Optional类

• 作用

主要解决的问题是空指针异常（NullPointerException）
本质是⼀个包含有可选值的包装类，这意味着 Optional 类既可以含有对象也可以为空

• of()

# null 值作为参数传递进去,则会抛异常
 
import java.util.Optional;
 
public class Main {
    public static void main(String[] args) throws Exception {
        # 会抛出异常
        Student student = null
        Optional<Student> opt = Optional.of(student);
        
        # 不会抛出异常
        Student student = new Student();
        Optional<Student> opt = Optional.of(student);
    }
}

• ofNullable()

# 如果对象即可能是 null 也可能是⾮ null，应该使⽤ ofNullable() ⽅法
# 如果值存在则isPresent()⽅法会返回true，调⽤get()⽅法会返回该对象⼀般使⽤get之前需要先验证是否有值，不然还会报错
 
import java.util.Optional;
 
public class Main {
    public static void main(String[] args) throws Exception {
        Student student = new Student();
        Optional<Student> opt = Optional.ofNullable(student);
        if(opt.isPresent()){
            System.out.println("optional不为空");
            Student s = opt.get();
        }else {
            System.out.println("optional为空");
        }
    }
}

• orElse()

# 设置默认值，用于兜底
# 如果有值则返回该值，否则返回传递给它的参数值
 
import java.util.Optional;
 
public class Main {
    public static void main(String[] args) throws Exception {
        Student student1 = null;
        Student student2 = new Student(2);
        Student result = Optional.ofNullable(student1).orElse(student2);
        System.out.println(result.getAge());
    }
}

• 写法2

import java.util.Optional;
 
public class Main {
    public static void main(String[] args) throws Exception {
        Student student = null;
        int result = Optional.ofNullable(student).map(obj->obj.getAge()).orElse(4);
        System.out.println(result);
    }
}