Java 流 .orElseThrow
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/34349261/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Java streams .orElseThrow
提问by spakai
I want convert a piece of code from a Connection Pool project i have been working on to use streams
我想从我一直致力于使用流的连接池项目中转换一段代码
the original code is
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");
I have got to the point of this
我已经到了这个地步
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;
The above can compile but the moment i add orElseThrowafter IfPresenti am getting the following
以上可以编译，但orElseThrow在IfPresent我得到以下内容后添加的那一刻
/home/prakashs/connection_pool/src/main/java/com/spakai/ConnectionPool.java:83: error: void cannot be dereferenced
.orElseThrow(ConnectionPoolException::new);
采纳答案by David Conrad
That's because ifPresentreturns void. It can't be chained. You could do something like:
那是因为ifPresent返回无效。不能上链。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());
What you were looking for would read well:
您正在寻找的内容会很好读：
.findFirst().ifPresent(value -> use(value)).orElseThrow(Exception::new);
But for it to work, ifPresentwould have to return the Optional, which would be a little odd. It would mean you could chain one ifPresentafter another, doing multiple operations on the value. That might have been a good design, but it isn't the one the creators of Optionalwent with.
但是要让它工作，ifPresent就必须返回Optional，这有点奇怪。这意味着你可以一个ifPresent接一个地链接起来，对值进行多次操作。这可能是一个很好的设计，但它不是创造者所Optional采用的设计。
回答by ZoltanTheHun
Use map instead of isPresent, and return with an Optional instead of an exception.
使用 map 而不是 isPresent，并返回一个 Optional 而不是异常。
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;
})
