Streams - Beginning Java 8 Language Features - page 612

The following snippet of code obtains a stream of strings by splitting a string using a regular expression (“ , ”).
The matched strings are printed on the standard output.

String str = "Ken,Jeff,Lee";
Pattern.compile(",")
.splitAsStream(str)
.forEach(System.out::println);

Ken
Jeff
Lee

Representing an Optional Value
In Java, null is used to represent “nothing” or an “empty” result. Most often, a method returns null if it does not have
a result to return. This has been a source of frequent NullPointerException in Java programs. Consider printing the
year of birth of a person, like so:

Person ken = new Person(1, "Ken", Person.Gender.MALE, null, 6000.0);
int year = ken.getDob().getYear(); // Throws a NullPointerException
System.out.println("Ken was born in the year " + year);

The code throws a NullPointerException at runtime. The problem is in the return value of the ken.getDob()
method that returns null . Calling the getYear() method on a null reference results in the NullPointerException .
So, what is the solution? In fact, there is no real solution to this. Java 8 has introduced an Optional<T> class in the
java.util package to deal with NullPointerException gracefully. Methods that may return nothing should return an
Optional instead of null .
An Optional is a wrapper for a non-null value that may or may not contain a non-null value. Its isPresent()
method returns true if it contains a non-null value, false otherwise. Its get() method returns the non-null value if
it contains a non-null value, and throws a NoSuchElementException otherwise. This implies that when a method
returns an Optional , you must, as a practice, check if it contains a non-null value before asking it for the value. If
you use the get() method before making sure it contains a non-null value, you may get a NoSuchElementException
instead of getting a NullPointerException . This is why I said in the previous paragraph that there is no real solution
to the NullPointerException . However, returning an Optional is certainly a better way to deal with nulls as
developers will get used to using the Optional objects in the way they are designed to be used.
How do you create an Optional<T> object? The Optional<T> class provides three static factory methods to create
its objects.

<T> Optional<T> empty() : Returns an empty Optional . That is, the Optional<T> returned
from this method does not contain a non-null value.

•

<T> Optional<T> of(T value) : Returns an Optional containing the specified value as the
non-null value. If the specified value is null , it throws a NullPointerException .

•

<T> Optional<T> ofNullable(T value) : Returns an Optional containing the specified value
if the value is non-null. If the specified value is null , it returns an empty Optional .
The following snippet of code shows how to create Optional objects:

•

// Create an empty Optional
Optional<String> empty = Optional.empty();