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📜  Java 9-可选的类改进

📅  最后修改于: 2020-11-15 03:16:48             🧑  作者: Mango

Java 8中引入了可选类，以避免空检查和NullPointerException问题。在Java 9中，添加了三个新方法来改善其功能。

  • 流()
  • ifPresentOrElse()
  • 要么()

stream()方法

句法

public Stream stream()

如果存在值，则它返回仅包含该值的顺序Stream，否则返回空Stream。

例

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> 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 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 filteredListJava9 = list.stream()
         .flatMap(Optional::stream)
         .collect(Collectors.toList());

      System.out.println(filteredList);
      System.out.println(filteredListJava9);
   }  
}

输出

[A, B]
[A, B]

ifPresentOrElse()方法

句法

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

import java.util.Optional;

public class Tester {
   public static void main(String[] args) {
      Optional 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()方法

句法

public Optional or(Supplier extends Optional extends T>> supplier)

如果存在值，则返回描述该值的Optional，否则返回由supplying函数产生的Optional。

例

import java.util.Optional;
import java.util.function.Supplier;

public class Tester {
   public static void main(String[] args) {
      Optional optional1 = Optional.of("Mahesh");

      Supplier> 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