Java8 Optional类介绍

x33g5p2x  于2020-09-30 发布在 Java  
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1. Optional解决空指针问题

Optional: 进行非空判断,避免空指针。
典型应用场景:

1.1. ofNullable判空

很low的写法:

@Test
public void test4() {
    Employee employee = new Employee();
    if (employee != null) {
        if (employee.getName() != null) {
            System.out.println(employee.getName());
        }
    }
}

Optional写法

    @Test
    public void test3() {
        Employee employee = new Employee();
        Optional.ofNullable(employee).map(u -> u.getName()).orElse("UnKnow");

         Optional.ofNullable(employee).map(Employee::getName).orElse("UnKnow");
    }

1.2. ifPresent()替代if判断

    public static void printName(Employee employee){
        Optional.ofNullable(employee).ifPresent(u ->
                System.out.println("The employee name is:"+ employee.getName()) );
    }

    // 重点掌握
    @Test
    public void test1(){
        User user = new User(1,"hadoop");
        Optional.ofNullable(user).
                map(User::getName).
                ifPresent(u ->{
            System.out.println(u);
            // 一系列的操作
            System.out.println("一系列的操作");
        });
    }

【说明】
ifPresent()方法接受一个Consumer对象(消费函数)如果包装对象的值非空,运行Consumer对象的accept()方法

2. filter()方法筛选判断

很low的写法

    public static void filterAge(Employee employee){
        if(employee != null){
            if(employee.getAge() > 18){
                System.out.println("The student age is more than 18.");
            }
        }
    }

filter()引入

    public static void filterAge2(Employee employee){
        Optional.ofNullable(employee).
                filter(u -> u.getAge() > 18).
                ifPresent(u -> System.out.println("The student age is more than 18."));
    }

【说明】
filter()方法接受参数为Predicate对象,用于对Optional对象进行过滤,如果符合Predicate的条件,返回Optional对象本身,否则返回一个空的Optional对象。

3. map()方法

map()方法的参数为Function(函数式接口)对象,map()方法将Optional中的包装对象用Function函数进行运算,并包装成新的Optional对象(包装对象的类型可能改变)。

    public static Optional<Integer> searchAge(Employee employee){
        return Optional.ofNullable(employee).map(u -> u.getAge());
    }

【说明】
Optional<T> 类型的变量如何使用:

    @Test
    public void test5(){
        Optional<Employee> op = Optional.of(new Employee());
        Employee emp = op.get();
        System.out.println(emp);
    }

    @Test
    public void test6(){
        Optional<Integer> opInt = Optional.of(1);
        System.out.println(opInt.get());
    }

4. orElseGet()和orElse()

orElseGet()方法与orElse()方法类似,区别在于orElseGet()方法的入参为一个Supplier对象,用Supplier对象的get()方法的返回值作为默认值.

    public static String getName(Employee employee){
        return Optional.ofNullable(employee)
                .map(u -> u.getName())
                .orElseGet(() -> "Unknow");
    }
    
    public static String getName2(Employee employee){
        return Optional.ofNullable(employee)
                .map(u -> u.getName())
                .orElse("Unknow");
    }

5. orElseThrow()方法

orElseThrow()方法其实与orElseGet()方法非常相似了,入参都是Supplier对象,只不过orElseThrow()的Supplier对象必须返回一个Throwable异常,并在orElseThrow()中将异常抛出

public static String getGender1(Student student)
    {
        return Optional.ofNullable(student).map(u -> u.getGender()).orElseThrow(() -> new RuntimeException("Unkown"));      
    }

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