Writing an equivalent java function in scala with return value

Question

I am trying to convert a java based line of code to scla but not geeting the same functionality

def isParameterSet(parameter: String): Boolean = {

//java based

val iterator=newConfig.entrySet().iterator()

while (iterator.hasNext())
{
  if (iterator.next().getKey()==parameter)
    return true

}
return false
}

converting this to more scala friendly version:

def isParameterSet(parameter: String): Boolean = {

    import scala.collection.JavaConversions._


val iterator=newConfig.entrySet().foreach(u=>if (u.getKey()=="parameter") return true)
return false
}

the above code always returns false

Do you care if the value is null? In general Scala does not want null in its system but since the newConfig is a Java map there is a chance that you can have a value null — Jono, Aug 11 '16 at 02:20

score 2 · Answer 1 · answered Aug 11 '16 at 01:58

2

using exists function，there is a simple sample, I think you will get the idea

scala> var l = List(1,2,3,4,5)
l: List[Int] = List(1, 2, 3, 4, 5)

scala> l.exists( x=>x==5)
res4: Boolean = true

answered Aug 11 '16 at 01:58

Jade Tang

321
4
23

i would prefer it to be val rather than var. Any specific reason to use var?? – Rakshith Aug 11 '16 at 02:41
1

No specific reason, and I do not think it is relevant to this question. – Jade Tang Aug 11 '16 at 03:49
It's not relevant but it is important. The spaces are also all over the place: `List(1, 2, 3, 4, 5)` and `l.exists(x => x == 5)` are easier to read – Fabien Warniez Sep 16 '16 at 21:41

score 0 · Answer 2 · answered Aug 11 '16 at 02:46

forEach is a sideEffecting function and will iterate over the entire Map. The closure passed into forEach cannot be considered the same an an imperative while loop. Scala and functional programming are declarative. So all you are doing is declaring some function to run on each Tuple of the map.

u=>if (u.getKey()=="parameter") return true

This is a function declared that will be executed over every element of the Map (maps can be traversed like lists). The lambda also is running in a different context than isParameterSet. I think the equivalent code segment to your scala code is:

def checkParameter(parameter: String, iterator: Iterator): Unit = {
 if (iterator.next().getKey() == parameter)
        return true

}

def isParameterSet(parameter: String): Boolean = {

    //java based

    val iterator = newConfig.entrySet().iterator()

    while (iterator.hasNext()) {
     checkParameter(parameter, iterator)
    }
    return false
  }

So here the return true does not affect isParameterSet since its in a different context.

Look at how closures and Lambda expressions work. The idea is not Scala specific so you can learn the concepts in something you are more familiar with (Java 8 has closures now). Learning this stuff on top of a unfamiliar language can be frustrating.

Below is some code to deal with how to check if your parameter is set. I made some assumptions here where a null value means a parameter is not set. I also tried to comment my intentions for you. Let me know if its not clear

    package parameter_set

    // Its usually considered ideal to keep all imports at the top of the code just like in Java
    import java.util

    import scala.collection.JavaConverters._

    object ParameterSet extends App {
      val javaConfig: util.Map[String, String] = new util.HashMap[String, String]()
      javaConfig.put("param1", "value1")
      javaConfig.put("param2", "value2")
      javaConfig.put("param3", "value3")
      javaConfig.put("param4", null)

      // The Option will make sense in a second
      val newConfig: Map[String, String] = javaConfig
        // converts it to a mutable map
        .asScala
        // converts it to an immutable map
        .toMap
        // I am a bit paranoid since this was a Java map and I need to make sure all null values are handled as Option
        .mapValues(value => Option(value))
        // This resolves a known issue since mapValues creates a view over the underlying Map (http://stackoverflow.com/questions/14882642/scala-why-mapvalues-produces-a-view-and-is-there-any-stable-alternatives)
        .view
        .force
        // Now we need to only take the key -> values
        .collect {
          case (key, Some(value)) => key -> value
        }



      def isParameterSet(parameter: String): Boolean = newConfig.exists(parameters => parameters._1 == parameter)

      println(isParameterSet("param2")) //true
      println(isParameterSet("param7")) //false
      println(isParameterSet("param4")) //false



    }

score 0 · Answer 3 · answered Aug 11 '16 at 15:47

Unwrap your Config object (root) to get a Java map, then convert that to a Scala map, then test for the parameter key:

    def isParameterSet(parameter: String): Boolean = {

      import scala.collection.JavaConversions._


      mapAsScalaMap(newConfig.root().unwrapped()).contains(parameter)
    }

Writing an equivalent java function in scala with return value

3 Answers3