Optional

转为Optinal对象 car为null也行
Optional<Car> optionalCar=Optional.ofNullable(car);

//人不一定有车
public class Person {

    private Optional<Car> car;

    public Optional<Car> getCar() {
        return car;
    }
}

//车没保险也正常
public class Car {

    private Optional<Insurance> insurance;

    public Optional<Insurance> getInsurance() {

        return insurance;
    }
}

//保险一定有名字!
public class Insurance {

    private String name;

    public String getName() {
        return name;
    }
}

层层get可能为空 拿到最里面的值

 public String getCarInsuranceName(Optional<Person> optionalPerson) {

//不能用map 会包2层Optional,因为getCar返回Optional<Car>
      //  Optional<Optional<Car>> car = optionalPerson.map(Person::getCar);
        
        return optionalPerson
                     .flatMap(Person::getCar)
                     .flatMap(Car::getInsurance)
                     .map(Insurance::getName)
                     .orElse("Unknown");//没有值时的默认值
    }

2个Optional都不null才执行xxx

Optional<Insurance> findCheapestInsurance(Optional<Person> optionalPerson,Optional<Car> optionalCar){
        
        return optionalPerson
                //如果是null不会调用
                .flatMap(p->optionalCar
                        //如果是null不会调用
                        .map(c->xxx(p,c))
                );
    }

    //2个都不是null才会执行到这
    private Insurance xxx(Person p, Car c) {
        //todo;
    }

如果不是null,并且满足条件,才执行....

Optional<Insurance> optInsurance = Optional.ofNullable(new Insurance());
        optInsurance
                 //存在并且满足条件,才会返回原值 不能后面就是null
                .filter(insurance -> "某名字".equals(insurance.getName()))
                .ifPresent(insurance->System.out.println("就是"));

string 转 Optional<Integer>

  public static  Optional<Integer> string2Integer(String str){
        try {
            return Optional.of(Integer.parseInt(str));
        } catch (NumberFormatException e) {
            return Optional.empty();
        }
    }