Guide To Java 8 Optional – Java 8备选指南

最后修改: 2016年 12月 8日


1. Overview


In this tutorial, we’re going to show the Optional class that was introduced in Java 8.

在本教程中,我们将展示Java 8中引入的Optional类。

The purpose of the class is to provide a type-level solution for representing optional values instead of null references.


To get a deeper understanding of why we should care about the Optional class, take a look at the official Oracle article.


2. Creating Optional Objects


There are several ways of creating Optional objects.


To create an empty Optional object, we simply need to use its empty() static method:


public void whenCreatesEmptyOptional_thenCorrect() {
    Optional<String> empty = Optional.empty();

Note that we used the isPresent() method to check if there is a value inside the Optional object. A value is present only if we have created Optional with a non-null value. We’ll look at the isPresent() method in the next section.


We can also create an Optional object with the static method of():


public void givenNonNull_whenCreatesNonNullable_thenCorrect() {
    String name = "baeldung";
    Optional<String> opt = Optional.of(name);

However, the argument passed to the of() method can’t be null. Otherwise, we’ll get a NullPointerException:


@Test(expected = NullPointerException.class)
public void givenNull_whenThrowsErrorOnCreate_thenCorrect() {
    String name = null;

But in case we expect some null values, we can use the ofNullable() method:


public void givenNonNull_whenCreatesNullable_thenCorrect() {
    String name = "baeldung";
    Optional<String> opt = Optional.ofNullable(name);

By doing this, if we pass in a null reference, it doesn’t throw an exception but rather returns an empty Optional object:


public void givenNull_whenCreatesNullable_thenCorrect() {
    String name = null;
    Optional<String> opt = Optional.ofNullable(name);

3. Checking Value Presence: isPresent() and isEmpty()


When we have an Optional object returned from a method or created by us, we can check if there is a value in it or not with the isPresent() method:


public void givenOptional_whenIsPresentWorks_thenCorrect() {
    Optional<String> opt = Optional.of("Baeldung");

    opt = Optional.ofNullable(null);

This method returns true if the wrapped value is not null.


Also, as of Java 11, we can do the opposite with the isEmpty method:

另外,从Java 11开始,我们可以用isEmpty方法做相反的事情。

public void givenAnEmptyOptional_thenIsEmptyBehavesAsExpected() {
    Optional<String> opt = Optional.of("Baeldung");

    opt = Optional.ofNullable(null);

4. Conditional Action With ifPresent()


The ifPresent() method enables us to run some code on the wrapped value if it’s found to be non-null. Before Optional, we’d do:


if(name != null) {

This code checks if the name variable is null or not before going ahead to execute some code on it. This approach is lengthy, and that’s not the only problem — it’s also prone to error.


Indeed, what guarantees that after printing that variable, we won’t use it again and then forget to perform the null check?


This can result in a NullPointerException at runtime if a null value finds its way into that code. When a program fails due to input issues, it’s often a result of poor programming practices.


Optional makes us deal with nullable values explicitly as a way of enforcing good programming practices.


Let’s now look at how the above code could be refactored in Java 8.

现在让我们来看看在Java 8中如何重构上述代码。

In typical functional programming style, we can execute perform an action on an object that is actually present:


public void givenOptional_whenIfPresentWorks_thenCorrect() {
    Optional<String> opt = Optional.of("baeldung");
    opt.ifPresent(name -> System.out.println(name.length()));

In the above example, we use only two lines of code to replace the five that worked in the first example: one line to wrap the object into an Optional object and the next to perform implicit validation as well as execute the code.


5. Default Value With orElse()


The orElse() method is used to retrieve the value wrapped inside an Optional instance. It takes one parameter, which acts as a default value. The orElse() method returns the wrapped value if it’s present, and its argument otherwise:


public void whenOrElseWorks_thenCorrect() {
    String nullName = null;
    String name = Optional.ofNullable(nullName).orElse("john");
    assertEquals("john", name);

6. Default Value With orElseGet()


The orElseGet() method is similar to orElse(). However, instead of taking a value to return if the Optional value is not present, it takes a supplier functional interface, which is invoked and returns the value of the invocation:


public void whenOrElseGetWorks_thenCorrect() {
    String nullName = null;
    String name = Optional.ofNullable(nullName).orElseGet(() -> "john");
    assertEquals("john", name);

7. Difference Between orElse and orElseGet()


To a lot of programmers who are new to Optional or Java 8, the difference between orElse() and orElseGet() is not clear. As a matter of fact, these two methods give the impression that they overlap each other in functionality.

