[WIP] Java Optional

创建: 2021/10/4

 

想想当年遇到新的，都会系统看一遍，把扁平的文字总结成一整张表格一样的博客。这一年很少能静下来这么做了，年纪也增长了不少，那种单纯的总结技术的时代似乎渐行渐远了。

不管怎样吧，从今天开始，以后也要静下心老老实实总结。

 

references: 

https://www.baeldung.com/java-optional

https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html

 

 assign

 

static Optional<T> empty()	Returns an empty Optional instance.
static Optional<T> of()	Returns an Optional with the specified present non-null value.
static Optional<T> ofNullable()	Returns an Optional describing the specified value, if non-null, otherwise returns an empty Optional.

 Value Presense

 

isPresent

boolean isPresent()	This method returns true if the wrapped value is not null.
void ifPresent(Consumer<? super T> consumer)

isEmpty

>= Java 11

Default Value

 

T orElse(T other)
T orElseGet(Supplier<? extends T> other)
<X extends Throwable>T orElseThrow(Suppiler<? extend X> exceptionSupplier)

get 

 

T get()

If a value is present in this Optional, returns the value, otherwise throws NoSuchElementException.

filter

 

Optional<T> filter(Predicate<? super T> predicate)

If a value is present, and the value matches the given predicate, return an Optional describing the value, otherwise return an empty Optional.

map and flatMap

 

<U> Optional<U> map(Function(? super T, ? extends U>)mapper)	If a value is present, apply the provided mapping function to it, and if the result is non-null, return an Optional describing the result.
<U> Optional<U> flatMap(Function(? super T, ? extends Optional<U>) mapper)	If a value is present, apply the provided Optional-bearing mapping function to it, return that result, otherwise return an empty Optional.

Chaining Options

in Java 8