Convert Java Stream to List
October 17, 2020
Stream to List. To collect stream into List, we need to use Java Stream.collect that performs a mutable reduction operation on the elements using a Collector.
<R,A> R collect(Collector<? super T,A,R> collector)
List, we need to pass Collector as argument achieved by calling following methods.
1.
Collectors.toList
2.
Collectors.toUnmodifiableList
3.
Collectors.toCollection
Now let us discuss the use of above methods to convert stream into list.
Collectors.toList
TheCollectors.toList returns a Collector that accumulates the input elements into a new List.
public static <T> Collector<T,?,List<T>> toList()
ExampleToList.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;
public class ExampleToList {
public static void main(String[] args) {
List<Integer> list1 = Stream.of(15, 20, 30)
.collect(Collectors.toList());
System.out.println(list1);
List<Integer> list2 = IntStream.range(15, 20).boxed()
.collect(Collectors.toList());
System.out.println(list2);
List<Integer> list3 = Arrays.asList(2, 4, 6).stream().map(i -> i * 2)
.collect(Collectors.toList());
System.out.println(list3);
List<String> list4 = Stream.of("Mahesh", "Krishn", "Shiv")
.collect(Collectors.toList());
System.out.println(list4);
}
}
[15, 20, 30] [15, 16, 17, 18, 19] [4, 8, 12] [Mahesh, Krishn, Shiv]
Collectors.toUnmodifiableList
TheCollectors.toUnmodifiableList returns a Collector that accumulates the input elements into an unmodifiable List in encounter order.
public static <T> Collector<T,?,List<T>> toUnmodifiableList()
ExampleToUnmodifiableList.java
package com.concretepage;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class ExampleToUnmodifiableList {
public static void main(String[] args) {
List<Integer> list1 = Stream.of(10, 15, 20).collect(Collectors.toUnmodifiableList());
System.out.println(list1);
List<Integer> list2 = Stream.of(3, 4, 5).map(i -> i * 2).collect(Collectors.toUnmodifiableList());
System.out.println(list2);
}
}
[10, 15, 20] [6, 8, 10]
Collectors.toCollection
TheCollectors.toCollection returns a Collector that accumulates the input elements into a new Collection, in encounter order.
public static <T,C extends Collection<T>> Collector<T,?,C> toCollection(Supplier<C> collectionFactory)
Stream into ArrayList and LinkedList.
ExampleToCollection.java
package com.concretepage;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class ExampleToCollection {
public static void main(String[] args) {
List<Integer> list1 = Stream.of(10, 15, 20).map(i -> i * 2)
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(list1);
List<String> list2 = Stream.of("Mahesh", "Sohan", "Kishan")
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(list2);
LinkedList<Integer> lnkList = Stream.of(40, 15, 35).sorted()
.collect(Collectors.toCollection(LinkedList::new));
System.out.println(lnkList);
}
}
[20, 30, 40] [Mahesh, Sohan, Kishan] [15, 35, 40]
References
StreamCollectors
