Java 8 – Optional

本文主要介绍Java 8的 Optional 的简单使用

Adresse

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

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@Test(expected = NoSuchElementException.class) public void emptyTest() { // 声明一个空的Optional对象 Optional 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); }

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@Data

@AllArgsConstructor

@NoArgsConstructor

public class Adresse {

privat String-Provinz;

private String-Stadt;

}

of + ofNullable相关代码托管在java8_demo

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@Test(erwartet = NoSuchElementException.class) public void emptyTest() { // 声明一个空的Optional对象 Optional nameOptional = Optional.empty(); // java.util.NoSuchElementException: Kein Wert vorhanden 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)Optional.empty() assertEquals(Optional.empty( ), Optional.ofNullable(null)); // java.lang.NullPointerException span> Optional.of(null); }

Karte

1. Stream.map Definition: <r> Stream<r> map(Function super T, ? extends R> mapper)</r></r>

2. Optional.map Definition: <u> Optional<u> map(Function super T, ? extends U> mapper)</u></u>

3. Die beiden oben genannten Definitionen sind sehr ähnlich, Optional kann als 最多包含一个元素s Stream

angesehen werden

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

flatMap

1. Stream.flatMap Definition: <r> Stream<r> flatMap(Function super T, ? extends Stream extends R>> mapper)</r></r>

2. Optional.flatMap Definition: <u> Optional<u> flatMap(Function super T, Optional<u>> mapper)</u></u></u>

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

Dereferenzierung

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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 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()); }

Basistyp + Optional

1. OptionalInt, OptionalLong, OptionalDouble

2. hat höchstens 只有一个-Elemente und hat nicht den gleichen Leistungsvorteil wie StreamInt gegenüber Stream<integer></integer>

3. wird nicht unterstützt map , flatmap , filter

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