今天 code review 的时候看到总监用 Java 8 stream 写了一个神奇的流程控制代码. 试了一下, 有点神奇, 像是不断处理, 每一步都可能失败, 最后有一个 fallback 来救场. 这里是一个示例:
// 保存为 StreamTest.java, 然后用下面两句编译运行
// $ javac StreamTest.java
// $ java StreamTest
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
public class StreamTest {
static String testStreamFallback(Integer number) {
String ret = Optional.ofNullable(number)
.flatMap(n -> Optional.ofNullable(n % 5 == 0 ? null : n))
.flatMap(n -> Optional.ofNullable(n % 3 == 0 ? null : n))
.map(score -> score % 2 != 0 ? "not x2, x3, x5" : "not x3, x5")
.orElse(number == null ? "null" : "x3 or x5");
return String.format("%s is %s", number, ret);
}
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(
0, 1, 2, 3, 4,
5, 6, 7, 8, 9,
null
);
System.out.println(String.join("\n",
numbers.stream()
.map(i -> testStreamFallback(i))
.collect(Collectors.toList()))
);
}
}
编译运行:
▶ javac StreamTest.java
▶ java StreamTest
0 is x3 or x5
1 is not x2, x3, x5
2 is not x3, x5
3 is x3 or x5
4 is not x3, x5
5 is x3 or x5
6 is x3 or x5
7 is not x2, x3, x5
8 is not x3, x5
9 is x3 or x5
null is null
flatMap 的文档:
/**
* If a value is present, apply the provided {@code Optional}-bearing
* mapping function to it, return that result, otherwise return an empty
* {@code Optional}. This method is similar to {@link #map(Function)},
* but the provided mapper is one whose result is already an {@code Optional},
* and if invoked, {@code flatMap} does not wrap it with an additional
* {@code Optional}.
*
* @param <U> The type parameter to the {@code Optional} returned by
* @param mapper a mapping function to apply to the value, if present
* the mapping function
* @return the result of applying an {@code Optional}-bearing mapping
* function to the value of this {@code Optional}, if a value is present,
* otherwise an empty {@code Optional}
* @throws NullPointerException if the mapping function is null or returns
* a null result
*/
public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Objects.requireNonNull(mapper.apply(value));
}
}
从 stream 的角度, 经过 mapper 的时候, 如果这个 optional 元素实际为 null, 返回 optional null (不作处理), 否则取出实际内容, 扔到 mapper 函数, 然后得到另一个 optional. 这个 stream 一直往后流, 直到 orElseGet.
说错了, 这不是 stream 的 fallback, 是 optional 的 fallback. Anyway, 不改了.
more to read:
