Java 8 Optional类 · BeginnersBook 中文系列教程

# Java 8 `Optional`类 > 原文： [https://beginnersbook.com/2017/10/java-8-optional-class/](https://beginnersbook.com/2017/10/java-8-optional-class/) 在 Java 8 中，我们在`java.util`包中新引入了`Optional`类。引入此类是为了避免在我们的代码中不执行空检查时经常遇到的`NullPointerException`。使用这个类，我们可以很容易地检查变量是否具有`null`值，通过这样做，我们可以避免`NullPointerException`。在本指南中，我们将了解如何使用`Optional`类以及此类的各种方法的用法。在我们看到`Optional`类的示例之前，让我们看看当我们不使用`Optional`类并且不执行`null`检查时会发生什么。 ## Java 示例：不使用`Optional`类在这个例子中，我们没有将值赋给`String str`，我们试图从中获取[子串](https://beginnersbook.com/2013/12/java-string-substring-method-example/)。由于`str`中没有值，程序抛出`NullPointerException`。 ```java public class Example { public static void main(String[] args) { String[] str = new String[10]; //Getting the substring String str2 = str[9].substring(2, 5); //Displaying substring System.out.print(str2); } } ``` 输出： ```java Exception in thread "main" java.lang.NullPointerException at Example.main(Example.java:5) ``` ## 解决方案：使用`Optional`类 `Optional`类的`Optional.ofNullable()`方法，如果给定对象有值，则返回非空`Optional`，否则返回空`Optional`。我们可以使用`isPresent()`方法检查返回的`Optional`值是空还是非空。 ```java //Importing Optional class import java.util.Optional; public class Example { public static void main(String[] args) { String[] str = new String[10]; Optional<String> isNull = Optional.ofNullable(str[9]); if(isNull.isPresent()){ //Getting the substring String str2 = str[9].substring(2, 5); //Displaying substring System.out.print("Substring is: "+ str2); } else{ System.out.println("Cannot get the substring from an empty string"); } str[9] = "AgraIsCool"; Optional<String> isNull2 = Optional.ofNullable(str[9]); if(isNull2.isPresent()){ //Getting the substring String str2 = str[9].substring(2, 5); //Displaying substring System.out.print("Substring is: "+ str2); } else{ System.out.println("Cannot get the substring from an empty string"); } } } ``` 输出： ```java Cannot get the substring from an empty string Substring is: raI ``` ## 示例：`Optional`的`isPresent()`和`ifPresent()`方法在上面的例子中，我们已经看到通过使用`isPresent()`方法，我们可以检查特定的`Optional`对象（或实例）是空还是非空。 `Optional`类中还有另一种方法，只有在给定的`Optional`对象为非空时才执行，方法为`ifPresent()`。让我们看一个例子来理解差异。 ```java //Importing Optional class import java.util.Optional; public class Example { public static void main(String[] args) { //Creating Optional object from a String Optional<String> GOT = Optional.of("Game of Thrones"); //Optional.empty() creates an empty Optional object Optional<String> nothing = Optional.empty(); /* isPresent() method: Checks whether the given Optional * Object is empty or not. */ if (GOT.isPresent()) { System.out.println("Watching Game of Thrones"); } else { System.out.println("I am getting Bored"); } /* ifPresent() method: It executes only if the given Optional * object is non-empty. */ //This will print as the GOT is non-empty GOT.ifPresent(s -> System.out.println("Watching GOT is fun!")); //This will not print as the nothing is empty nothing.ifPresent(s -> System.out.println("I prefer getting bored")); } } ``` 输出： ```java Watching Game of Thrones Watching GOT is fun! ``` ## Java 8 - `Optional`的`orElse()`和`orElseGet()`方法如果该对象为空，则这两个方法`orElse()`和`orElseGet()`返回`Optional`对象的值，如果该对象为空，则返回作为参数传递给此方法的默认值。 ```java //Importing Optional class import java.util.Optional; public class Example { public static void main(String[] args) { //Creating Optional object from a String Optional<String> GOT = Optional.of("Game of Thrones"); //Optional.empty() creates an empty Optional object Optional<String> nothing = Optional.empty(); //orElse() method System.out.println(GOT.orElse("Default Value")); System.out.println(nothing.orElse("Default Value")); //orElseGet() method System.out.println(GOT.orElseGet(() -> "Default Value")); System.out.println(nothing.orElseGet(() -> "Default Value")); } } ``` 输出： ```java Game of Thrones Default Value Game of Thrones Default Value ``` ## Java 8 - `Optional.map`和`Optional.flatMap` 在这个例子中，我们将看到`Optional`如何与`map`和`flatMap`一起使用。 ```java //Importing Optional class import java.util.Optional; public class Example { public static void main(String[] args) { //Creating Optional object from a String Optional<String> GOT = Optional.of("Game of Thrones"); //Optional.empty() creates an empty Optional object Optional<String> nothing = Optional.empty(); System.out.println(GOT.map(String::toLowerCase)); System.out.println(nothing.map(String::toLowerCase)); Optional<Optional<String>> anotherOptional = Optional.of(Optional.of("BreakingBad")); System.out.println("Value of Optional object"+anotherOptional); System.out.println("Optional.map: " +anotherOptional.map(gender -> gender.map(String::toUpperCase))); //Optional<Optional<String>> -> flatMap -> Optional<String> System.out.println("Optional.flatMap: " +anotherOptional.flatMap(gender -> gender.map(String::toUpperCase))); } } ``` 输出： ```java Optional[game of thrones] Optional.empty Value of Optional objectOptional[Optional[BreakingBad]] Optional.map: Optional[Optional[BREAKINGBAD]] Optional.flatMap: Optional[BREAKINGBAD] ``` ## 示例：`Optional`和过滤器在这个例子中，我们将看到`Optional`如何与过滤器一起使用。要阅读有关过滤器的信息，请参阅本指南： [Java 过滤器](https://beginnersbook.com/2017/10/java-8-stream-filter/)。 **有关过滤器的更多教程：** 1. [Java - 过滤映射](https://beginnersbook.com/2017/10/java-8-filter-a-map-by-keys-and-values/) 2. [从 Java 的流中过滤空值](https://beginnersbook.com/2017/10/java-8-filter-null-values-from-a-stream/) ```java //Importing Optional class import java.util.Optional; public class Example { public static void main(String[] args) { //Creating Optional object from a String Optional<String> GOT = Optional.of("Game of Thrones"); /* Filter returns an empty Optional instance if the output doesn't * contain any value, else it returns the Optional object of the * given value. */ System.out.println(GOT.filter(s -> s.equals("GAME OF THRONES"))); System.out.println(GOT.filter(s -> s.equalsIgnoreCase("GAME OF THRONES"))); } } ``` 输出： ```java Optional.empty Optional[Game of Thrones] ``` **参考文献：** [Java 8 - `Optional`类 JavaDoc](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html)