Java 8 Streams Collect Examples
Collecting to a list
package xyz.byexample.java8;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Collect {
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
System.out.println(numbers.stream().collect(Collectors.toList()));
}
}
Output
[1, 2, 3, 4, 5]
Collecting Average of Integers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
System.out.println( numbers.stream().collect(Collectors.averagingInt(Integer::intValue)));
Output
3.0
Collecting Sum of Integers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
System.out.println(numbers.stream().collect(Collectors.summingInt(Integer::intValue)));
Output
15
Collecting Maximum of Integers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
System.out.println(numbers.stream().collect(Collectors.maxBy(Integer::compare)));
Output
Optional[5]
Note that here we’re just using Integer’s compare() function as our
comparator. If you’d like you could define your own Comparator<? super T>
instead.
Another point worth noting is that we get an Optional<Integer> as a response.
If the stream had been empty, we’d have got a result of Optional.empty here
since there is no maximum value if there are no values (for the summing and
average operators, we would get 0 or 0.0 respectively since those are the
sums and averages of an empty list, so no Optional needed there).
Make sure you check out our examples on Optional if you’d
like to see some more examples of Optional in Java 8.
Collecting into two partitions
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
System.out.println(numbers.stream().collect(Collectors.partitioningBy(number -> number < 3)));
System.out.println(letters.stream().collect(Collectors.partitioningBy(letter -> letter.matches("(a|e|i|o|u)"))));
Output
{false=[3, 4, 5], true=[1, 2]}
{false=[b, c, d], true=[a, e]}
Note that this is different from a basic filter() operation - filter() will
return only those elements that match the filter, where partitioningBy will
return two partitions: one containing those elements that do meet the requirement
and one that does not.
Joining Strings
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
System.out.println(letters.stream().collect(Collectors.joining("!")));
Output
a!b!c!d!e