Java 8——Optional

本文主要介绍Java 8的 Optional 的简单使用

Address

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@Data @AllArgsConstructor @NoArgsConstructor public class Address { private String province; private String city; }

of + ofNullable

相关代码托管在java8_demo

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@Test(expected = NoSuchElementException.class) public void emptyTest() { // 声明一个空的Optional对象 Optional<Address> nameOptional = Optional.empty(); // java.util.NoSuchElementException: No value present nameOptional.get(); }   @Test(expected = NullPointerException.class) public void ofNullTest() { // 依据实例创建Optional对象 Optional.of(new Address("Guangdong", "Zhongshan"));   // Optional.ofNullable(null)返回Optional.empty() assertEquals(Optional.empty(), Optional.ofNullable(null)); // java.lang.NullPointerException Optional.of(null); }

map

1. Stream.map的定义：<R> Stream<R> map(Function<? super T, ? extends R> mapper)
2. Optional.map的定义：<U> Optional<U> map(Function<? super T, ? extends U> mapper)
3. 上面两者定义非常类似，Optional可看成最多包含一个元素的 Stream

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@Test public void mapTest() { String province = "Guangdong"; String city = "Zhongshan"; Optional<Address> addressOptional = Optional.of(new Address(province, city)); // mapper : Address -> String // Optional<Address> -> Optional<String> Optional<String> stringOptional = addressOptional.map(Address::getCity); assertTrue(stringOptional.isPresent()); assertEquals(city, stringOptional.get()); }

flatMap

1. Stream.flatMap的定义：<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
2. Optional.flatMap的定义：<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper)

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@Test public void flatmapTest() { String province = "Guangdong"; String city = "Zhongshan"; Optional<Address> addressOptional = Optional.of(new Address(province, city)); // 对于 mapper 为 T -> Optional<U>时，调用Optional.map，生成的是Optional<Optional<U>> Optional<Optional<String>> optionalOptional = addressOptional.map(address -> Optional.ofNullable(address.getCity())); // 对于 mapper 为 T -> Optional<U>时，调用Optional.map，生成的是Optional<U>,被扁平化 Optional<String> stringOptional = addressOptional.flatMap(address -> Optional.ofNullable(address.getCity())); assertTrue(stringOptional.isPresent()); assertEquals(city, stringOptional.get()); }

解引用

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@Test(expected = UnsupportedOperationException.class) public void dereferenceTest() { // get：最简单 + 最不安全 Address address = addressOptional.get(); assertNotNull(address);   address = null; Optional<Address> emptyAddressOptional = Optional.ofNullable(address);   String defaultValue = "Unknown"; // orElse：设置默认值 Address elseAddress = emptyAddressOptional.orElse(new Address(defaultValue, defaultValue)); assertEquals(defaultValue, elseAddress.getProvince()); assertEquals(defaultValue, elseAddress.getCity());   // orElseGet：orElse的延迟调用版本 Address elseGetAddress = emptyAddressOptional.orElseGet(Address::new); assertNull(elseGetAddress.getProvince()); assertNull(elseGetAddress.getCity());   // ifPresent：存在值则运行consumer，否则不进行任何操作 emptyAddressOptional.ifPresent(System.out::println);   // orElseThrow：不存在时，抛出异常 emptyAddressOptional.orElseThrow(UnsupportedOperationException::new); }

filter

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@Test public void filterTest() { assertTrue(addressOptional.filter(address -> address.getCity().contains("Z")).isPresent()); }

基础类型 + Optional

1. OptionalInt、OptionalLong、OptionalDouble
2. 最多只有一个元素，并没有像StreamInt那样相对于 Stream<Integer> 有性能优势
3. 不支持 map 、 flatmap 、 filter