This tutorial is going to cover about new methods of the Optional<T> class in Java 9. We’re going to get through 3 methods which recently added, have the syntax as follows:
- void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction)
- Optional<T> or(Supplier<? extends Optional<? extends T>> supplier)
- Stream<T> stream()
For more detail about the Optional<T> class, please visit its Javadocs on Oracle web site.
1. The stream() Method
The stream() method has been added to the Option<T> class from Java 9.When we call the method on an Optional object, if a value is present it returns a sequential Stream containing only that value, otherwise, returns an empty Stream.
Let’s see ist syntax as follows:
Stream<T> stream()
And examples:
@Test
public void testOptionalStream() {
Optional<String> maleOpt = Optional.of("Male");
Optional<String> feMaleOpt = Optional.of("FeMale");
Optional<String> OtherOpt = Optional.empty();
assertEquals("Male",maleOpt.stream().findFirst().get());
assertTrue(maleOpt.stream().count() == 1);
assertTrue(OtherOpt.stream().count() == 0);
}
2. The ifPresentOrElse() Method
Firstly, let’s take a look again at the syntax of the ifPresentOrElse() method:
void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction)
Parameters:
- action: the action to be performed, if a value is present.
- emptyAction: the empty-based action to be performed, if no value is present.
Next, let’s see some examples:
List<Optional<String>> days = List.of(Optional.of("Monday"),
Optional.of("Tuesday"),
Optional.empty(),
Optional.ofNullable(null),
Optional.of("Friday"),
Optional.of("Saturday"),
Optional.of("Sunday")
);
days.stream().forEach(p -> p.ifPresentOrElse(System.out::println,
() -> System.out.println("Day not available")));
Firstly, we define a list of days which contains 7 days, the 3rd, 4th elements are empty or NULL.
And next, we leverage Java Stream API to print out the list. The ifPresentOrElse() provides us two alternatives actions for both cases: data presents and data is NULL or empty. Running the example will give the result as follows:
Monday Tuesday Day not available Day not available Friday Saturday Sunday
3. The or() Method
Let’s take a look at the or(), a new method of the Optional<T> class introduced in Java 9:
Optional<T> or(Supplier<? extends Optional<? extends T>> supplier)
Parameters:
- supplier – the supplying function that produces an Optional to be returned
The method returns an Optional describing the value of this Optional, if a value is present, otherwise an Optional produced by the supplying function.
Let’s see an example:
List<Optional<Integer>> studentAges = List.of(Optional.of(20),
Optional.of(21),
Optional.empty(),
Optional.ofNullable(null),
Optional.of(22),
Optional.of(18),
Optional.of(19)
);
studentAges.stream().map(p -> p.or(() -> Optional.of(20)))
.forEach(System.out::println);
Firstly, we create a list of ages of students in a class with several students whose ages are not clear, is NULL or empty. Next, we will replace all those students (whose ages are not clear) by a default age 20 and then print out.
Running the example will print out the result:
Optional[20] Optional[21] Optional[20] Optional[20] Optional[22] Optional[18] Optional[19]
4. Conclusions
The tutorial has illustrated how to use the new methods of the Optional<T> class in Java 9. Those methods are useful for us in manipulating with the Option<T> class, especially with Stream API.
Below are other Java 9 related tutorials for your references:
Install Oracle Java 9 on CentOS, RHEL 7
Install Oracle Java 9 on Ubuntu 16.04 LTS (Xenial Xerus)
Set Up Eclipse, IntelliJ And NetBeans For Java 9
Create Immutable Lists In Java 9 By Static Factory Methods
Private Interface Methods In Java 9
Using The InputStream.transferTo() To Copy Streams In Java 9
How To Compare Arrays In Java 9