日拱一卒：Lamdba 之 Optional 操作

1. Optional 类

public final class Optional<T> {
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

    public T orElse(T other) {
        return value != null ? value : other;
    }

    public T orElseGet(Supplier<? extends T> other) {
        return value != null ? value : other.get();
    }
}

1.1 Optional to Stream

@Test
public void optionalToStream() {
    List<OrderDTO> orders = Stream.of(new OrderDTO("A001", "1001", 0),
                    new OrderDTO("A002", "1002", 1),
                    new OrderDTO("A003", "1003", 1))
            .collect(Collectors.toList());

    String orderCode = Optional.ofNullable(orders)
            .orElse(Collections.emptyList())
            .stream()
            .findFirst()
            .map(OrderDTO::getOrderCode)
            .orElse(null);

    String orderCode2 = Optional.ofNullable(orders)
            .orElseGet(Collections::emptyList)
            .stream()
            .findFirst()
            .map(OrderDTO::getOrderCode)
            .orElse(null);

    // if (Optional.ofNullable(orders).isPresent()) {
    if (orders != null) {
        String orderCode3 = Optional.ofNullable(orders)
                .get()
                .stream()
                .findFirst()
                .map(OrderDTO::getOrderCode)
                .orElse(null);
    }
}

image.png

1.2 orElseGet(Supplier)

@Test
public void optionalElse() {
    OrderDTO orderDTO = new OrderDTO("A001", "1001");
    String orderCode = Optional.ofNullable(orderDTO)
            .orElseGet(OrderDTO::new)
            .getOrderCode();
}

2. map()

@Test
public void optionalMap() {
    List<OrderDTO> orders = Stream.of(new OrderDTO("A001", "1001", 0),
                    new OrderDTO("A002", "1002", 1),
                    new OrderDTO("A003", "1003", 1))
            .collect(Collectors.toList());

    String orderCode = Optional.ofNullable(orders)
            .map(list -> list.stream().findFirst().orElse(new OrderDTO()))
            .map(OrderDTO::getOrderCode)
            .orElse(null);
}

Optional.map() 源码

public final class Optional<T> {
    public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent())
            return empty();
        else {
            return Optional.ofNullable(mapper.apply(value));
        }
    }
}