Java streams .orElseThrow

Question

我想从连接池项目转换一段代码，我一直在使用流

原始代码是

for (Map.Entry<JdbConnection,Instant> entry : borrowed.entrySet()) {
  Instant leaseTime = entry.getValue();
  JdbConnection jdbConnection = entry.getKey();
  Duration timeElapsed = Duration.between(leaseTime, Instant.now());
  if (timeElapsed.toMillis() > leaseTimeInMillis) {
    //expired, let's close it and remove it from the map
    jdbConnection.close();
    borrowed.remove(jdbConnection);

    //create a new one, mark it as borrowed and give it to the client
    JdbConnection newJdbConnection = factory.create();
    borrowed.put(newJdbConnection,Instant.now());
    return newJdbConnection;
  }
}

throw new ConnectionPoolException("No connections available");

我已经说到这一点了

borrowed.entrySet().stream()
                   .filter(entry -> Duration.between(entry.getValue(), Instant.now()).toMillis() > leaseTimeInMillis)
                   .findFirst()
                   .ifPresent(entry -> {
                     entry.getKey().close();
                     borrowed.remove(entry.getKey());
                   });


JdbConnection newJdbConnection = factory.create();
borrowed.put(newJdbConnection,Instant.now());
return newJdbConnection;

上面的代码可以编译，但当我在IfPresent之后添加orElseThrow时，我会得到以下结果

/home/prakashs/connection_pool/src/main/java/com/spakai/ConnectionPool.java:83: error: void cannot be dereferenced
                       .orElseThrow(ConnectionPoolException::new);

Answer 1

这是因为ifPresent返回空值。它不能被链式连接。你可以这样做：

Entry<JdbConnection, Instant> entry =
    borrowed.entrySet().stream()
        .filter(entry -> Duration.between(entry.getValue(), Instant.now())
                            .toMillis() > leaseTimeInMillis)
        .findFirst()
        .orElseThrow(ConnectionPoolException::new));
entry.getKey().close();
borrowed.remove(entry.getKey());

你正在寻找的东西读起来会很好：

.findFirst().ifPresent(value -> use(value)).orElseThrow(Exception::new);

但要让它正常工作，ifPresent必须返回Optional，这可能有点奇怪。这意味着您可以将一个ifPresent链接到另一个，对值执行多个操作。这可能是一个很好的设计，但它不是Optional的创建者所采用的设计。

Answer 2

使用map而不是isPresent，返回一个可选的值而不是一个异常。

borrowed.entrySet().stream()
               .filter(entry -> Duration.between(entry.getValue(), Instant.now()).toMillis() > leaseTimeInMillis)
               .findFirst()
               .map(entry -> {
                 entry.getKey().close();
                 borrowed.remove(entry.getKey());
                 JdbConnection newJdbConnection = factory.create();
                 borrowed.put(newJdbConnection,Instant.now());
                 return newJdbConnection;
               })