Optional Class was introduced in Java 8 to avoid null checks and NullPointerException issues. In java 9, three new methods are added to improve its functionality.
- stream()
- ifPresentOrElse()
- or()
stream() method
Syntax
public Stream<T> stream()
If a value is present, it returns a sequential Stream containing only that value, otherwise returns an empty Stream.
Example
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Tester {
public static void main(String[] args) {
List<Optional<String>> list = Arrays.asList (
Optional.empty(),
Optional.of("A"),
Optional.empty(),
Optional.of("B"));
//filter the list based to print non-empty values
//if optional is non-empty, get the value in stream, otherwise return empty
List<String> filteredList = list.stream()
.flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
.collect(Collectors.toList());
//Optional::stream method will return a stream of either one
//or zero element if data is present or not.
List<String> filteredListJava9 = list.stream()
.flatMap(Optional::stream)
.collect(Collectors.toList());
System.out.println(filteredList);
System.out.println(filteredListJava9);
}
}
Output
[A, B]
[A, B]
ifPresentOrElse() method
Syntax
public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction)
If a value is present, performs the given action with the value, otherwise performs the given empty-based action.
Example
import java.util.Optional;
public class Tester {
public static void main(String[] args) {
Optional<Integer> optional = Optional.of(1);
optional.ifPresentOrElse( x -> System.out.println("Value: " + x),() ->
System.out.println("Not Present."));
optional = Optional.empty();
optional.ifPresentOrElse( x -> System.out.println("Value: " + x),() ->
System.out.println("Not Present."));
}
}
Output
Value: 1
Not Present.
or() method
Syntax
public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier)
If a value is present, returns an Optional describing the value, otherwise returns an Optional produced by the supplying function.
Example
import java.util.Optional;
import java.util.function.Supplier;
public class Tester {
public static void main(String[] args) {
Optional<String> optional1 = Optional.of("Mahesh");
Supplier<Optional<String>> supplierString = () -> Optional.of("Not Present");
optional1 = optional1.or( supplierString);
optional1.ifPresent( x -> System.out.println("Value: " + x));
optional1 = Optional.empty();
optional1 = optional1.or( supplierString);
optional1.ifPresent( x -> System.out.println("Value: " + x));
}
}
Output
Value: Mahesh
Value: Not Present