Java 9 Optional类改进
Java 9 Optional类改进
Java 8中引入了Optional类,以避免空检查和空指针异常问题。在Java 9中,添加了三种新方法来改进其功能。
stream()
ifPresentOrElse()
or()
stream() method
语法
public Stream<T> stream()
如果存在值,它将返回仅包含该值的顺序流,否则将返回空流。
例子
import java.util.Arrays; import java.util.List; import java.util.Optional; import java.util.stream.Collectors; import java.util.stream.Stream; public class Tester { public static void main(String[] args) { List<Optional<String>> list = Arrays.asList ( Optional.empty(), Optional.of("A"), Optional.empty(), Optional.of("B")); //filter the list based to print non-empty values //if optional is non-empty, get the value in stream, otherwise return empty List<String> filteredList = list.stream() .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty()) .collect(Collectors.toList()); //Optional::stream method will return a stream of either one //or zero element if data is present or not. List<String> filteredListJava9 = list.stream() .flatMap(Optional::stream) .collect(Collectors.toList()); System.out.println(filteredList); System.out.println(filteredListJava9); } }
输出
[A, B] [A, B]
ifPresentOrElse() method
语法
public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction)
如果存在值,则使用该值执行给定的操作,否则执行给定的基于空的操作。
例子
import java.util.Optional; public class Tester { public static void main(String[] args) { Optional<Integer> optional = Optional.of(1); optional.ifPresentOrElse( x -> System.out.println("Value: " + x),() -> System.out.println("Not Present.")); optional = Optional.empty(); optional.ifPresentOrElse( x -> System.out.println("Value: " + x),() -> System.out.println("Not Present.")); } }
输出
Value: 1 Not Present.
or() method
语法
public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier)
如果存在值,则返回描述该值的可选值,否则返回由供应函数生成的可选值。
例子
import java.util.Optional; import java.util.function.Supplier; public class Tester { public static void main(String[] args) { Optional<String> optional1 = Optional.of("Mahesh"); Supplier<Optional<String>> supplierString = () -> Optional.of("Not Present"); optional1 = optional1.or( supplierString); optional1.ifPresent( x -> System.out.println("Value: " + x)); optional1 = Optional.empty(); optional1 = optional1.or( supplierString); optional1.ifPresent( x -> System.out.println("Value: " + x)); } }
输出
Value: Mahesh Value: Not Present