Java 8——Optional

This article mainly introduces the simple use of Optional of Java 8

Address

##of + ofNullable

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

Related code hosting In java8_demo

##1@Test(expected = NoSuchElementException.class)

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##public

void

emptyTest() { //Declare an empty Optional object

Optional

nameOptional = Optional.empty();

// java.util.NoSuchElementException: No value present

nameOptional.get();}

@Test(expected = NullPointerException.class)

public

void

ofNullTest() {// Create an Optional object based on the instance

Optional.of(new Address("Guangdong",

"Zhongshan"));

// Optional.ofNullable(null) returns Optional.empty()

assertEquals(Optional.empty(), Optional.ofNullable(null));

// java.lang.NullPointerException

Optional.of(null);

}

map

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

2. ##Definition of Optional.map: Optional map(Function< ;? super T, ? extends U> mapper)

3. The above two definitions are very similar,

   Optional can be regarded as containing at most one element Stream

##1flatMap

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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()); }

##Definition of Stream.flatMap

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

   ##Optional.flatMap## Definition of #:

   Optional flatMap(Function super T, Optional> mapper)

##1

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@Test

public

void flatmapTest() {

String province = "Guangdong";String city = "Zhongshan";

Optional

addressOptional = Optional.of(

new Address(province, city));

// When mapper is T -> Optional, Calling Optional.map generates Optional>

##Optional> optionalOptional = addressOptional.map(address -> Optional.ofNullable(address.getCity ()));

// When mapper is T -> Optional, when Optional.map is called, Optional is generated and is flattened

Optional 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 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></integer> 有性能优势

3. 不支持 map 、 flatmap 、 filter

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