How to convert java 8 map, filter and streams to scala?

Question

I am very new to scala trying to understand by changing the equivalent java to scala so that I get better understanding.

How to convert java 8 map, filter and streams to scala ?

I have the following java 8 code which I am trying to convert to 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());                                                         
 }  

I tried :

 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
}

But getting error while doing filter and stream. Can some one point to direct direction ? Any references ? I am getting error on getValidUsages :

 compile error “cannot resolve reference project with such signature

Answer 1

This is somewhat difficult to answer since I am unfamiliar with some of the types you use. But if I guess there are types like the following:

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]]
}

then you could just do this:

  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
  }

In your original code there was presumably some items value, but in the above I just assume you do TestWindows testWindows = items.get(itemId).getTestWindows() outside the getValidUsages function.

My example doesn't use java structures at all and just uses the scala core collections. The other main difference is that I use immutable data structures which is, I think, a little easier to follow and generally safer.

Some items of note: 1) The Option.toSet operation result in an empty set when called upon a None. 2) There is an example of function currying used for the withinDateRange method. 3) I obviously have no idea what your original types do and had to guess at the relevant parts.