对于很多刚接触Optional或Java 8的程序员来说,orElse()orElseGet()之间的区别并不明显。事实上,这两个方法给人的印象是它们在功能上相互重叠。

However, there’s a subtle but very important difference between the two that can affect the performance of our code drastically if not well understood.


Let’s create a method called getMyDefault() in the test class, which takes no arguments and returns a default value:


public String getMyDefault() {
    System.out.println("Getting Default Value");
    return "Default Value";

Let’s see two tests and observe their side effects to establish both where orElse() and orElseGet() overlap and where they differ:


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);

In the above example, we wrap a null text inside an Optional object and attempt to get the wrapped value using each of the two approaches.


The side effect is:


Getting default value...
Getting default value...

The getMyDefault() method is called in each case. It so happens that when the wrapped value is not present, then both orElse() and orElseGet() work exactly the same way.


Now let’s run another test where the value is present, and ideally, the default value should not even be created:


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);

In the above example, we are no longer wrapping a null value, and the rest of the code remains the same.


Now let’s take a look at the side effect of running this code:


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.

在这个简单的例子中,创建默认对象并没有明显的成本,因为 JVM 知道如何处理这种情况。然而,当诸如getMyDefault()这样的方法需要调用网络服务甚至查询数据库时,成本就变得非常明显。

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(

Method references in Java 8 come in handy here, to pass in the exception constructor.

Java 8中的方法引用在这里派上了用场,可以传递给异常构造函数。

Java 10 introduced a simplified no-arg version of orElseThrow() method. In case of an empty Optional it throws a NoSuchElementException:

Java 10引入了一个简化的无参数版本的orElseThrow()方法。如果有一个空的Optional,它会抛出一个NoSuchElementException

@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:


public void givenOptional_whenGetsValue_thenCorrect() {
    Optional<String> 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<String> 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.

这就是get()方法的主要缺陷。理想情况下,Optional 应该帮助我们避免这种不可预见的异常。因此,这种方法违背了Optional的目标,并且可能会在未来的版本中被弃用。

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:


public void whenOptionalFilterWorks_thenCorrect() {
    Integer year = 2016;
    Optional<Integer> yearOptional = Optional.of(year);
    boolean is2016 = yearOptional.filter(y -> y == 2016).isPresent();
    boolean is2017 = yearOptional.filter(y -> y == 2017).isPresent();

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:


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)));

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)
       .filter(p -> p >= 10)
       .filter(p -> p <= 15)

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:


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)));

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:


public void givenOptional_whenMapWorks_thenCorrect() {
    List<String> companyNames = Arrays.asList(
      "paypal", "oracle", "", "microsoft", "", "apple");
    Optional<List<String>> listOptional = Optional.of(companyNames);

    int size = listOptional
    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 an Optional describing this value only if it matches the given predicate. Otherwise returns an empty Optional. 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:


public void givenOptional_whenMapWorks_thenCorrect2() {
    String name = "baeldung";
    Optional<String> nameOptional = Optional.of(name);

    int len = nameOptional
    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:


public void givenOptional_whenMapWorksWithFilter_thenCorrect() {
    String password = " password ";
    Optional<String> passOpt = Optional.of(password);
    boolean correctPassword = passOpt.filter(
      pass -> pass.equals("password")).isPresent();

    correctPassword = passOpt
      .filter(pass -> pass.equals("password"))

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<String> getName() {
        return Optional.ofNullable(name);

    public Optional<Integer> getAge() {
        return Optional.ofNullable(age);

    public Optional<String> 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<Person> personOptional = Optional.of(person);

Notice now that when we wrap a Person object, it will contain nested Optional instances:


public void givenOptional_whenFlatMapWorks_thenCorrect2() {
    Person person = new Person("john", 26);
    Optional<Person> personOptional = Optional.of(person);

    Optional<Optional<String>> nameOptionalWrapper  
    Optional<String> nameOptional  
      = nameOptionalWrapper.orElseThrow(IllegalArgumentException::new);
    String name1 = nameOptional.orElse("");
    assertEquals("john", name1);

    String name = personOptional
    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

13.在Java 8中串联Optionals

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.

