Java8 Optional库实用操作【简化if else】

本文介绍一下Optional在实际开发应用中的使用情况

一、概述

官方API-Optional。
Optional是JKD1.8发布的一个类库，主要是解决烦人的NullPointerException问题以及if、else，提升代码简洁度。

二、应用

ofNullable

static <T> Optional<T> ofNullable(T value)
返回一个T类型的Optional对象

String string = "123";
Optional<String> stringOptional = Optional.ofNullable(string);

//输出：Optional[123]

---

ifPresent

void ifPresent(Consumer<? super T> consumer)

String str2 = "二哈";
Optional.ofNullable(str2).ifPresent(var -> {
    System.out.println(var);
});

//输出：二哈

//等同于：

if (Objects.nonNull(str2)) {
    System.out.println(str2);
}

---

orElse

T orElse(T other) 

String str2 = "二哈";
String result = Optional.ofNullable(str2).orElse("大哈");
System.out.println(result);

//输出：二哈

//等同于:

if (Objects.nonNull(str2)) {
    System.out.println(str2);
} else {
    System.out.println("大哈");
}

---

orElseGet

T orElseGet(Supplier<? extends T> other)

String str3 = null;
String result = Optional.ofNullable(str3).orElseGet(() -> {
    return "233";
});
System.out.println(result);

//输出：233

//等同于:
String str3 = null;
if (str3 == null) {
    return "233";
} else {
    return str3;
}

---

orElseThrow

 <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X

String str4 = "233";
String result2 = Optional.ofNullable(str4).orElseThrow(RuntimeException::new);
System.out.println(result2);

//输出：233

//等同于：

if (str4 != null) {
    System.out.println(str4);
} else {
    throw new RuntimeException();
}

---

map

<U> Optional<U> map(Function<? super T, ? extends U> mapper)

@Builder
@Data
public class Animal {
    //猫
    private Cat cat;
    //狗
    private Dog dog;
}

@Builder
@Data
public class Dog {
    //种类
    private String species;
    private Action action;
}

@Data
@Builder
public class Cat {
    //种类
    private String species;
    private Action action;
}

@Data
@Builder
public class Action {
    private String eat;
    private String sleep;
}

public class TestMain {
   public static void main(String[] args) {
      Action dogAction = Action.builder().eat("我喜欢吃狗粮").sleep("喜欢睡觉").build();
      Action catAction = Action.builder().eat("我喜欢吃猫粮").sleep("喜欢睡觉").build();
      Dog dog = Dog.builder().species("哈士奇").action(dogAction).build();
      Cat cat = Cat.builder().species("蓝猫").action(catAction).build();

      Animal animal = Animal.builder().cat(cat).dog(dog).build();

      //从对象里获取值
      Optional.ofNullable(animal)
          .map(Animal::getDog)
          .map(Dog::getAction)
          .ifPresent(var -> {
              System.out.println("【Optional】" + var.getSleep());
        });
    
      //等同于
      if (Objects.nonNull(animal)) {
          Dog dogD = animal.getDog();
          if (Objects.nonNull(dogD)) {
              Action actionA = dogD.getAction();
              if (Objects.nonNull(actionA)) {
                   System.out.println("【if】" + var.getSleep());
              }
          }
      }
   }
}

---

filter

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

String str = "233";
Optional.ofNullable(str).filter(var -> var.length() == 3).ifPresent(var-> {
    System.out.println("长度为3");
});

//输出：长度为3

//等同于：

if (null != str) {
    if (str.length() == 3) {
        System.out.println("长度为3");
    }
}

---

flatMap

<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper)

public class TestMain {
   public static void main(String[] args) {
    Optional.ofNullable(animal)
         .map(Animal::getDog)
         .flatMap(TestMain::getDogSleep)
         .ifPresent(var -> {
            System.out.println(var);
         });
     private static Optional<String> getDogSleep(Dog dog) {
         return Optional.ofNullable(dog).map(Dog::getAction).map(Action::getSleep);
     }
  }
}

...emmm这个我使用的不太多，因为他返回一个Optional<T>，所以可以像上边一样作代码抽取。