[英]Java streams: Optional to stream
這是我的代碼:
Optional<Application> application = this.applicationDao.findById(id);
其中, Application
類是:
public class Application {
private String code;
private Collection<ApplicationQuote> quotes;
}
我需要從return Optional<Application>
創建一個流,如下所示:
(app, quote-1) > (app, quote-2) > ... > (app, quote-n)
每個quote-n
在其中返回Optional<Application>.quotes
。
我希望我解釋得那么好。
到目前為止,我已經能夠編寫這段代碼了,但我覺得這並不舒服:
Optional<Application> application = this.applicationDao.findById(id);
application.map(app -> Pair.of(app, Optional.ofNullable(app.getQuotes())))
.filter(quote -> quote.getValue().isPresent())
.map(quote -> quote.getValue().get().stream().map(q -> Pair.of(quote.getKey(), q)));
你在這里過分復雜。 即你實際上不需要將app.quotes
包裝成一個可選項來檢查它是否為非null(實際上濫用了Optional的主要目的)等等......
因為你想要一個Stream<Pair<Application, ApplicationQuote>>
你可以這樣做:
JDK8:
application.filter(app -> app.getQuotes() != null)
.map(Stream::of).orElseGet(Stream::empty)
.flatMap(app -> app.getQuotes().stream().map(quote -> Pair.of(app, quote)));
JDK9:
application.filter(app -> app.getQuotes() != null).stream()
.flatMap(app -> app.getQuotes().stream().map(quote -> Pair.of(app, quote)));
Optional.orElse
理想情況下,您目前擁有的是Optional<Stream<Pair<Application, ApplicationQuote>>> optionalPairStream
以及您可能正在尋找的內容,只需添加一個默認大小寫,並將Stream
作為:
Stream<Pair<Application, ApplicationQuote>> pairStream = application
.map(app -> Pair.of(app, Optional.ofNullable(app.getQuotes())))
.filter(quote -> quote.getValue().isPresent())
.map(quote -> quote.getValue().get().stream().map(q -> Pair.of(quote.getKey(), q)))
.orElse(Stream.empty());
Optional.stream
使用Java9,您可以更新相同的代碼:
Stream<Pair<Application, ApplicationQuote>> pairStream = application
.map(app -> Pair.of(app, Optional.ofNullable(app.getQuotes())))
.filter(quote -> quote.getValue().isPresent())
.stream() // new API
.flatMap(quote -> quote.getValue().orElse(Collections.emptyList())
.stream().map(q -> Pair.of(quote.getKey(), q)));
像這樣的東西?
Optional<Pair<String,List<String>> quotesForApp =
app.map(a -> a.getQuotes()
.stream()
.map(Quote::getValue())
.collect(toList()) // optional list of quotes now
// we can call app.getCode() safely, or we wouldn't be mapping
.map(quotes -> Pair.of(app.getCode(), quotes));
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