1. Información general
En este tutorial, mostraremos la clase opcional que se introdujo en Java 8.
El propósito de la clase es proporcionar una solución a nivel de tipo para representar valores opcionales en lugar de referencias nulas .
Para obtener una comprensión más profunda de por qué debería importarnos la clase opcional , consulte el artículo oficial de Oracle.
2. Creación de objetos opcionales
Hay varias formas de crear objetos opcionales .
Para crear un objeto opcional vacío , simplemente necesitamos usar su método estático vacío () :
@Test public void whenCreatesEmptyOptional_thenCorrect() { Optional empty = Optional.empty(); assertFalse(empty.isPresent()); }Tenga en cuenta que usamos el método isPresent () para verificar si hay un valor dentro del objeto opcional . Un valor está presente solo si hemos creado Opcional con un valor no nulo . Veremos el método isPresent () en la siguiente sección.
También podemos crear un objeto opcional con el método estático de () :
@Test public void givenNonNull_whenCreatesNonNullable_thenCorrect() { String name = "baeldung"; Optional opt = Optional.of(name); assertTrue(opt.isPresent()); }Sin embargo, el argumento pasado al método of () no puede ser nulo. De lo contrario, obtendremos una NullPointerException :
@Test(expected = NullPointerException.class) public void givenNull_whenThrowsErrorOnCreate_thenCorrect() { String name = null; Optional.of(name); }Pero en caso de que esperemos algunos valores nulos , podemos usar el método ofNullable () :
@Test public void givenNonNull_whenCreatesNullable_thenCorrect() { String name = "baeldung"; Optional opt = Optional.ofNullable(name); assertTrue(opt.isPresent()); }Al hacer esto, si pasamos una referencia nula , no lanza una excepción, sino que devuelve un objeto opcional vacío :
@Test public void givenNull_whenCreatesNullable_thenCorrect() { String name = null; Optional opt = Optional.ofNullable(name); assertFalse(opt.isPresent()); }3. Comprobación de la presencia del valor: isPresent () y isEmpty ()
Cuando tenemos un objeto Opcional devuelto por un método o creado por nosotros, podemos verificar si hay un valor en él o no con el método isPresent () :
@Test public void givenOptional_whenIsPresentWorks_thenCorrect() { Optional opt = Optional.of("Baeldung"); assertTrue(opt.isPresent()); opt = Optional.ofNullable(null); assertFalse(opt.isPresent()); }Este método devuelve verdadero si el valor envuelto no es nulo.
Además, a partir de Java 11, podemos hacer lo contrario con el método isEmpty :
@Test public void givenAnEmptyOptional_thenIsEmptyBehavesAsExpected() { Optional opt = Optional.of("Baeldung"); assertFalse(opt.isEmpty()); opt = Optional.ofNullable(null); assertTrue(opt.isEmpty()); }4. Acción condicional con ifPresent ()
El método ifPresent () nos permite ejecutar algún código en el valor envuelto si se encuentra que no es nulo . Antes de Opcional , haríamos:
if(name != null) { System.out.println(name.length()); }Este código verifica si la variable de nombre es nula o no antes de seguir adelante para ejecutar algún código en ella. Este enfoque es largo y ese no es el único problema, también es propenso a errores.
De hecho, ¿qué garantiza que después de imprimir esa variable, no la volveremos a usar y luego nos olvidemos de realizar la verificación nula?
Esto puede resultar en una NullPointerException en tiempo de ejecución si un valor nulo encuentra su camino en ese código. Cuando un programa falla debido a problemas de entrada, a menudo es el resultado de prácticas de programación deficientes.
Opcional nos hace tratar con valores que aceptan valores NULL explícitamente como una forma de hacer cumplir las buenas prácticas de programación.
Veamos ahora cómo se podría refactorizar el código anterior en Java 8.
En el estilo de programación funcional típico, podemos ejecutar realizar una acción en un objeto que está realmente presente:
@Test public void givenOptional_whenIfPresentWorks_thenCorrect() { Optional opt = Optional.of("baeldung"); opt.ifPresent(name -> System.out.println(name.length())); }En el ejemplo anterior, usamos solo dos líneas de código para reemplazar las cinco que funcionaron en el primer ejemplo: una línea para envolver el objeto en un objeto Opcional y la siguiente para realizar una validación implícita y ejecutar el código.
