionUtils.collect(Collection, Transformer) - to turn a collection in to collection of strings when the default toString() is lacking

  • Method Summary

    Modifier and Type
    Method
    Description
    static <T> T
    any(Iterable<T> iterable)
    Returns an element from the given iterable; null if the iterable is null or empty.
    static <T> T
    any(Collection<T> c)
    Returns an element from the given collection; null if the collection is null or empty.
    static <T> Iterable<T>
    asCancellableIterable(TaskMonitor monitor, Iterable<T>... iterables)
    Combines all collections passed-in into a pass-through (not creating a new collection) Iterable that uses the given task monitor.
    static <T> Collection<T>
    asCollection(Collection<?> collection, Class<T> clazz)
    Checks that the elements in the given collection are of the type specified by clazz and then casts the collection to be of the specified type.
    static <T> Collection<T>
    asCollection(Collection<T> c)
    Returns the given collection if not null, an empty collection otherwise.
    static <T> Iterable<T>
    asIterable(Iterable<T>... iterables)
    Combines all collections passed-in into a pass-through (not creating a new collection) Iterable.
    static <T> Iterable<T>
    asIterable(Iterator<T> iterator)
    Returns an iterable over an iterator
    static <T> Iterable<T>
    asIterable(T t)
    Turns the given item into an iterable
    static <T> List<T>
    asList(Iterable<T> it)
     
    static <T> List<T>
    asList(Collection<T> c)
    A convenient way to check for null and whether the given collection is a List.
    static <T> List<T>
    asList(Enumeration<T> enumeration)
     
    static <T> List<T>
    asList(Iterator<T> it)
     
    static <T> List<T>
    asList(List<?> list, Class<T> clazz)
    Checks that the elements in the given list are of the type specified by clazz and then casts the list to be of the specified type.
    static <T> List<T>
    asList(List<T> list)
    Returns the given list if not null, otherwise returns an empty list.
    static <T> List<T>
    asList(T... items)
    Similar to Arrays.asList(Object...), except that this method will turn a single null parameter into an empty list.
    static <T> Set<T>
    asSet(Iterable<T> iterable)
    Turns the given iterable into a new Set, returning it directly if it is a set, draining it into a set if it is not already.
    static <T> Set<T>
    asSet(Collection<T> c)
     
    static <T> Set<T>
    asSet(Iterator<T> it)
    Drains the given iterator into a new Set
    static <T> Set<T>
    asSet(T... items)
    Turns the given items into a set.
    static <T> Stream<T>
    asStream(Iterable<T>... iterables)
    Combines all iterables passed-in into a pass-through (not creating a new collection) Stream.
    static <T> Stream<T>
    asStream(Iterator<T> iterator)
    Turns the given iterator into a stream
    static boolean
    isAllNull(Object... objects)
    Returns true if all the given objects are null.
    static <T> boolean
    isAllNull(Collection<T> c)
    Returns true if all the given objects are null.
    static <T> boolean
    isAllSameType(Collection<?> list, Class<T> clazz)
    Returns true if each item in the list is of type clazz.
    static <T> boolean
    isBlank(Collection<T> c)
    Returns true if the given array is null or has 0 length
    static <T> boolean
    isBlank(T... t)
    Returns true if the given array is null or has 0 length
    static <T> boolean
    isOneOf(T t, T... possibles)
    Returns true if the given item is in the collection of possible items
    static <T> Collection<T>
    nonNull(Collection<T> c)
    Returns the given collection if not null, an empty collection (a Set) otherwise.

    Methods inherited from class java.lang.Object

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

  • Method Details

    • asSet

      @SafeVarargs public static <T> Set<T> asSet(T... items)
      Turns the given items into a set. If there is only a single item and it is null, then an empty set will be returned.
      Parameters:
      items - the items to put in the set
      Returns:
      the list of items

    • asSet

      public static <T> Set<T> asSet(Collection<T> c)

    • asSet

      public static <T> Set<T> asSet(Iterator<T> it)
      Drains the given iterator into a new Set
      Parameters:
      it - the iterator
      Returns:
      the set

    • asSet

      public static <T> Set<T> asSet(Iterable<T> iterable)
      Turns the given iterable into a new Set, returning it directly if it is a set, draining it into a set if it is not already.
      Parameters:
      iterable - the iterable
      Returns:
      the set

    • asList

      @SafeVarargs public static <T> List<T> asList(T... items)
      Similar to Arrays.asList(Object...), except that this method will turn a single null parameter into an empty list. Also, this method creates a new, mutable array, whereas the former's array is not mutable.
      Parameters:
      items - the items to add to the list
      Returns:
      the list

    • asList

      public static <T> List<T> asList(List<T> list)
      Returns the given list if not null, otherwise returns an empty list. This is useful for clients avoid null checks.
      Parameters:
      list - the list to check
      Returns:
      a non-null collection

    • asList

      public static <T> List<T> asList(Collection<T> c)
      A convenient way to check for null and whether the given collection is a List. If the value is a list, then it is returned. If the value is null, an empty list is returned. Otherwise, a new list is created from the given collection.
      Parameters:
      c - the collection to check
      Returns:
      a list

