Java 8--optional

This article mainly introduces the Java 8 Optional simple use of

Address

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

of + ofnullable

Related code hosted in Java8_demo

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@Test (expected = nosuchelementexception.class)public void emptytest() { //Declare an empty optional objectoptional<address> nameoptional = Optional.empty ();//Java.util.NoSuchElementException:No value presentNameoptional.get ();}@Test (expected = nullpointerexception.class)public void ofnulltest() { //Create a optional object from an instanceOptional.of (New Address ("Guangdong", "Zhongshan")); //optional.ofnullable (NULL) returns Optional.empty ()Assertequals (Optional.empty (), Optional.ofnullable (null));//Java.lang.NullPointerExceptionOptional.of (null);}

Map

1. Stream.mapDefinition of:<R> Stream<R> map(Function<? super T, ? extends R> mapper)
2. Optional.mapDefinition of:<U> Optional<U> map(Function<? super T, ? extends U> mapper)
3. The above two definitions are very similar and Optional can be seen 最多包含一个元素 asStream

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@Test public void maptest() {String province = "Guangdong"; String City = "Zhongshan"; optional<address> addressoptional = optional.of (new Address (province, city)); //mapper:address, String //optional<address>-optional<string> optional<string> stringoptional = Addressoptional.map (address::getcity); Asserttrue (stringOptional.isPresent ()); Assertequals (city, Stringoptional.get ());}

FlatMap

1. Stream.flatMapDefinition of:<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
2. Optional.flatMapDefinition of:<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper)

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@Testpublic void flatmaptest() { String province ="Guangdong";String City ="Zhongshan";optional<address> addressoptional = Optional.of (New Address (province, city));//For mapper T-optional<u>, call Optional.map, generate optional<optional<u>>optional<optional<string>> optionaloptional = addressoptional.map (Address-optional.ofnullable ( Address.getcity ()));//For mapper T-optional<u>, call Optional.map, generate Optional<u>optional<string> stringoptional = addressoptional.flatmap (Address-optional.ofnullable (address.getCity () ); Asserttrue (Stringoptional.ispresent ()); Assertequals (city, Stringoptional.get ());}

Solution Reference

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@Test (expected = unsupportedoperationexception.class)public void dereferencetest() { //Get: Simplest + least secureAddress address = Addressoptional.get (); Assertnotnull (address); Address =null;optional<address> emptyaddressoptional = optional.ofnullable (Address); String defaultvalue ="Unknown";///OrElse: Set default valueAddress elseaddress = Emptyaddressoptional.orelse (New Address (DefaultValue, DefaultValue));Assertequals (DefaultValue, Elseaddress.getprovince ()); Assertequals (DefaultValue, Elseaddress.getcity ());delayed call version of//Orelseget:orelseAddress elsegetaddress = Emptyaddressoptional.orelseget (address::new);Assertnull (Elsegetaddress.getprovince ()); Assertnull (Elsegetaddress.getcity ());///ifpresent: The presence value runs consumer, otherwise no action is madeEmptyaddressoptional.ifpresent (System.out::p rintln);///Orelsethrow: Throws an exception if it does not existEmptyaddressoptional.orelsethrow (unsupportedoperationexception::new);}

Filter

1234	@Test public void filtertest() {asserttrue (addressoptional.filter (Address- Address.getcity (). Contains ("Z")). IsPresent ());}

Base type + Optional

1. OptionalInt, OptionalLong ,OptionalDouble
2. Most 只有一个 elements, and there is no StreamInt Stream<Integer> performance advantage like that
3. Not supported map , flatmap ,filter

Java 8--optional