Java 8——可選

本文主要介紹Java 8的 Optional 的簡單使用

Address

}

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

#of + ofNullable

相關程式碼託管在java8_demo#

 

map

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

2. Optional.map的定義：<u> Optional<u> map(Function&lt ;? super T, ? extends U> mapper)</u></u>

3. 上面兩者定義非常類似，Optional可看成最多包含一個元素的 Stream

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@Test(expected = NoSuchElementException.class) #public void emptyTest() { // 宣告一個空的Optional物件 Optional nameOptional = Optional.empty(); // java.util.NoSuchElementException: No value present nameOptional.get(); ##} #  #@Test(expected = NullPointerException.class) #public void ofNullTest() { // 依據實例建立Optional物件 Optional.of( new Address("Guangdong", "Zhongshan"));   // Optional.ofNullable(null)回傳Optional.empty() assertEquals(Optional.empty(), Optional.ofNullable( null)); // java.lang.NullPointerException Optional.of( null); }

8Optional stringOptional = addressOptional.map(Address::getCity);

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9 1011#@Test ##public void mapTest () { #String province = "Guangdong"; ##String city = "Zhongshan"; Optional addressOptional = Optional.of( new Address(province, city)); // mapper : Address -> String // Optional -> Optional

assertTrue(stringOptional.isPresent());

assertEquals(city, stringOptional.get());

1. assertEquals(city, stringOptional。 ##}

flatMap

2. #Stream.flatMap的定義：

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

Optional.flatMap"Guangdong";

##Optional.flatMap ##Optional.flatMap 的定義： Optional flatMap(Function super T, Optional> mapper)

12#3##4##5#6 78 910 1112 @Test public void flatmapTest##() { String province =

String city = ###"Zhongshan";######### #Optional

addressOptional = Optional.of(###new Address(province, city));############// 對於mapper 為T -> Optional時，呼叫Optional.map，生成的是Optional>#########Optional> optionalOptional = addressOptional.map(address ->> optionalOptional = addressOptional.map(address -> Optional.ofNullable). ()));#########// 對於mapper 為T -> Optional時，呼叫Optional.map，產生的是Optional,被扁平化###### ####Optional stringOptional = addressOptional.flatMap(address -> Optional.ofNullable(address.getCity()));######assertTrue(stringOptional.isPresent());######assertTrue(stringOptional.isPresent());#### #assertEquals(city, stringOptional.get());######}###############

解引用

20emptyAddressOptional.ifPresent(System.out::println) ; 

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2122 #23#24#25#26 @Test(expected = UnsupportedOperationException.class) public void dereferenceTest () { ## // 取得：最簡單+ 最不安全 位址address = 位址可選.get(); assertNotNull(address);   address = null; #可選; emptyAddressOptional =Optional.ofNullable(address);   String defaultValue = "Unknown"; // orElse：設定預設值 ##位址elseAddress = emptyAddressOptional.orElse(new Address(defaultValue, defaultValue)); assertEquals(defaultValue, elseAddress.getProvince()); assertEquals(defaultValue, elseAddress.getCity());   #// orElseGet：orElse 的延遲呼叫版本 Address elseGetAddress =emptyAddressOptional.orElseGet(Address:: new); #assertNull(elseGetAddress.getProvince());assertNull(elseGetAddress.getCity());   #// ifPresent：存在值則運行消費者，否則不進行任何操作

#// orElseThrow：不存在時，發送異常#emptyAddressOptional.orElseThrow(UnsupportedOperationException::

new); ##} 過濾器

##1 234##@測試 # #公共void filterTest () {

assertTrue(addressOptional.filter(address ->位址.getCity().contains(

"Z")).isPresent());

1. ##}基礎類型+ 可選

2. ##OptionalInt、OptionalLong、OptionalDouble

################################### #最多###只有一個###元素，並沒有​​像###StreamInt###那樣相對於###Stream###有效能優勢###########不支援###地圖###、###平面地圖###、###過濾#############

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