Java：Optional

创建Optional对象

1. 创建一个空的Optional对象

Optional<String> empty = Optional.empty();

使用 isPresent() 方法检查Optional对象中是否有值。

@Test
public void givenNonNull_whenCreatesNonNullable_thenCorrect() {
    String name = "baeldung";
    Optional<String> opt = Optional.of(name);
    assertTrue(opt.isPresent());
}

　

2. 创建一个Optional对象

String name = "baeldung";
Optional<String> opt = Optional.of(name);

但是，传递给 of() 方法的参数不能为 null。 否则，我们将得到一个 NullPointerException。

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

 

3. 创建一个可以接受null值的Optional对象

@Test
public void givenNonNull_whenCreatesNullable_thenCorrect() {
    String name = "baeldung";
    Optional<String> opt = Optional.ofNullable(name);
    assertTrue(opt.isPresent());
}

使用null值创建：

@Test
public void givenNull_whenCreatesNullable_thenCorrect() {
    String name = null;
    Optional<String> opt = Optional.ofNullable(name);
    assertFalse(opt.isPresent());
}

 

判断Optional是否为空

1. isPresent()，如果值不为 null，则返回 true。

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

    opt = Optional.ofNullable(null);
    assertFalse(opt.isPresent());
}

2. isEmpty()，如果值为null，则返回true

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

    opt = Optional.ofNullable(null);
    assertTrue(opt.isEmpty());
}

 

 Optional的使用

Optional<String> opt = Optional.of("baeldung");
if(opt != null) {
    System.out.println(opt.length());
}

优化后

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

　

使用orElse()

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

使用orElseGet()

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

它们的不同

@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);
}

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

输出结果：

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

在每种情况下都会调用 getMyDefault() 方法。 在值为null时，orElse() 和 orElseGet() 的工作方式完全相同。当在值不为null时，它们的运行结果就会不同：

@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);
}

输出结果：

Using orElseGet:
Using orElse:
Getting default value...

所以在使用像orMyDefault()的方法是不管值是否为空方法都会被运行。

方法会被运行但是返回的还是optional中的值。

 

使用orElseThrow()

@Test(expected = IllegalArgumentException.class)
public void whenOrElseThrowWorks_thenCorrect() {
    String nullName = null;
    String name = Optional.ofNullable(nullName).orElseThrow(IllegalArgumentException::new);
}

Java10的更新中可以不传输parameter

@Test(expected = NoSuchElementException.class)
public void whenNoArgOrElseThrowWorks_thenCorrect() {
    String nullName = null;
    String name = Optional.ofNullable(nullName).orElseThrow();
}

 

使用get()返回值

@Test
public void givenOptional_whenGetsValue_thenCorrect() {
    Optional<String> opt = Optional.of("baeldung");
    String name = opt.get();
    assertEquals("baeldung", name);
}　

get() 仅当包装对象不为 null 时才能返回值； 否则，它会抛出 no such element 异常

@Test(expected = NoSuchElementException.class)
public void givenOptionalWithNull_whenGetThrowsException_thenCorrect() {
    Optional<String> opt = Optional.ofNullable(null);
    String name = opt.get();
}

 

使用filter()

@Test
public void whenOptionalFilterWorks_thenCorrect() {
    Integer year = 2016;
    Optional<Integer> yearOptional = Optional.of(year);

    boolean is2016 = yearOptional.filter(y -> y == 2016).isPresent();
    assertTrue(is2016);

    boolean is2017 = yearOptional.filter(y -> y == 2017).isPresent();
    assertFalse(is2017);
}

例子

public boolean priceIsInRange1(Modem modem) {
    boolean isInRange = false;

    if (modem != null && modem.getPrice() != null 
      && (modem.getPrice() >= 10 
        && modem.getPrice() <= 15)) {

        isInRange = true;
    }
    return isInRange;
}

优化后

public boolean priceIsInRange2(Modem modem2) {
     return Optional.ofNullable(modem2)
       .map(Modem::getPrice)
       .filter(p -> p >= 10)
       .filter(p -> p <= 15)
       .isPresent();
 }

　　

 .map() 的使用

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

    int size = listOptional
      .map(List::size)
      .orElse(0);
    assertEquals(6, size);
}

例子2.

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

    int len = nameOptional
     .map(String::length)
     .orElse(0);
    assertEquals(8, len);
}

　　

例子3.

@Test
public void givenOptional_whenMapWorksWithFilter_thenCorrect() {
    String password = " password ";
    Optional<String> 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);
}

　　

使用 .flatMap()

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
}

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

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

    String name = personOptional
      .flatMap(Person::getName)
      .orElse("");
    assertEquals("john", name);
}