5. Valor predeterminado con orElse ()
El método orElse () se usa para recuperar el valor envuelto dentro de una instancia opcional . Toma un parámetro, que actúa como valor predeterminado. El método orElse () devuelve el valor envuelto si está presente, y su argumento en caso contrario:
@Test public void whenOrElseWorks_thenCorrect() { String nullName = null; String name = Optional.ofNullable(nullName).orElse("john"); assertEquals("john", name); }6. Valor predeterminado con orElseGet ()
El método orElseGet () es similar a orElse () . Sin embargo, en lugar de tomar un valor para devolver si el valor opcional no está presente, toma una interfaz funcional de proveedor, que se invoca y devuelve el valor de la invocación:
@Test public void whenOrElseGetWorks_thenCorrect() { String nullName = null; String name = Optional.ofNullable(nullName).orElseGet(() -> "john"); assertEquals("john", name); }7. Diferencia entre orElse y orElseGet ()
Para muchos programadores que son nuevos en Optional o Java 8, la diferencia entre orElse () y orElseGet () no está clara. De hecho, estos dos métodos dan la impresión de que se superponen en funcionalidad.
Sin embargo, hay una diferencia sutil pero muy importante entre los dos que puede afectar el rendimiento de nuestro código drásticamente si no se comprende bien.
Creemos un método llamado getMyDefault () en la clase de prueba, que no toma argumentos y devuelve un valor predeterminado:
public String getMyDefault() { System.out.println("Getting Default Value"); return "Default Value"; }Veamos dos pruebas y observemos sus efectos secundarios para establecer dónde se superponen orElse () y orElseGet () y dónde difieren:
@Test public void whenOrElseGetAndOrElseOverlap_thenCorrect() { String text = null; String defaultText = Optional.ofNullable(text).orElseGet(this::getMyDefault); assertEquals("Default Value", defaultText); defaultText = Optional.ofNullable(text).orElse(getMyDefault()); assertEquals("Default Value", defaultText); }En el ejemplo anterior, envolvemos un texto nulo dentro de un objeto opcional e intentamos obtener el valor envuelto utilizando cada uno de los dos enfoques.
El efecto secundario es:
Getting default value... Getting default value...En cada caso, se llama al método getMyDefault () . Sucede que cuando el valor envuelto no está presente, tanto orElse () como orElseGet () funcionan exactamente de la misma manera.
Ahora ejecutemos otra prueba donde el valor está presente, e idealmente, el valor predeterminado ni siquiera debería crearse:
@Test public void whenOrElseGetAndOrElseDiffer_thenCorrect() { String text = "Text present"; System.out.println("Using orElseGet:"); String defaultText = Optional.ofNullable(text).orElseGet(this::getMyDefault); assertEquals("Text present", defaultText); System.out.println("Using orElse:"); defaultText = Optional.ofNullable(text).orElse(getMyDefault()); assertEquals("Text present", defaultText); }En el ejemplo anterior, ya no estamos ajustando un valor nulo y el resto del código sigue siendo el mismo.
Ahora echemos un vistazo al efecto secundario de ejecutar este código:
Using orElseGet: Using orElse: Getting default value...Tenga en cuenta que cuando se usa orElseGet () para recuperar el valor envuelto, el método getMyDefault () ni siquiera se invoca ya que el valor contenido está presente.
Sin embargo, cuando se usa orElse () , ya sea que el valor envuelto esté presente o no, se crea el objeto predeterminado. Entonces, en este caso, acabamos de crear un objeto redundante que nunca se usa.
En este ejemplo simple, no hay un costo significativo para crear un objeto predeterminado, ya que la JVM sabe cómo manejarlo. Sin embargo, cuando un método como getMyDefault () tiene que realizar una llamada a un servicio web o incluso consultar una base de datos, el costo se vuelve muy obvio.
8. Excepciones con orElseThrow ()
The orElseThrow() method follows from orElse() and orElseGet() and adds a new approach for handling an absent value.
Instead of returning a default value when the wrapped value is not present, it throws an exception:
@Test(expected = IllegalArgumentException.class) public void whenOrElseThrowWorks_thenCorrect() { String nullName = null; String name = Optional.ofNullable(nullName).orElseThrow( IllegalArgumentException::new); }Method references in Java 8 come in handy here, to pass in the exception constructor.
