de.larssh.utils

Class Collectors

java.lang.Object

de.larssh.utils.Collectors

public final class Collectors
extends Object

This class contains helper methods for Collector.

Method Summary

Modifier and Type	Method and Description
static <T> BinaryOperator<T>	throwingMerger() Returns a merge function, suitable for use in Map.merge() or Collectors.toMap(Function, Function, BinaryOperator), which always throws IllegalStateException.
static <K,V> Collector<Map.Entry<K,V>,?,Map<K,V>>	toLinkedHashMap() Returns a Collector that accumulates Map.Entry elements into a LinkedHashMap.
static <T,K,U> Collector<T,?,Map<K,U>>	toLinkedHashMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper) Returns a Collector that accumulates elements into a LinkedHashMap whose keys and values are the result of applying the provided mapping functions to the input elements.
static <T,K,U> Collector<T,?,Map<K,U>>	toLinkedHashMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction) Returns a Collector that accumulates elements into a LinkedHashMap whose keys and values are the result of applying the provided mapping functions to the input elements.
static <T> Collector<T,?,Set<T>>	toLinkedHashSet() Returns a Collector that accumulates the input elements into a new LinkedHashSet, in encounter order.
static <T,K,U> Collector<T,?,Map<K,U>>	toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper) Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
static <T,K,U> Collector<T,?,Map<K,U>>	toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction) Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
static <T,K,U,M extends Map<K,U>> Collector<T,?,M>	toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier) Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
static <T,K,U,M extends Map<K,U>> Collector<T,?,M>	toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, Supplier<M> mapSupplier) Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
static <K,V,M extends Map<K,V>> Collector<Map.Entry<K,V>,?,M>	toMap(Supplier<M> mapSupplier) Returns a Collector that accumulates Map.Entry elements into a Map.

Methods inherited from class java.lang.Object

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

Method Detail

throwingMerger

public static <T> BinaryOperator<T> throwingMerger()

Returns a merge function, suitable for use in Map.merge() or Collectors.toMap(Function, Function, BinaryOperator), which always throws IllegalStateException. This can be used to enforce the assumption that the elements being collected are distinct.

Type Parameters:

T - the type of input arguments to the merge function

Returns:

a merge function which always throw IllegalStateException

toLinkedHashMap

public static <K,V> Collector<Map.Entry<K,V>,?,Map<K,V>> toLinkedHashMap()

Returns a Collector that accumulates Map.Entry elements into a LinkedHashMap.

Type Parameters:

K - the type of the key

V - the type of the value

Returns:

a Collector which collects Map.Entry elements into a LinkedHashMap

toLinkedHashMap

public static <T,K,U> Collector<T,?,Map<K,U>> toLinkedHashMap(Function<? super T,? extends K> keyMapper,
                                                              Function<? super T,? extends U> valueMapper)

Returns a Collector that accumulates elements into a LinkedHashMap whose keys and values are the result of applying the provided mapping functions to the input elements.

Type Parameters:

T - the type of the input elements

K - the output type of the key mapping function

U - the output type of the value mapping function

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

Returns:

a Collector which collects elements into a LinkedHashMap whose keys and values are the result of applying mapping functions to the input elements

toLinkedHashMap

public static <T,K,U> Collector<T,?,Map<K,U>> toLinkedHashMap(Function<? super T,? extends K> keyMapper,
                                                              Function<? super T,? extends U> valueMapper,
                                                              BinaryOperator<U> mergeFunction)

Returns a Collector that accumulates elements into a LinkedHashMap whose keys and values are the result of applying the provided mapping functions to the input elements.

Type Parameters:

T - the type of the input elements

K - the output type of the key mapping function

U - the output type of the value mapping function

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, java.util.function.BiFunction)

Returns:

a Collector which collects elements into a LinkedHashMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

toLinkedHashSet

public static <T> Collector<T,?,Set<T>> toLinkedHashSet()

Returns a Collector that accumulates the input elements into a new LinkedHashSet, in encounter order.

Type Parameters:

T - the type of the input elements

Returns:

a Collector which collects all the input elements into a LinkedHashSet

toMap

public static <K,V,M extends Map<K,V>> Collector<Map.Entry<K,V>,?,M> toMap(Supplier<M> mapSupplier)

Returns a Collector that accumulates Map.Entry elements into a Map.

Type Parameters:

K - the type of the key

V - the type of the value

M - the type of the resulting Map

Parameters:

mapSupplier - a function which returns a new, empty Map into which the results will be inserted

Returns:

a Collector which collects Map.Entry elements into a Map created by mapSupplier

toMap

public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper,
                                                    Function<? super T,? extends U> valueMapper)

Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

If the mapped keys contains duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

Type Parameters:

T - the type of the input elements

K - the output type of the key mapping function

U - the output type of the value mapping function

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

Returns:

a Collector which collects elements into a Map whose keys and values are the result of applying mapping functions to the input elements

toMap

public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper,
                                                    Function<? super T,? extends U> valueMapper,
                                                    BinaryOperator<U> mergeFunction)

Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

If the mapped keys contains duplicates (according to Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.

Type Parameters:

T - the type of the input elements

K - the output type of the key mapping function

U - the output type of the value mapping function

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, java.util.function.BiFunction)

Returns:

a Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

toMap

public static <T,K,U,M extends Map<K,U>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper,
                                                                Function<? super T,? extends U> valueMapper,
                                                                Supplier<M> mapSupplier)

Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

If the mapped keys contains duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

Type Parameters:

T - the type of the input elements

K - the output type of the key mapping function

U - the output type of the value mapping function

M - the type of the resulting Map

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mapSupplier - a function which returns a new, empty Map into which the results will be inserted

Returns:

a Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

toMap

public static <T,K,U,M extends Map<K,U>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper,
                                                                Function<? super T,? extends U> valueMapper,
                                                                BinaryOperator<U> mergeFunction,
                                                                Supplier<M> mapSupplier)

Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

If the mapped keys contains duplicates (according to Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. The Map is created by a provided supplier function.

Type Parameters:

T - the type of the input elements

K - the output type of the key mapping function

U - the output type of the value mapping function

M - the type of the resulting Map

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, java.util.function.BiFunction)

mapSupplier - a function which returns a new, empty Map into which the results will be inserted

Returns:

a Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function