Java流.orElseThrow

发布时间：2020-12-15 05:13:28 所属栏目：Java 来源：网络整理

导读：我想从我一直在努力使用流的连接池项目中转换一段代码原始代码是 for (Map.EntryJdbConnection,Instant entry : borrowed.entrySet()) { Instant leaseTime = entry.getValue(); JdbConnection jdbConnection = entry.getKey(); Duration timeElapsed = Dur

我想从我一直在努力使用流的连接池项目中转换一段代码

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");

我已经明白了这一点

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;

以上可以编译但是我在IfPresent之后添加orElseThrow的那一刻我得到以下内容

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

解决方法

那是因为ifPresent返回void.它无法链接.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());

你在寻找什么会读得很好：

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

但是为了工作,ifPresent必须返回Optional,这有点奇怪.这意味着您可以将一个ifPresent链接到另一个,对该值执行多个操作.这可能是一个很好的设计,但它不是Optional的创造者所采用的.

（编辑：李大同）

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