Java 8——Optional

巴扎黑

Release： 2017-06-23 15:48:29

Original

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

Address

##of + ofNullable

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

Related code hosting In java8_demo

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

##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

Definition of Stream.map: <r> Stream<r> map(Function super T, ? extends R> mapper)</r></r>
##Definition of Optional.map: Optional map(Function< ;? super T, ? extends U> mapper)
The above two definitions are very similar,
Optional can be regarded as containing at most one element Stream

Java 8——Optional

##1flatMap

@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

< ;R> Stream flatMap(Function super T, ? extends Stream extends R>> mapper)
##Optional.flatMap## Definition of #:
Optional flatMap(Function super T, Optional> mapper)

##1 Java 8——Optional

@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

@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

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

基础类型 + Optional

OptionalInt、OptionalLong、OptionalDouble
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
不支持 map 、 flatmap 、 filter

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