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背景

Optional是一个容器，用来管理一个对象。主要是用来解决null和NullPointerException问题。

Java7 -> Java8 Optional是如何简化你的代码

假设有一个Student类

@Datapublic class Student {
       private String name;    private Integer age;    public Student() {
       }    public Student(String name, Integer age) {
           this.name = name;        this.age = age;    }}

举例一

//java7    private String getJava7UpperCaseStudentName(Student student) {
           if (student != null) {
               String name = student.getName();            if (name != null) {
                   return name.toUpperCase();            }        }        return null;    }    //java8    private String getJava8UpperCaseStudentName(Student student) {
           return Optional.ofNullable(student).map(Student::getName).map(String::toUpperCase).orElse(null);    }

举例二

@Test    public void testOptionalOrElse() {
           Student origin = null;        Student defaultStudent = new Student("default", -1);        //java7        Student java7Student = origin != null ? origin : defaultStudent;        //java8        Student java8Student = Optional.ofNullable(origin).orElse(defaultStudent);        System.out.println("java7Student : " + java7Student);        System.out.println("java8Student : " + java8Student);    }

可见，使用Optional可以简化代码。

用法及测试

of()、ofNullable()和empty()

@Test    public void testEmptyOptional() {
           Optional
   
     emptyOptional = Optional.empty();        System.out.println("emptyOptional.isPresent:" + emptyOptional.isPresent());    }    @Test    public void testOptional() {
           Optional
    
      nullStudentOptional = Optional.ofNullable(null);        Optional
     
       studentOptional = Optional.of(new Student("a", 1));        System.out.println("nullStudentOptional:" + nullStudentOptional.orElse(null));        System.out.println("studentOptional:" + studentOptional.orElse(null));    }
     
    
   

输出结果

emptyOptional.isPresent:falsenullStudentOptional:nullstudentOptional:Student(name=a, age=1)

of()函数传入的Object不能为空，否则会直接抛出空指针异常

ofNullable()函数传入的Object可为空

empty()函数会创建一个空的Optional对象，其value为null

isPresent()

@Test    public void testOptionalIsPresent() {
           Optional
   
     nullStudentOptional = Optional.ofNullable(null);        Optional
    
      studentOptional = Optional.of(new Student("a", 1));        System.out.println("nullStudentOptional.isPresent():" + nullStudentOptional.isPresent());        System.out.println("studentOptional.isPresent():" + studentOptional.isPresent());    }
    
   

输出结果

nullStudentOptional.isPresent():falsestudentOptional.isPresent():true

isPresent()函数会判断当前Optional容器保存的对象是否不为空，true表示不为空，false表示为空

IfPresent()

@Test    public void testOptionalIfPresent() {
           Optional
   
     studentOptional1 = Optional.of(new Student("a", 1));        studentOptional1.ifPresent(student -> {
               System.out.println("accept student:" + student);            student.setAge(student.getAge() * 2);            student.setName(student.getName() + "-accept");        });        System.out.println("after accept student:" + studentOptional1.orElse(null));    }
   

运行结果

accept student:Student(name=a, age=1)after accept student:Student(name=a-accept, age=2)

orElseThrow()、orElseGet()

@Test    public void testOptionalOrElseThrow() {
           Optional
   
     studentOptional = Optional.ofNullable(null);        Student student = studentOptional.orElseThrow(() -> new NullPointerException("studentOptional is null"));        System.out.println("student : " + student);    }    @Test    public void testOptionalOrElseGet() {
           Optional
    
      studentOptional = Optional.ofNullable(null);        Student student = studentOptional.orElseGet(() -> new Student("zz", -1));        System.out.println("student : " + student);    }
    
   

输出结果

结果1：java.lang.NullPointerException: studentOptional is null	at com.common.java8.OptionalTest.lambda$testOptionalOrElseThrow$7(OptionalTest.java:56)	at java.util.Optional.orElseThrow(Optional.java:290)	at com.common.java8.OptionalTest.testOptionalOrElseThrow(OptionalTest.java:56)...结果2：student : Student(name=zz, age=-1)

map()、flatMap()

public Optional map(Function
      mapper) {
           Objects.requireNonNull(mapper);        if (!isPresent())            return empty();        else {
               return Optional.ofNullable(mapper.apply(value));        }    }    public Optional flatMap(Function
       > mapper) {
           Objects.requireNonNull(mapper);        if (!isPresent())            return empty();        else {
               return Objects.requireNonNull(mapper.apply(value));        }    }

从方法签名上看，两者返回参数相同，入参Function的返回参数不一样，一个是U，一个是Optional< U >, 但是他们都返回Optional< U >

@Test    public void testOptionalMapAndFlatMap() {
           Optional
   
     studentOptional = Optional.of(new Student("aa", 11));        System.out.println("map:" + studentOptional.map(Student::getName).orElse(null));        System.out.println("flatMap:" + studentOptional.flatMap(s -> Optional.of(s.getName())).orElse(null));        Optional
    
      nameNullOptional = Optional.of(new Student(null, 11));        System.out.println("map:" + nameNullOptional.map(Student::getName).orElse(null));        System.out.println("flatMap:" + nameNullOptional.flatMap(s -> Optional.ofNullable(s.getName())).orElse(null));        System.out.println("flatMap:" + nameNullOptional.flatMap(s -> null).orElse(null));    }
    
   

输出结果

map:aaflatMap:aamap:nullflatMap:nulljava.lang.NullPointerException	at java.util.Objects.requireNonNull(Objects.java:203)	at java.util.Optional.flatMap(Optional.java:241)	at com.common.java8.OptionalTest.testOptionalMapAndFlatMap(OptionalTest.java:23)

从上面这种用法两者基本相同。但是在Optional类中，两个方法真正的区别是flatMap返回的必须是一个非空Optional值，否则会抛出NullPointException.

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