如何將java 8 map，filter和streams轉換為scala？

Question

我是scala嘗試理解的新手，通過將等效的java更改為scala來讓我更好地理解。

如何將java 8 map，filter和streams轉換為scala？

我有以下java 8代碼，我試圖轉換為Scala：

 public Set<String> getValidUsages(String itemId, long sNo, Date timeOfAccess) {                                                     
     Set<String> itemSet = Sets.newHashSet();                                                                                                   
     TestWindows testWindows = items.get(itemId).getTestWindows();                                                          

     final boolean isTV = existsEligibleTestWindow(testWindows.getTV(), timeOfAccess);                                       
     if (isTV) {                                                                                                                              
         itemSet.add(TV);                                                                                                              
     } else {                                                                                                                                    
         final boolean isCableUseable = existsEligibleTestWindow(testWindows.getCableUse(), timeOfAccess);                         
         final boolean isWifi = existsEligibleTestWindow(testWindows.getWifi(), timeOfAccess);                               
         if (isCableUseable || isWifi) {                                                                                                     
             itemSet.add(MOVIE);                                                                                                           
         }                                                                                                                                       
     }                                                                                                                                           

     if (testWindows.getUsageIds() != null) {                                                                                           
         itemSet.addAll(testWindows.getUsageIds()                                                                                      
                 .entrySet()                                                                                                                     
                 .stream()                                                                                                                       
                 .filter(entry -> existsEligibleTestWindow(entry.getValue(), timeOfAccess))                                               
                 .map(Map.Entry::getKey)                                                                                                         
                 .collect(Collectors.toSet()));                                                                                                  
     }                                                                                                                                           

     return itemSet;                                                                                                                            
 }                                                                                                                                               

 private boolean existsEligibleTestWindow(List<TestWindow> windows, Date timeOfAccess) {                                           
     if (windows != null) {                                                                                                                      
         return windows.stream()                                                                                                                 
                 .filter(w -> withinDateRange(timeOfAccess, w))                                                                                  
                 .findAny()                                                                                                                      
                 .isPresent();                                                                                                                   
     }                                                                                                                                           
     return false;                                                                                                                               
 }                                                                                                                                               

 private boolean withinDateRange(Date toCheck, TestWindow window) {                                                                       
     return toCheck.after(window.getStartTime()) && toCheck.before(window.getEndTime());                                                         
 }  

我試過了 ：

 def withinDateRange(toCheck: Date, window: TestWindow): Boolean = {
    toCheck.after( window.getStartTime ) && toCheck.before( window.getEndTime )
  }


  def getValidUsages(itemId: String, sNo: Long, timeOfAccess: Date): Set[String] = {
    var itemSet = Sets.newHashSet()
    val testWindows = items.value(itemId).getTestWindows
    val isTV = existsEligibleTestWindow(testWindows.get(0).getTV, timeOfAccess)
    if (isTV) {
      itemSet += TV
    } else {
      val isCableUseable = existsEligibleTestWindow(testWindows.get(0).getCableUse, timeOfAccess)
      val isWifi = existsEligibleTestWindow(testWindows.get(0).getWifi, timeOfAccess)
      if (isCableUseable || isWifi) {
        itemSet += MOVIE
      }
    }
    if (testWindows.get(0).getUsageIds != null) {
      itemSet.addAll(testWindows.get(0).getUsageIds.entrySet().stream()
        .filter((x) => existsEligibleTestWindow(x._2, timeOfAccess)).map(x => Map.Entry._1 )
        .collect(Collectors.toSet()))
    }
    itemSet
  }


   def existsEligibleConsumptionWindow(windows: List[ConsumptionWindow], timeOfAccess: Date): Boolean = {
if (windows != null) {
  return windows.exists((x) => withinDateRange(timeOfAccess, x))
}
false
}

