Optional类

Optional类是为了尽可能的避免空指针

常用方法：

Optional.of(T t) : 创建一个 Optional 实例

Optional<Employee> op = Optional.of(new Employee());
Employee emp = op.get();
System.out.println(emp);

Optional.empty() : 创建一个空的 Optional 实例

	Optional<Employee> op = Optional.empty();
	System.out.println(op.get());//java.util.NoSuchElementException: No value present

Optional.ofNullable(T t):若 t 不为 null,创建 Optional 实例,否则创建空实例

Optional<Employee> op = Optional.ofNullable(null);
System.out.println(op.get());//java.util.NoSuchElementException: No value present

isPresent() : 判断是否包含值

Optional<Employee> op = Optional.ofNullable(new Employee());
if(op.isPresent()){//若op有值
	System.out.println(op.get());
}

orElse(T t) : 如果调用对象包含值，返回该值，否则返回t

Optional<Employee> op = Optional.ofNullable(new Employee());
//若上方op中有值了就返回值，没值就默认new一个张三
Employee emp = op.orElse(new Employee("张三"));
System.out.println(emp);

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<Employee> op = Optional.ofNullable(new Employee());
//若上方op中有值了就返回值，没值就默认new一个张三
Employee emp2 = op.orElseGet(() -> new Employee("张三"));//函数式接口
System.out.println(emp2);

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<Employee> op = Optional.of(new Employee(101, "张三", 18, 9999.99));		
Optional<String> op2 = op.map(Employee::getName);
System.out.println(op2.get());//张三

flatMap(Function mapper):与 map 类似，要求返回值必须是Optional

Optional<Employee> op = Optional.of(new Employee(101, "张三", 18, 9999.99));
Optional<String> op3 = op.flatMap((e) -> Optional.of(e.getName()));
System.out.println(op3.get());