深入学习Java8中的Optional

什么是Optional？

Optional是java8中引入的一个包含或者不包含一个非空对象的容器对象

Optional有哪些方法？

of(T)与ofNullable(T)方法

两者区别，ofNullable可以传null，of方法不能传null

• 使用of方法传null

@Test
    public void testOptionalNull() {
        Optional<Object> objectOptional = Optional.of(null);
    }

当使用of方法包装空对象时会抛出NullPointException

get()方法

• 使用Optional包装非空对象，调用get方法

    private User user;

    @BeforeEach
    public void createUser() {
        user = User.builder().id(1L)
                .name("user1")
                .build();
    }

    @Test
    public void testOptionalOf() {
        Optional<User> userOptional = Optional.of(this.user);
        User user = userOptional.get();
        System.out.println("get:" + user);
    }

执行结果：

• 使用Optional包装空对象，调用get方法

@Test
public void testOptional() {
    Optional<User> emptyObj = Optional.empty();
    User user = emptyObj.get();
    System.out.println(user);
}

执行结果,会抛出NoSuchElementException

isPresent()方法

    private User user;

    @BeforeEach
    public void createUser() {
        user = User.builder().id(1L)
                .name("user1")
                .build();
    }

    @Test
    public void testOptionalIsPresent() {
        Optional<User> userOptional = Optional.ofNullable(this.user);
        System.out.println("user isPresent:" + userOptional.isPresent());

        System.out.println("=============");

        Optional<Object> emptyOptional = Optional.ofNullable(null);
        System.out.println("Empty isPresent:" + emptyOptional.isPresent());
    }

执行结果：

ifPresent(Consumer<? super T>)方法

    private User user;

    @BeforeEach
    public void createUser() {
        user = User.builder().id(1L)
                .name("user1")
                .build();
    }

    @Test
    public void testOptionalIfPresent() {
        Optional<User> userOptional = Optional.ofNullable(this.user);

        userOptional.ifPresent(user -> System.out.println("Optional value存在：" + user));

        System.out.println("=============");

        Optional<Object> emptyOptional = Optional.ofNullable(null);
        emptyOptional.ifPresent(obj -> System.out.println("Optional value存在：" + obj));
    }

执行结果：

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

    private User user;

    @BeforeEach
    public void createUser() {
        user = User.builder().id(1L)
                .name("user1")
                .build();
    }

    @Test
    public void testOptionalMap() {
        String name = Optional.of(user).map(User::getName).get();
        log.info("name:{}", name);
    }

执行的结果：

orElse(T) 与 orElseGet(Supplier<? extends T>)的区别

    private User user;

    @BeforeEach
    public void createUser() {
        user = User.builder().id(1L)
                .name("user1")
                .build();
    }

    @Test
    public void testOptionalOrElse() {
        User user = Optional.of(this.user).orElse(this.createNewUser(10L));
        System.out.println("or else user:" + user);
        User user1 = Optional.of(this.user).orElseGet(() -> this.createNewUser(10L));
        System.out.println("or else get user:" + user1);

        System.out.println("===================");
        User user2 = Optional.<User>empty().orElse(this.createNewUser(10L));
        System.out.println("empty or else user:" + user2);
        User user3 = Optional.<User>empty().orElseGet(() -> this.createNewUser(10L));
        System.out.println("empty or else get user:" + user3);

    }

    private User createNewUser(Long id) {
        System.out.println("Create new user!");
        return User.builder()
                .id(id)
                .name("user" + id)
                .build();
    }

执行结果：

当Optional包含的对象都是空时，orElse与orElseGet方法都会返回参数中获取到的对象，区别是当Optional包含的对象不为空时，orElseGet中createNewUser是懒加载的

orElseThrow(Supplier<? extends X>)方法

    @Test
    public void testOptionalOrElseThrow() {
        User user = null;
        Object o = Optional.ofNullable(user).orElseThrow(IllegalArgumentException::new);
    }

执行的结果：

filter(Predicate<? super T>)方法

    @Test
    public void testOptionalFilter() {
        Optional<User> userOptional = Optional.of(this.user);
        Optional<User> filterOptional = userOptional.filter(user -> user.getName() != null && user.getName().contains("1"));
        User user = filterOptional.get();
        System.out.println("filter存在的结果：" + user);
        System.out.println("==============");

        Optional<User> emptyOptional = Optional.empty();
        Optional<User> filterEmptyOptional = emptyOptional.filter(emptyUser -> emptyUser.getName() != null && emptyUser.getName().contains("1"));
        User emptyUser = filterEmptyOptional.get();
        System.out.println("empty filter存在的结果：" + emptyUser);

    }

执行结果：

文章永久链接：https://tech.souyunku.com/26521