有时,我们可能需要从一些Optional中获取第一个非空的Optional对象。在这种情况下,使用类似orElseOptional()的方法会非常方便。不幸的是,这样的操作在Java 8中不被直接支持。

Let’s first introduce a few methods that we’ll be using throughout this section:


private Optional<String> getEmpty() {
    return Optional.empty();

private Optional<String> getHello() {
    return Optional.of("hello");

private Optional<String> getBye() {
    return Optional.of("bye");

private Optional<String> 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:

为了在Java 8中连锁几个Optional对象并获得第一个非空对象,我们可以使用Stream API。

public void givenThreeOptionals_whenChaining_thenFirstNonEmptyIsReturned() {
    Optional<String> found = Stream.of(getEmpty(), getHello(), getBye())
    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:


public void givenThreeOptionals_whenChaining_thenFirstNonEmptyIsReturnedAndRestNotEvaluated() {
    Optional<String> found =
      Stream.<Supplier<Optional<String>>>of(this::getEmpty, this::getHello, this::getBye)

    assertEquals(getHello(), found);

In case we need to use methods that take arguments, we have to resort to lambda expressions:


public void givenTwoOptionalsReturnedByOneArgMethod_whenChaining_thenFirstNonEmptyIsReturned() {
    Optional<String> found = Stream.<Supplier<Optional<String>>>of(
      () -> createOptional("empty"),
      () -> createOptional("hello")

    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():


public void givenTwoEmptyOptionals_whenChaining_thenDefaultIsReturned() {
    String found = Stream.<Supplier<Optional<String>>>of(
      () -> createOptional("empty"),
      () -> createOptional("empty")
      .orElseGet(() -> "default");

    assertEquals("default", found);

14. JDK 9 Optional API

14.JDK 9 Optional API

The release of Java 9 added even more new methods to the Optional API:

Java 9的发布为Optional API增加了更多新方法。

  • 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<Person> search(List<Person> people, String name, Optional<Integer> age) {
    // Null checks for people and name
            .filter(p -> p.getName().equals(name))
            .filter(p -> p.getAge().get() >= age.orElse(0))

Then we release our method, and another developer tries to use it:

然后我们发布了我们的方法,而另一个开发者试图使用它。, "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.

现在,开发人员执行其代码并得到一个NullPointerException。 在这里,我们不得不对我们的可选参数进行空检查,这违背了我们想要避免这种情况的最初目的。

Here are some possibilities we could have done to handle it better:


public static List<Person> search(List<Person> people, String name, Integer age) {
    // Null checks for people and name
    final Integer ageFilter = age != null ? age : 0;

            .filter(p -> p.getName().equals(name))
            .filter(p -> p.getAge().get() >= ageFilter)

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<Person> search(List<Person> people, String name) {
    return doSearch(people, name, 0);

public static List<Person> search(List<Person> people, String name, int age) {
    return doSearch(people, name, age);

private static List<Person> doSearch(List<Person> people, String name, int age) {
    // Null checks for people and name
            .filter(p -> p.getName().equals(name))
            .filter(p -> p.getAge().get().intValue() >= age)

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.

此外,在可序列化的类中使用Optional将导致NotSerializableException我们的文章Java Optional作为返回类型进一步探讨了序列化的问题。

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.

在这篇文章中,我们介绍了Java 8 Optional类的大部分重要特性。

We briefly explored some reasons why we would choose to use Optional instead of explicit null checking and input validation.


We also learned how to get the value of an Optional, or a default one if empty, with the get(), orElse() and orElseGet() methods (and saw the important difference between the last two).


Then we saw how to transform or filter our Optionals with map(), flatMap() and filter(). We discussed what a fluent API Optional offers, as it allows us to chain the different methods easily.


Finally, we saw why using Optionals as method parameters is a bad idea and how to avoid it.


The source code for all examples in the article is available over on GitHub.