但是在進行過濾和流時會出錯。 有人能指出方向嗎？ 任何參考？ 我在getValidUsages上收到錯誤：

 compile error “cannot resolve reference project with such signature

Answer 1

這有點難以回答，因為我不熟悉您使用的某些類型。 但是，如果我猜有類似如下的類型：

trait Window {
  def getStartTime: LocalDate
  def getEndTime: LocalDate
}
trait TestWindows extends Window {
  def getTV: List[Window]
  def getCableUse: List[Window]
  def getWifi: List[Window]
  def getUsageIds: Map[String, List[Window]]
}

然后你可以這樣做：

  def withinDateRange(toCheck: LocalDate)(window: Window): Boolean =
    window.getStartTime.isBefore(toCheck) && window.getEndTime.isAfter(toCheck)

  // Nothing should ever be null in Scala. If it's possible you don't have any ConsumptionWindows you should either
  // model it as an empty list or an Option[List[ConsumptionWindow]]
  def existsEligibleTestWindow(windows: List[Window],
                               timeOfAccess: LocalDate): Boolean =
  windows.exists(withinDateRange(timeOfAccess))


  def getValidUsages(testWindows: TestWindows, timeOfAccess: LocalDate): Set[String] = {
    val isTV = existsEligibleTestWindow(testWindows.getTV, timeOfAccess)
    val isCableUse = existsEligibleTestWindow(testWindows.getCableUse, timeOfAccess)
    val isWifi = existsEligibleTestWindow(testWindows.getWifi, timeOfAccess)
    val tvOrMovie: Option[String] = if (isTV) Some("TV")
                                    else if (isCableUse || isWifi) Some("MOVIE")
                                    else None

    val byUsageId = testWindows.getUsageIds.collect { case (key, windows) if existsEligibleTestWindow(windows, timeOfAccess) => key }.toSet

    tvOrMovie.toSet ++ byUsageId
  }

在您的原始代碼中，可能有一些items值，但在上面我假設您在getValidUsages函數之外執行TestWindows testWindows = items.get(itemId).getTestWindows() 。

我的示例根本不使用java結構，只使用scala核心集合。 另一個主要區別是我使用不可變數據結構，我認為這種結構更易於遵循並且通常更安全。

一些注意事項：1）當調用None時，Option.toSet操作導致空集。 2）有一個用於withinDateRange方法的函數currying的例子。 3）我顯然不知道你的原始類型做了什么，不得不猜測相關部分。

Answer 2

問題似乎是你在Scala中使用Java類型，同時依賴於scala的map和filter操作。 這有其自身的麻煩但是如果你首先將列表/集合轉換為Scala的集合 （警告，Scala類型默認是不可變的），那么你應該能夠只使用map / filter操作而不必調用java的stream()方法。

Answer 3

def getValidUsages(itemId: String, sNo: long, timeOfAcess: Date): Set[String] = {
  var itemSet: Set[String] = Sets.newHashSet()
  val testWindows: TestWindows = items.get(itemId).getTestWindows()
  val isTV: Boolean  = existsEligibleTestWindow(testWindows.getTV(), timeOfAccess)

  isTV match {
    case true => itemSet.add(TV)
    case false => {
      val isCableUseable: Boolean = existsEligibleTestWindow(testWindows.getCableUse(), timeOfAcess)
      val isWifi: Boolean = existsEligibleTestWindow(testWindows.getWifi(), timeOfAccess)
      if(isCableUseable || isWifi) {
        itemSet.add(MOVIE)
      }
    }
  }

  if(testWindows.getUsageIds() != null) {
    itemSet.addAll(testWindows.getUsageIds()
                .stream.
                .filter(entry => existsEligibleTestWindow(entry._2, timeOfAccess))
                .map(filteredData => Map.Entry._1)
                .collect Collectors.toSet())
  }
  itemSet
}

def existsEligibleTestWindow(windows: List[TestWindow], timeOfAcess: Date): Boolean = {
  windows match {
    case null => false
    case _ => windows.stream.filter(data => withinDateRange(timeOfAcess), data).findAny().isPresent
  }
}

祝好運 ：）