1. Gambaran keseluruhan
Dalam tutorial ini, kita akan menunjukkan kelas Pilihan yang diperkenalkan di Java 8.
Tujuan kelas adalah untuk menyediakan penyelesaian tahap-jenis untuk mewakili nilai pilihan dan bukannya rujukan nol .
Untuk mendapatkan pemahaman yang lebih mendalam mengenai mengapa kita harus mementingkan kelas Pilihan , lihat artikel rasmi Oracle.
2. Membuat Objek Pilihan
Terdapat beberapa cara untuk membuat objek Pilihan .
Untuk membuat kosong Pilihan objek, kita hanya perlu menggunakan yang () kosong kaedah statik:
@Test public void whenCreatesEmptyOptional_thenCorrect() { Optional empty = Optional.empty(); assertFalse(empty.isPresent()); }Perhatikan bahawa kami menggunakan kaedah isPresent () untuk memeriksa apakah ada nilai di dalam objek Pilihan . Nilai hadir hanya jika kita membuat Pilihan dengan nilai bukan nol . Kami akan melihat kaedah isPresent () di bahagian seterusnya.
Kami juga dapat membuat objek Pilihan dengan kaedah statik () :
@Test public void givenNonNull_whenCreatesNonNullable_thenCorrect() { String name = "baeldung"; Optional opt = Optional.of(name); assertTrue(opt.isPresent()); }Walau bagaimanapun, argumen yang diteruskan ke kaedah () tidak boleh dibatalkan. Jika tidak, kami akan mendapat NullPointerException :
@Test(expected = NullPointerException.class) public void givenNull_whenThrowsErrorOnCreate_thenCorrect() { String name = null; Optional.of(name); }Tetapi sekiranya kita menjangkakan beberapa nilai nol , kita dapat menggunakan kaedah ofNullable () :
@Test public void givenNonNull_whenCreatesNullable_thenCorrect() { String name = "baeldung"; Optional opt = Optional.ofNullable(name); assertTrue(opt.isPresent()); }Dengan melakukan ini, jika kita memberikan rujukan nol , ia tidak membuang pengecualian tetapi mengembalikan objek Pilihan kosong :
@Test public void givenNull_whenCreatesNullable_thenCorrect() { String name = null; Optional opt = Optional.ofNullable(name); assertFalse(opt.isPresent()); }3. Memeriksa Kehadiran Nilai: isPresent () dan isEmpty ()
Apabila kita mempunyai objek Pilihan yang dikembalikan dari kaedah atau dibuat oleh kita, kita dapat memeriksa apakah ada nilai di dalamnya atau tidak dengan kaedah isPresent () :
@Test public void givenOptional_whenIsPresentWorks_thenCorrect() { Optional opt = Optional.of("Baeldung"); assertTrue(opt.isPresent()); opt = Optional.ofNullable(null); assertFalse(opt.isPresent()); }Kaedah ini kembali benar jika nilai yang dibungkus tidak nol.
Juga, pada Java 11, kita dapat melakukan sebaliknya dengan kaedah isEmpty :
@Test public void givenAnEmptyOptional_thenIsEmptyBehavesAsExpected() { Optional opt = Optional.of("Baeldung"); assertFalse(opt.isEmpty()); opt = Optional.ofNullable(null); assertTrue(opt.isEmpty()); }4. Tindakan Bersyarat Dengan ifPresent ()
Kaedah ifPresent () membolehkan kita menjalankan beberapa kod pada nilai terbungkus sekiranya didapati tidak nol . Sebelum Pilihan , kami akan melakukan:
if(name != null) { System.out.println(name.length()); }Kod ini memeriksa sama ada pemboleh ubah nama adalah nol atau tidak sebelum meneruskan untuk melaksanakan beberapa kod di atasnya. Pendekatan ini panjang lebar, dan itu bukan satu-satunya masalah - ini juga terdedah kepada kesilapan.
Betul, apa yang menjamin bahawa setelah mencetak pembolehubah itu, kita tidak akan menggunakannya lagi dan kemudian lupa untuk melakukan pemeriksaan nol?
Ini boleh menghasilkan NullPointerException pada waktu runtime jika nilai null masuk ke dalam kod tersebut. Apabila program gagal kerana masalah input, ia sering kali disebabkan oleh amalan pengaturcaraan yang buruk.
Dengan pilihan, kita dapat menangani nilai-nilai yang tidak dapat dijelaskan secara eksplisit sebagai kaedah untuk melaksanakan amalan pengaturcaraan yang baik.
Sekarang mari kita lihat bagaimana kod di atas dapat dipantulkan di Java 8.