    • nonNull

      public static <T> Collection<T> nonNull(Collection<T> c)
      Returns the given collection if not null, an empty collection (a Set) otherwise. This is useful for clients avoid null checks.
      Parameters:
      c - the collection to check
      Returns:
      a non-null collection

    • asCollection

      public static <T> Collection<T> asCollection(Collection<T> c)
      Returns the given collection if not null, an empty collection otherwise. This is useful for clients avoid null checks.
      Parameters:
      c - the collection to check
      Returns:
      a non-null collection

    • asList

      public static <T> List<T> asList(Enumeration<T> enumeration)

    • asList

      public static <T> List<T> asList(Iterable<T> it)

    • asList

      public static <T> List<T> asList(Iterator<T> it)

    • asList

      public static <T> List<T> asList(List<?> list, Class<T> clazz)
      Checks that the elements in the given list are of the type specified by clazz and then casts the list to be of the specified type.
      Parameters:
      list - the source list
      clazz - the class of T
      Returns:
      a casted list of type T
      Throws:
      IllegalArgumentException - if the given list contains elements that are not of the type specified by clazz.

    • asCollection

      public static <T> Collection<T> asCollection(Collection<?> collection, Class<T> clazz)
      Checks that the elements in the given collection are of the type specified by clazz and then casts the collection to be of the specified type.
      Parameters:
      collection - the source collection
      clazz - the class of T
      Returns:
      a casted list of type T
      Throws:
      IllegalArgumentException - if the given collection contains elements that are not of the type specified by clazz.

    • isAllSameType

      public static <T> boolean isAllSameType(Collection<?> list, Class<T> clazz)
      Returns true if each item in the list is of type clazz.
      Type Parameters:
      T - the type
      Parameters:
      list - the list to inspect
      clazz - the class type
      Returns:
      true if each item in the list is of type clazz

    • isOneOf

      @SafeVarargs public static <T> boolean isOneOf(T t, T... possibles)
      Returns true if the given item is in the collection of possible items
      Parameters:
      t - the item in question
      possibles - the set of things
      Returns:
      true if the given item is in the collection of possible items

    • isAllNull

      public static boolean isAllNull(Object... objects)
      Returns true if all the given objects are null.

      See also apache ObjectUtils.anyNotNull(Object...) and ObjectUtils.allNotNull(Object...)

      Parameters:
      objects - the objects to check
      Returns:
      true if all the given objects are null

    • isAllNull

      public static <T> boolean isAllNull(Collection<T> c)
      Returns true if all the given objects are null.

      See also apache ObjectUtils.anyNotNull(Object...) and ObjectUtils.allNotNull(Object...)

      Parameters:
      c - the objects to check
      Returns:
      true if all the given objects are null

    • isBlank

      public static <T> boolean isBlank(Collection<T> c)
      Returns true if the given array is null or has 0 length
      Parameters:
      c - the collection to check
      Returns:
      true if blank

    • isBlank

      public static <T> boolean isBlank(T... t)
      Returns true if the given array is null or has 0 length
      Parameters:
      t - the items to check
      Returns:
      true if blank

    • asIterable

      public static <T> Iterable<T> asIterable(T t)
      Turns the given item into an iterable
      Parameters:
      t - the object from which to create an iterable
      Returns:
      an iterable over the given iterator

    • asIterable

      public static <T> Iterable<T> asIterable(Iterator<T> iterator)
      Returns an iterable over an iterator
      Parameters:
      iterator - the iterator to create an iterable from
      Returns:
      an iterable over the given iterator

    • asIterable

      @SafeVarargs public static <T> Iterable<T> asIterable(Iterable<T>... iterables)
      Combines all collections passed-in into a pass-through (not creating a new collection) Iterable.
      Parameters:
      iterables - the iterables to combine
      Returns:
      the iterable

    • asCancellableIterable

      @SafeVarargs public static <T> Iterable<T> asCancellableIterable(TaskMonitor monitor, Iterable<T>... iterables)
      Combines all collections passed-in into a pass-through (not creating a new collection) Iterable that uses the given task monitor.
      Parameters:
      monitor - a task monitor that allows for cancelling iteration
      iterables - the iterables to combine
      Returns:
      the iterable

    • asStream

      public static <T> Stream<T> asStream(Iterator<T> iterator)
      Turns the given iterator into a stream
      Parameters:
      iterator - the iterator
      Returns:
      the stream

    • asStream

      @SafeVarargs public static <T> Stream<T> asStream(Iterable<T>... iterables)
      Combines all iterables passed-in into a pass-through (not creating a new collection) Stream.
      Parameters:
      iterables - the iterables to combine
      Returns:
      the stream

    • any

      public static <T> T any(Collection<T> c)
      Returns an element from the given collection; null if the collection is null or empty. This is meant for clients that have a collection with any number of items and just need to get one.
      Parameters:
      c - the collection
      Returns:
      the item

    • any

      public static <T> T any(Iterable<T> iterable)
      Returns an element from the given iterable; null if the iterable is null or empty. This is meant for clients that have a collection with any number of items and just need to get one.
      Parameters:
      iterable - the items
      Returns:
      the item