Java 10 introduced a simplified no-arg version of orElseThrow() method. In case of an empty Optional it throws a NoSuchElelementException:
@Test(expected = NoSuchElementException.class) public void whenNoArgOrElseThrowWorks_thenCorrect() { String nullName = null; String name = Optional.ofNullable(nullName).orElseThrow(); }9. Returning Value With get()
The final approach for retrieving the wrapped value is the get() method:
@Test public void givenOptional_whenGetsValue_thenCorrect() { Optional opt = Optional.of("baeldung"); String name = opt.get(); assertEquals("baeldung", name); }However, unlike the previous three approaches, get() can only return a value if the wrapped object is not null; otherwise, it throws a no such element exception:
@Test(expected = NoSuchElementException.class) public void givenOptionalWithNull_whenGetThrowsException_thenCorrect() { Optional opt = Optional.ofNullable(null); String name = opt.get(); }This is the major flaw of the get() method. Ideally, Optional should help us avoid such unforeseen exceptions. Therefore, this approach works against the objectives of Optional and will probably be deprecated in a future release.
So, it's advisable to use the other variants that enable us to prepare for and explicitly handle the null case.
10. Conditional Return With filter()
We can run an inline test on our wrapped value with the filter method. It takes a predicate as an argument and returns an Optional object. If the wrapped value passes testing by the predicate, then the Optional is returned as-is.
However, if the predicate returns false, then it will return an empty Optional:
@Test public void whenOptionalFilterWorks_thenCorrect() { Integer year = 2016; Optional yearOptional = Optional.of(year); boolean is2016 = yearOptional.filter(y -> y == 2016).isPresent(); assertTrue(is2016); boolean is2017 = yearOptional.filter(y -> y == 2017).isPresent(); assertFalse(is2017); }The filter method is normally used this way to reject wrapped values based on a predefined rule. We could use it to reject a wrong email format or a password that is not strong enough.
Let's look at another meaningful example. Say we want to buy a modem, and we only care about its price.
We receive push notifications on modem prices from a certain site and store these in objects:
public class Modem { private Double price; public Modem(Double price) { this.price = price; } // standard getters and setters }We then feed these objects to some code whose sole purpose is to check if the modem price is within our budget range.
Let's now take a look at the code without Optional:
public boolean priceIsInRange1(Modem modem) { boolean isInRange = false; if (modem != null && modem.getPrice() != null && (modem.getPrice() >= 10 && modem.getPrice() <= 15)) { isInRange = true; } return isInRange; }Pay attention to how much code we have to write to achieve this, especially in the if condition. The only part of the if condition that is critical to the application is the last price-range check; the rest of the checks are defensive:
@Test public void whenFiltersWithoutOptional_thenCorrect() { assertTrue(priceIsInRange1(new Modem(10.0))); assertFalse(priceIsInRange1(new Modem(9.9))); assertFalse(priceIsInRange1(new Modem(null))); assertFalse(priceIsInRange1(new Modem(15.5))); assertFalse(priceIsInRange1(null)); }Apart from that, it's possible to forget about the null checks over a long day without getting any compile-time errors.
Now let's look at a variant with Optional#filter:
public boolean priceIsInRange2(Modem modem2) { return Optional.ofNullable(modem2) .map(Modem::getPrice) .filter(p -> p >= 10) .filter(p -> p <= 15) .isPresent(); }The map call is simply used to transform a value to some other value. Keep in mind that this operation does not modify the original value.
In our case, we are obtaining a price object from the Model class. We will look at the map() method in detail in the next section.
First of all, if a null object is passed to this method, we don't expect any problem.
Secondly, the only logic we write inside its body is exactly what the method name describes — price-range check. Optional takes care of the rest:
@Test public void whenFiltersWithOptional_thenCorrect() { assertTrue(priceIsInRange2(new Modem(10.0))); assertFalse(priceIsInRange2(new Modem(9.9))); assertFalse(priceIsInRange2(new Modem(null))); assertFalse(priceIsInRange2(new Modem(15.5))); assertFalse(priceIsInRange2(null)); }The previous approach promises to check price range but has to do more than that to defend against its inherent fragility. Therefore, we can use the filter method to replace unnecessary if statements and reject unwanted values.
11. Transforming Value With map()
In the previous section, we looked at how to reject or accept a value based on a filter.
We can use a similar syntax to transform the Optional value with the map() method:
@Test public void givenOptional_whenMapWorks_thenCorrect() { List companyNames = Arrays.asList( "paypal", "oracle", "", "microsoft", "", "apple"); Optional
listOptional = Optional.of(companyNames); int size = listOptional .map(List::size) .orElse(0); assertEquals(6, size); }
In this example, we wrap a list of strings inside an Optional object and use its map method to perform an action on the contained list. The action we perform is to retrieve the size of the list.
