Java 8 – Optional

巴扎黑

Freigeben： 2017-06-23 15:48:29

Original

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本文主要介绍Java 8的 Optional 的简单使用

Adresse

@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);

}

7

@Data

@AllArgsConstructor

@NoArgsConstructor

public class Adresse {

privat String-Provinz;

private String-Stadt;

}

of + ofNullable相关代码托管在java8_demo

@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

Stream.map Definition: <r> Stream<r> map(Function super T, ? extends R> mapper)</r></r>
Optional.map Definition:  Optional map(Function super T, ? extends U> mapper)
Die beiden oben genannten Definitionen sind sehr ähnlich, Optional kann als 最多包含一个元素s Stream

Java 8 – Optional angesehen werden

@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

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

Java 8 – Optional

@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

@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());
}

Basistyp + Optional

OptionalInt, OptionalLong, OptionalDouble
hat höchstens 只有一个-Elemente und hat nicht den gleichen Leistungsvorteil wie StreamInt gegenüber Stream<integer></integer>
wird nicht unterstützt map , flatmap , filter

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