Collectors.GroupingBy Method

Definition

Namespace:

Java.Util.Streams

Assembly:

Mono.Android.dll

Overloads

GroupingBy(IFunction)	Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.
GroupingBy(IFunction, ICollector)	Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
GroupingBy(IFunction, ISupplier, ICollector)	Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

GroupingBy(IFunction)

Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.

[Android.Runtime.Register("groupingBy", "(Ljava/util/function/Function;)Ljava/util/stream/Collector;", "", ApiSince=24)]
[Java.Interop.JavaTypeParameters(new System.String[] { "T", "K" })]
public static Java.Util.Streams.ICollector? GroupingBy (Java.Util.Functions.IFunction? classifier);

[<Android.Runtime.Register("groupingBy", "(Ljava/util/function/Function;)Ljava/util/stream/Collector;", "", ApiSince=24)>]
[<Java.Interop.JavaTypeParameters(new System.String[] { "T", "K" })>]
static member GroupingBy : Java.Util.Functions.IFunction -> Java.Util.Streams.ICollector

Parameters

classifier

IFunction

the classifier function mapping input elements to keys

Returns

ICollector

a Collector implementing the group-by operation

Attributes

RegisterAttribute JavaTypeParametersAttribute

Remarks

Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.

The classification function maps elements to some key type K. The collector produces a Map<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are Lists containing the input elements which map to the associated key under the classification function.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.

Java documentation for java.util.stream.Collectors.groupingBy(java.util.function.Function<? super T, ? extends K>).

Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.

GroupingBy(IFunction, ICollector)

Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

[Android.Runtime.Register("groupingBy", "(Ljava/util/function/Function;Ljava/util/stream/Collector;)Ljava/util/stream/Collector;", "", ApiSince=24)]
[Java.Interop.JavaTypeParameters(new System.String[] { "T", "K", "A", "D" })]
public static Java.Util.Streams.ICollector? GroupingBy (Java.Util.Functions.IFunction? classifier, Java.Util.Streams.ICollector? downstream);

[<Android.Runtime.Register("groupingBy", "(Ljava/util/function/Function;Ljava/util/stream/Collector;)Ljava/util/stream/Collector;", "", ApiSince=24)>]
[<Java.Interop.JavaTypeParameters(new System.String[] { "T", "K", "A", "D" })>]
static member GroupingBy : Java.Util.Functions.IFunction * Java.Util.Streams.ICollector -> Java.Util.Streams.ICollector

Parameters

classifier

IFunction

a classifier function mapping input elements to keys

downstream

ICollector

a Collector implementing the downstream reduction

Returns

ICollector

a Collector implementing the cascaded group-by operation

Attributes

RegisterAttribute JavaTypeParametersAttribute

Remarks

Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

The classification function maps elements to some key type K. The downstream collector operates on elements of type T and produces a result of type D. The resulting collector produces a Map<K, D>.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

For example, to compute the set of last names of people in each city:

{@code
            Map<City, Set<String>> namesByCity
              = people.stream().collect(
                groupingBy(Person::getCity,
                           mapping(Person::getLastName,
                                   toSet())));
            }

Java documentation for java.util.stream.Collectors.groupingBy(java.util.function.Function<? super T, ? extends K>, java.util.stream.Collector<? super T, A, D>).

Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.

GroupingBy(IFunction, ISupplier, ICollector)

Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

[Android.Runtime.Register("groupingBy", "(Ljava/util/function/Function;Ljava/util/function/Supplier;Ljava/util/stream/Collector;)Ljava/util/stream/Collector;", "", ApiSince=24)]
[Java.Interop.JavaTypeParameters(new System.String[] { "T", "K", "D", "A", "M extends java.util.Map<K, D>" })]
public static Java.Util.Streams.ICollector? GroupingBy (Java.Util.Functions.IFunction? classifier, Java.Util.Functions.ISupplier? mapFactory, Java.Util.Streams.ICollector? downstream);

[<Android.Runtime.Register("groupingBy", "(Ljava/util/function/Function;Ljava/util/function/Supplier;Ljava/util/stream/Collector;)Ljava/util/stream/Collector;", "", ApiSince=24)>]
[<Java.Interop.JavaTypeParameters(new System.String[] { "T", "K", "D", "A", "M extends java.util.Map<K, D>" })>]
static member GroupingBy : Java.Util.Functions.IFunction * Java.Util.Functions.ISupplier * Java.Util.Streams.ICollector -> Java.Util.Streams.ICollector

Parameters

classifier

IFunction

a classifier function mapping input elements to keys

mapFactory

ISupplier

a supplier providing a new empty Map into which the results will be inserted

downstream

ICollector

a Collector implementing the downstream reduction

Returns

ICollector

a Collector implementing the cascaded group-by operation

Attributes

RegisterAttribute JavaTypeParametersAttribute

Remarks

Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector. The Map produced by the Collector is created with the supplied factory function.

The classification function maps elements to some key type K. The downstream collector operates on elements of type T and produces a result of type D. The resulting collector produces a Map<K, D>.

For example, to compute the set of last names of people in each city, where the city names are sorted:

{@code
            Map<City, Set<String>> namesByCity
              = people.stream().collect(
                groupingBy(Person::getCity,
                           TreeMap::new,
                           mapping(Person::getLastName,
                                   toSet())));
            }

Java documentation for java.util.stream.Collectors.groupingBy(java.util.function.Function<? super T, ? extends K>, java.util.function.Supplier<M>, java.util.stream.Collector<? super T, A, D>).

Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.