streams - Optional

最新推荐文章于 2019-09-03 09:45:11 发布

原创最新推荐文章于 2019-09-03 09:45:11 发布 · 102 阅读

CC 4.0 BY-SA版权

本文深入探讨了Java中Optional类型的使用，展示了如何通过Optional处理空值，避免NullPointerException。通过多个案例，如从空流中查找元素、Optional的基本操作及便利函数，阐述了Optional在流操作和错误处理中的作用。

Is there some kind of object we can use that will act as a holder for a stream element, but can also kindly tell us (that is, no exceptions) if the element we’re looking for isn’t there?

This idea is implemented as the Optional type. Certain standard stream operations return Optional objects because they cannot guarantee the desired result will exist.

case 1:

// streams/OptionalsFromEmptyStreams.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

import java.util.*;
import java.util.stream.*;

class OptionalsFromEmptyStreams {
  public static void main(String[] args) {
    System.out.println(Stream.<String>empty().findFirst());
    System.out.println(Stream.<String>empty().findAny());
    System.out.println(Stream.<String>empty().max(String.CASE_INSENSITIVE_ORDER));
    System.out.println(Stream.<String>empty().min(String.CASE_INSENSITIVE_ORDER));
    System.out.println(Stream.<String>empty().reduce((s1, s2) -> s1 + s2));
    System.out.println(IntStream.empty().average());
  }
}
/* Output:
Optional.empty
Optional.empty
Optional.empty
Optional.empty
Optional.empty
OptionalDouble.empty
*/

case 2:

// streams/OptionalBasics.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

import java.util.*;
import java.util.stream.*;

class OptionalBasics {
  static void test(Optional<String> optString) {
    if (optString.isPresent()) {
      System.out.println(optString.get());
    } else {
      System.out.println("Nothing inside!");
    }
  }

  public static void main(String[] args) {
    test(Stream.of("Epithets").findFirst());
    test(Stream.<String>empty().findFirst());
  }
}
/* Output:
Epithets
Nothing inside!
*/

case 3:

convenience functions

// streams/Optionals.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Optionals {
  static void basics(Optional<String> optString) {
    if (optString.isPresent()) {
      System.out.println(optString.get());
    } else {
      System.out.println("Nothing inside!");
    }
  }

  static void ifPresent(Optional<String> optString) {
    optString.ifPresent(System.out::println);
  }

  static void orElse(Optional<String> optString) {
    System.out.println(optString.orElse("Nada"));
  }

  static void orElseGet(Optional<String> optString) {
    System.out.println(optString.orElseGet(() -> "Generated"));
  }

  static void orElseThrow(Optional<String> optString) {
    try {
      System.out.println(optString.orElseThrow(() -> new Exception("Supplied")));
    } catch (Exception e) {
      System.out.println("Caught " + e);
    }
  }

  static void test(String testName, Consumer<Optional<String>> cos) {
    System.out.println(" === " + testName + " === ");
    cos.accept(Stream.of("Epithets").findFirst());
    cos.accept(Stream.<String>empty().findFirst());
  }

  public static void main(String[] args) {
    test("basics", Optionals::basics);
    test("ifPresent", Optionals::ifPresent);
    test("orElse", Optionals::orElse);
    test("orElseGet", Optionals::orElseGet);
    test("orElseThrow", Optionals::orElseThrow);
  }
}
/* Output:
 === basics ===
Epithets
Nothing inside!
 === ifPresent ===
Epithets
 === orElse ===
Epithets
Nada
 === orElseGet ===
Epithets
Generated
 === orElseThrow ===
Epithets
Caught java.lang.Exception: Supplied
*/

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/streams/OptionalsFromEmptyStreams.java

3. https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#empty--

4. https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html#toString--

5. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Optional.java

6. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/streams/OptionalBasics.java

7. https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html#isPresent--