java8中的Optional容器类，避免空指针异常

public class Person {
    private Integer age;
    private Double salary;
    public Person() {
    }
    public Person(Integer age, double salary) {
        this.age = age;
        this.salary = salary;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public double getSalary() {
        return salary;
    }
    public void setSalary(double salary) {
        this.salary = salary;
    }
    @Override
    public String toString() {
        return "Person{" +
                "age=" + age +
                ", salary=" + salary +
                '}';
    }
}
//of方法 创建一个Optional对象
        Optional<Person> of = Optional.of(new Person());
        Optional<Person> of1 = Optional.of(null);
        System.out.println(of.get());
        System.out.println(of1.get());//java.lang.NullPointerException
        //empty方法 创建一个空的Optional实例
        Optional<Person> empty = Optional.empty();
        System.out.println(empty.get());  //java.util.NoSuchElementException: No value present
        //ofNullable(T t) 如果T不为空，创建Optional实例，如果为空，创建空实例，下面就是这个方法的实现，综合了上面两个方法
//        public static <T> Optional<T> ofNullable(T value) {
//            return value == null ? empty() : of(value);
//        }
        Optional<Person> ofNullable = Optional.ofNullable(new Person());
        //isPersent方法 判断是否包含空值
        Optional<Person> isPer= Optional.ofNullable(new Person());
            //如果不为空 输出
        if(isPer.isPresent()){
            System.out.println(isPer.get());
        }
        //orElse(T t) 如果调用对象包含值 返回该值，否则返回t
        Optional<Person> orEl = Optional.ofNullable(null);
        System.out.println(orEl.orElse(new Person(12, 222))); //输出 Person{age=12, salary=222.0}
        //orElseGet(Supplier s) 如果调用对象包含值 返回该值，否则返回s获取的值
        Optional<Object> orEG = Optional.ofNullable(null);
        System.out.println(orEG.orElseGet(() -> new Integer(33))); //输出33
        //map(Function f) 如果有值 对其处理，并返回处理后的Optional，否则返回Optional.empty()
        Optional<Person> oMap = Optional.ofNullable(new Person(12, 2222));
        Optional<Integer> oAge = oMap.map(e -> e.getAge());
        System.out.println(oAge.get()); //输出12
        //flatMap(Function mapper) 与map类似，就是返回值必须是Optional   进一步避免空指针异常
        Optional<Person> oFlatMap = Optional.ofNullable(new Person(12, 2222));
        Optional<Integer> oFlatAge = oMap.flatMap(e -> Optional.of(e.getAge()));
        System.out.println(oAge.get());  //输出12

public class Teacher {
    private Optional<Student> student = Optional.empty();
    public void setStudent(Optional<Student> student) {
        this.student = student;
    }
    public Optional<Student> getStudent() {
        return student;
    }
}

public class Student {
    private String name;
    public Student(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

public static void main(String[] args) {
        Optional<Teacher> op = Optional.ofNullable(null);
        String studentName = getStudentName(op);
        System.out.println(studentName);
    }
    public static String getStudentName(Optional<Teacher> teacher){
        return teacher.orElse(new Teacher())
                .getStudent()
                .orElse(new Student("小明"))
                .getName();
    }