Dalam gaya pengaturcaraan fungsional biasa, kita dapat melakukan melakukan tindakan pada objek yang sebenarnya ada:
@Test public void givenOptional_whenIfPresentWorks_thenCorrect() { Optional opt = Optional.of("baeldung"); opt.ifPresent(name -> System.out.println(name.length())); }Dalam contoh di atas, kami hanya menggunakan dua baris kod untuk menggantikan lima baris yang berfungsi pada contoh pertama: satu baris untuk membungkus objek menjadi objek Pilihan dan yang berikutnya untuk melakukan pengesahan tersirat serta melaksanakan kod tersebut.
5. Nilai Lalai Dengan orElse ()
Kaedah orElse () digunakan untuk mengambil nilai yang dibungkus di dalam Opsyen pilihan . Ia memerlukan satu parameter, yang bertindak sebagai nilai lalai. Kaedah orElse () mengembalikan nilai terbungkus jika ada, dan argumennya sebaliknya:
@Test public void whenOrElseWorks_thenCorrect() { String nullName = null; String name = Optional.ofNullable(nullName).orElse("john"); assertEquals("john", name); }6. Nilai Lalai Dengan orElseGet ()
Kaedah orElseGet () serupa dengan orElse () . Namun, daripada mengambil nilai untuk dikembalikan jika nilai Pilihan tidak ada, ia memerlukan antara muka fungsional pembekal, yang dipanggil dan mengembalikan nilai pemohon:
@Test public void whenOrElseGetWorks_thenCorrect() { String nullName = null; String name = Optional.ofNullable(nullName).orElseGet(() -> "john"); assertEquals("john", name); }7. Perbezaan Antara orElse dan orElseGet ()
Bagi banyak pengaturcara yang baru menggunakan Optional atau Java 8, perbezaan antara orElse () dan orElseGet () tidak jelas. Sebenarnya, kedua-dua kaedah ini memberi kesan bahawa mereka saling bertindih dalam fungsi.
Walau bagaimanapun, terdapat perbezaan yang halus tetapi sangat penting antara keduanya yang dapat mempengaruhi prestasi kod kami secara drastik jika tidak difahami dengan baik.
Mari buat kaedah yang dipanggil getMyDefault () di kelas ujian, yang tidak memerlukan argumen dan mengembalikan nilai lalai:
public String getMyDefault() { System.out.println("Getting Default Value"); return "Default Value"; }Mari kita lihat dua ujian dan perhatikan kesan sampingannya untuk menentukan di mana atauElse () dan atauElseGet () bertindih dan di mana mereka berbeza:
@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); }Dalam contoh di atas, kami membungkus teks kosong di dalam objek Pilihan dan berusaha mendapatkan nilai yang dibungkus menggunakan masing-masing dua pendekatan.
Kesan sampingannya adalah:
Getting default value... Getting default value...Kaedah getMyDefault () dipanggil dalam setiap kes. Kebetulan apabila nilai yang dibungkus tidak ada, maka kedua-dua orElse () dan atauElseGet () berfungsi dengan cara yang sama.
Sekarang mari kita jalankan ujian lain di mana nilainya ada, dan idealnya, nilai lalai bahkan tidak boleh dibuat:
@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); }Dalam contoh di atas, kami tidak lagi membungkus nilai nol , dan kod selebihnya tetap sama.
Sekarang mari kita lihat kesan sampingan menjalankan kod ini:
Using orElseGet: Using orElse: Getting default value...Notice that when using orElseGet() to retrieve the wrapped value, the getMyDefault() method is not even invoked since the contained value is present.
However, when using orElse(), whether the wrapped value is present or not, the default object is created. So in this case, we have just created one redundant object that is never used.
In this simple example, there is no significant cost to creating a default object, as the JVM knows how to deal with such. However, when a method such as getMyDefault() has to make a web service call or even query a database, the cost becomes very obvious.
8. Exceptions With 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.
Kami juga belajar bagaimana mendapatkan nilai Pilihan , atau nilai lalai jika kosong, dengan kaedah get () , atauElse () dan atauElseGet () (dan melihat perbezaan penting antara dua yang terakhir).
Kemudian kami melihat bagaimana mengubah atau menapis pilihan kami dengan peta (), flatMap () dan penapis () . Kami membincangkan apa yang ditawarkan oleh Opsional API yang lancar , kerana ia membolehkan kami mengaitkan kaedah yang berbeza dengan mudah.
Akhirnya, kami melihat mengapa menggunakan Opsional sebagai parameter kaedah adalah idea yang tidak baik dan bagaimana menjauhinya.
Kod sumber untuk semua contoh dalam artikel boleh didapati di GitHub.