Modern Java in Action 5 - Optional

Optional

Kamila Chyla

2022-06-20 266 words 2 minutes

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Contents

Null problem

Reasons why null is problematic:

source of coding errors (missing checks)
does not correctly model absence of value (null has no meaning)
breaks Java philosophy of hiding pointers (null is a pointer which is NOT hidden)

Handling in other languages

Groovy has safe navigation operator
Kotlin is known for its null-safety
Haskell has Maybe typeclass
Scala also has an Option class

Optional

Creating

Three static methods may create instances of Optional:

Optional.empty() - creates empty instance
Optional.of(T) - returns Optional with given non-null value
Optional.ofNullable(T) - returns either Optional with T value or Optional.empty() if value is null

java.util.Optional in Java 17 - compared to Java 8 - gains the following:

Version	Method	Desc
9	ifPresentOrElse	if present, gives value to `Consumetr<T>`, else tuns given `Runnable`
9	or	returns Optional that will use alternative Optional that will be used if this one is empty
9	stream()	if value is present, returns a sequential `Stream` with single value or empty `Stream`
10	orElseThrow	if present, returns value, else, throws Exception provided by `Supplier<? extends Exception>`
11	isEmpty	returns `true` if value is present

A chain of Optionals

In order to be able to navigate through a series of Optional values one needs to remember the difference between map and flatmap:

if a mapping function returns value, you want to use map
if a mapping function returns Optional, you want to use flatmap

Here, person’s car and car’s insurance are modelled as Optional values, hence the use of flatmap:

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public String getCarInsuranceName(Optional<Person> person) {
    return person.flatMap(Person::getCar)
                 .flatMap(Car::getInsurance)
                 .map(Insurance::getName)
                 .orElse("Unknown");              
}

Ten wpis jest częścią serii modern-java-in-action.

Wszystkie wpisy w tej serii:

2022-05-07 - Modern Java in Action 8 - concurrency and reactive programming
2022-02-07 - Modern Java in Action 7 - notes about the module system
2022-30-06 - Modern Java in Action 6 - Time and Date
2022-20-06 - Modern Java in Action 5 - Optional
2022-17-06 - Modern Java in Action 4 - refactoring and testing
2022-15-06 - Modern Java in Action 3 - collection API
2022-14-06 - Modern Java in Action 2 - fork-join and spliterators
2022-03-06 - Modern Java in Action 1 - Java 8 refresher