java - 如何以 Java 8 函数式风格重写仅一个变量类型不同的两个代码分支？

Question

我想以函数式风格重写以下逻辑，即使用 map、filter、ifPresent 编写单行逻辑、orElse、orElseGet、orElseThrow 等

String id;
if (entity.legacyIndicator().isPresent()) { // field is an Optional<Foo>
  if (entity.legacyId().isPresent()) { // field is an Optional<Long>
    id = entity.legacyId().toString(); // e.g. id = '01234567'
  } else {
    throw new IOException("No ID found.");
  }
} else {
  if (entity.newId().isPresent()) { // field is an Optional<UUID>
    id = entity.newId().toString(); // e.g. id = '01234567-89ab-cdef-0123-456789abcdef'
  } else {
    throw new IOException("No ID found.");
  }
}

对我来说主要的麻烦是 legacyId() 和 newId() 是不同的类型。作为程序员，我知道它们都共享 isPresent() 和 toString()，但不共享实际的接口(interface)，那么如何“统一”这两个分支？

我很快就会尽最大努力编辑这个问题，因为我仍在尽我所能。但我对统一分支完全感到困惑。

Answer 1

好吧，不是很干净，但我会选择类似的东西

    public String getId(Test.Entity entity) throws Throwable {
    return entity.legacyIndicator()
        .map(o -> (Optional) entity.legacyId())
        .orElseGet(() -> (Optional) entity.newId())
        .orElseThrow(() -> new IOException("No ID found."))
        .toString();
}

这里有一些 Junit 测试来验证执行

@org.junit.Test
public void test() throws Throwable {
    Entity entity = new Entity();

    entity.legacyIndicator = Optional.empty();
    final UUID uuid = UUID.randomUUID();
    entity.newId = Optional.of(uuid);

    String id = getId(entity);
    Assert.assertEquals(uuid.toString(), id);


    entity = new Entity();

    entity.legacyIndicator = Optional.of(Boolean.TRUE);
    entity.newId = Optional.of(uuid);
    entity.legacyId = Optional.of("SomeId");
    id = getId(entity);

    Assert.assertEquals("SomeId", id);
}

@org.junit.Test(expected = IOException.class)
public void testExceptionLegacy() throws Throwable {
    Entity entity = new Entity();

    entity.legacyIndicator = Optional.of(Boolean.TRUE);
    entity.legacyId = Optional.empty();
    String id = getId(entity);
}

@org.junit.Test(expected = IOException.class)
public void testExceptionNew() throws Throwable {
    Entity entity = new Entity();

    entity.legacyIndicator = Optional.empty();
    entity.legacyId = Optional.empty();
    entity.newId = Optional.empty();
    String id = getId(entity);
}

class Entity {
    Optional<Boolean> legacyIndicator;
    Optional<String> legacyId;
    Optional<UUID> newId;

    Optional<Boolean> legacyIndicator() {
        return legacyIndicator;
    }

    Optional<UUID> newId() {
        return newId;
    }

    Optional<String> legacyId() {
        return legacyId;
    }

}