The map method returns the result of the computation wrapped inside Optional. We then have to call an appropriate method on the returned Optional to retrieve its value.
Notice that the filter method simply performs a check on the value and returns a boolean. The map method however takes the existing value, performs a computation using this value, and returns the result of the computation wrapped in an Optional object:
@Test public void givenOptional_whenMapWorks_thenCorrect2() { String name = "baeldung"; Optional nameOptional = Optional.of(name); int len = nameOptional .map(String::length) .orElse(0); assertEquals(8, len); }We can chain map and filter together to do something more powerful.
Let's assume we want to check the correctness of a password input by a user. We can clean the password using a map transformation and check its correctness using a filter:
@Test public void givenOptional_whenMapWorksWithFilter_thenCorrect() { String password = " password "; Optional passOpt = Optional.of(password); boolean correctPassword = passOpt.filter( pass -> pass.equals("password")).isPresent(); assertFalse(correctPassword); correctPassword = passOpt .map(String::trim) .filter(pass -> pass.equals("password")) .isPresent(); assertTrue(correctPassword); }As we can see, without first cleaning the input, it will be filtered out — yet users may take for granted that leading and trailing spaces all constitute input. So, we transform a dirty password into a clean one with a map before filtering out incorrect ones.
12. Transforming Value With flatMap()
Just like the map() method, we also have the flatMap() method as an alternative for transforming values. The difference is that map transforms values only when they are unwrapped whereas flatMap takes a wrapped value and unwraps it before transforming it.
Previously, we created simple String and Integer objects for wrapping in an Optional instance. However, frequently, we will receive these objects from an accessor of a complex object.
To get a clearer picture of the difference, let's have a look at a Person object that takes a person's details such as name, age and password:
public class Person { private String name; private int age; private String password; public Optional getName() { return Optional.ofNullable(name); } public Optional getAge() { return Optional.ofNullable(age); } public Optional getPassword() { return Optional.ofNullable(password); } // normal constructors and setters }We would normally create such an object and wrap it in an Optional object just like we did with String.
Alternatively, it can be returned to us by another method call:
Person person = new Person("john", 26); Optional personOptional = Optional.of(person);Notice now that when we wrap a Person object, it will contain nested Optional instances:
@Test public void givenOptional_whenFlatMapWorks_thenCorrect2() { Person person = new Person("john", 26); Optional personOptional = Optional.of(person); Optional
nameOptionalWrapper = personOptional.map(Person::getName); Optional nameOptional = nameOptionalWrapper.orElseThrow(IllegalArgumentException::new); String name1 = nameOptional.orElse(""); assertEquals("john", name1); String name = personOptional .flatMap(Person::getName) .orElse(""); assertEquals("john", name); }
Here, we're trying to retrieve the name attribute of the Person object to perform an assertion.
Note how we achieve this with map() method in the third statement, and then notice how we do the same with flatMap() method afterwards.
The Person::getName method reference is similar to the String::trim call we had in the previous section for cleaning up a password.
The only difference is that getName() returns an Optional rather than a String as did the trim() operation. This, coupled with the fact that a map transformation wraps the result in an Optional object, leads to a nested Optional.
While using map() method, therefore, we need to add an extra call to retrieve the value before using the transformed value. This way, the Optional wrapper will be removed. This operation is performed implicitly when using flatMap.
13. Chaining Optionals in Java 8
Sometimes, we may need to get the first non-empty Optional object from a number of Optionals. In such cases, it would be very convenient to use a method like orElseOptional(). Unfortunately, such operation is not directly supported in Java 8.
Let's first introduce a few methods that we'll be using throughout this section:
private Optional getEmpty() { return Optional.empty(); } private Optional getHello() { return Optional.of("hello"); } private Optional getBye() { return Optional.of("bye"); } private Optional createOptional(String input) { if (input == null || "".equals(input) || "empty".equals(input)) { return Optional.empty(); } return Optional.of(input); }In order to chain several Optional objects and get the first non-empty one in Java 8, we can use the Stream API:
@Test public void givenThreeOptionals_whenChaining_thenFirstNonEmptyIsReturned() { Optional found = Stream.of(getEmpty(), getHello(), getBye()) .filter(Optional::isPresent) .map(Optional::get) .findFirst(); assertEquals(getHello(), found); }The downside of this approach is that all of our get methods are always executed, regardless of where a non-empty Optional appears in the Stream.
If we want to lazily evaluate the methods passed to Stream.of(), we need to use the method reference and the Supplier interface:
@Test public void givenThreeOptionals_whenChaining_thenFirstNonEmptyIsReturnedAndRestNotEvaluated() { Optional found = Stream.
>of(this::getEmpty, this::getHello, this::getBye) .map(Supplier::get) .filter(Optional::isPresent) .map(Optional::get) .findFirst(); assertEquals(getHello(), found); }
In case we need to use methods that take arguments, we have to resort to lambda expressions:
@Test public void givenTwoOptionalsReturnedByOneArgMethod_whenChaining_thenFirstNonEmptyIsReturned() { Optional found = Stream.
>of( () -> createOptional("empty"), () -> createOptional("hello") ) .map(Supplier::get) .filter(Optional::isPresent) .map(Optional::get) .findFirst(); assertEquals(createOptional("hello"), found); }
Often, we'll want to return a default value in case all of the chained Optionals are empty. We can do so just by adding a call to orElse() or orElseGet():
@Test public void givenTwoEmptyOptionals_whenChaining_thenDefaultIsReturned() { String found = Stream.
>of( () -> createOptional("empty"), () -> createOptional("empty") ) .map(Supplier::get) .filter(Optional::isPresent) .map(Optional::get) .findFirst() .orElseGet(() -> "default"); assertEquals("default", found); }
14. JDK 9 Optional API
The release of Java 9 added even more new methods to the Optional API:
- or() method for providing a supplier that creates an alternative Optional
- ifPresentOrElse() method that allows executing an action if the Optional is present or another action if not
- stream() method for converting an Optional to a Stream
Here is the complete article for further reading.
15. Misuse of Optionals
Finally, let's see a tempting, however dangerous, way to use Optionals: passing an Optional parameter to a method.
Imagine we have a list of Person and we want a method to search through that list for people with a given name. Also, we would like that method to match entries with at least a certain age, if it's specified.
With this parameter being optional, we come with this method:
public static List search(List people, String name, Optional age) { // Null checks for people and name return people.stream() .filter(p -> p.getName().equals(name)) .filter(p -> p.getAge().get() >= age.orElse(0)) .collect(Collectors.toList()); }Then we release our method, and another developer tries to use it:
someObject.search(people, "Peter", null);Now the developer executes its code and gets a NullPointerException.There we are, having to null check our optional parameter, which defeats our initial purpose in wanting to avoid this kind of situation.
Here are some possibilities we could have done to handle it better:
public static List search(List people, String name, Integer age) { // Null checks for people and name final Integer ageFilter = age != null ? age : 0; return people.stream() .filter(p -> p.getName().equals(name)) .filter(p -> p.getAge().get() >= ageFilter) .collect(Collectors.toList()); }There, the parameter's still optional, but we handle it in only one check.
Another possibility would have been to create two overloaded methods:
public static List search(List people, String name) { return doSearch(people, name, 0); } public static List search(List people, String name, int age) { return doSearch(people, name, age); } private static List doSearch(List people, String name, int age) { // Null checks for people and name return people.stream() .filter(p -> p.getName().equals(name)) .filter(p -> p.getAge().get().intValue() >= age) .collect(Collectors.toList()); }That way we offer a clear API with two methods doing different things (though they share the implementation).
So, there are solutions to avoid using Optionals as method parameters. The intent of Java when releasing Optional was to use it as a return type, thus indicating that a method could return an empty value. As a matter of fact, the practice of using Optional as a method parameter is even discouraged by some code inspectors.
16. Optional and Serialization
As discussed above, Optional is meant to be used as a return type. Trying to use it as a field type is not recommended.
Additionally, using Optional in a serializable class will result in a NotSerializableException. Our article Java Optional as Return Type further addresses the issues with serialization.
And, in Using Optional With Jackson, we explain what happens when Optional fields are serialized, along with a few workarounds to achieve the desired results.
17. Conclusion
In this article, we covered most of the important features of Java 8 Optional class.
We briefly explored some reasons why we would choose to use Optional instead of explicit null checking and input validation.
También aprendimos cómo obtener el valor de un Opcional , o uno predeterminado si está vacío, con los métodos get () , orElse () y orElseGet () (y vimos la importante diferencia entre los dos últimos).
Luego vimos cómo transformar o filtrar nuestros Opcionales con map (), flatMap () y filter () . Discutimos lo que ofrece una API fluida opcional , ya que nos permite encadenar los diferentes métodos fácilmente.
Finalmente, vimos por qué el uso de opcionales como parámetros de método es una mala idea y cómo evitarlo.
El código fuente de todos los ejemplos del artículo está disponible en GitHub.